COMBINING NATURAL AND SYNTHETIC MATERIALS TO PRODUCE MULTIFUNCTIONAL COMPOSITES By Mariana Desireé Reale Batista A DISSERTATION Submitted to Michigan State University in partial fulfillment of the requirements for the degree of M aterials Science and Engineering Doctor of Philosophy 201 9 ABSTRACT COMBINING NATURAL AND SYNTHETIC MATERIALS TO PRODUCE MULTIFUNCTIONAL COMPOSITES By Mariana Desireé Reale Batista Synthetic fiber reinforced composites offer excellent mechanical properties and performance. However, due to the environment awareness and the growing requirement for fuel economy there is a growing urgency for combining a polymer with natural materials to create an eco - friendly composite that is also light - w eight and low cost. This research investigates multiple ways of using cellulose fibers in applications ranging from composites for automotive applications to their use on electronic devices, so the broad potential of cellulose can be exploited. The first i nvestigation involves adding cellulose nanocrystals (CNCs) to a conventional carbon fiber - epoxy composite to simultaneously strengthen and toughen the composite. CNCs were functionalized with 3 - aminopropyltriethoxysilane (APTES) and distributed at the inte rphase between a carbon fiber (CF) and an epoxy matrix. Stronger fiber/matrix adhesion was achieved by sizing CFs with a layer of epoxy, and further increase in interfacial shear strength (IFSS) was achieved by adding the f unctionalized CNCs (APTES - CNCs) a t the interphase. Sizing CFs with APTES - CNCs at a concentration of 1.0 wt% resulted in 81% increase in IFSS compared to unsized CFs due to the establishment of covalent bonding and the stiffness of the interphase modulus. Cellulose fibers were investigated at the macro scale when combined with inorganic reinforcements (glass fiber or talc) in a hybrid composite. Hybrid composites were injected molded with total fiber content kept constant at 30 wt% and the properties of the composites were characterized ove r 0 30 wt% of cellulose in a polypropylene (PP) matrix. Tensile, flexural and notched Izod impact tests revealed that in general the mechanical properties decreased with increasing cellulose content. The crystallization temperature (T c ) of the composites i ncreased compared to neat PP, revealing the fibers ability to act as nucleating agents and speed rate of part production which will result in lowering the manufacturing cost. Overall, combining an optimum concentration of cellulose fiber with glass or talc is a promising alternative to reduce or replace the use of inorganic reinforcements on automotive under - the - hood and body interior components. The use of cellulose fiber in electronic devices was investigated. Sensors to detect ultraviolet (UV) radiation were fabricated on a rigid glass substrate as well as on flexible substrates composed of cellulose fiber in the form of a paper or a polyimide film (PI). Carbon Nanotubes (CNTs) were drop - cast between the electrodes of the sensor on each substrate. All se nsors respond immediately to UV On/Off cycles with a change in resistance due to the ability of CNTs to adsorb and desorb oxygen on their surface. Although the PI substrate yielded a sensor with the greater response, the cellulose paper proved to be effect ive to detect UV radiation, keeping its functionality even after being mechanically bent 1000 times, which is an advantage for practical applications. The final project investigated the use of bamboo fibers (BFs) as the main reinforcement in a high - fiber v olume fraction composite. Unidirectional long BF reinforced epoxy composites were made by compression molding and a process to surface treat the BF with sodium hydroxide (NaOH) was performed. Composites with 40v% NaOH modified BFs show a considerable incre ase of 29% for flexural modulus and 26% for flexural strength, compared to 40v% untreated BF reinforced composites. Coating the NaOH modified BF with Graphene Oxide (GO) resulted in composites with greater flexural properties, increasing modulus at 43% and strength at 29%. This research has explored using cellulose fibers at both the nano and macro scales as an addition to synthetic fibers and also as a potential ecofriendly alternative to replace synthetic fiber in reinforced composites. Copyright by MARIANA DESIREÉ REALE BATISTA 201 9 v D edicated to my m om Aguida, my d ad Carlos , my sister Carla , and Zira . vi ACKNOWLEDGEMENTS There are many people that have made completing my graduate studies possible. I t hank you Dr . Lawrence D r zal for being so patient with me and giving me all the guidance, encouragement and support to build on me confidence to finish my doctoral program . It has a lways been a pleasure learning from hi s experience. Thanks to my c ommittee members Dr. Ramani Narayan, Dr. Mahmoodul Haq and Dr. K . Jayaraman for the ir advices and for their willingness to guide me . Many thanks to the Coordenação de Aperfeiçoamento de Pessoal de Nív el Superior (CAPES) for sponsoring the research conducted in this dissertation through the Brazil Scientific Mobility Program [99999.013655/2013 - 02] . I would like to thank NASA and Ford Motor Company for allowing me to improve m y skills through internship s . I am also grateful to the Agência Espacial Brasileira (AEB). To all th e CMSC research members, Per Askeland , Mike Rich, Brian Rook and Ed Drown who were always so helpful in discussing the projects and setting up the experiments. To the CHEMS office and the other graduate students at the CMSC who were there to support each other Zeyang, Markus, Nick, Keith, Yan, Dee . I am grateful for m y friends, specially Erick, Lívia, Juliana , Tha í s and Steven for the fun moments and for always supporting me. I also express my deepest gratitude to all the professors that taught me through my life. Lastly , I am extremely thankful to my beloved parents, sister, David, Mirian , Adna and all my family who were always there to give me emotional support and f or the care and guidance throughout my life . I am a lso thankful to God who gave me faith to finish this journey. vii TABLE OF CONTENTS .. . x LIST OF ... . . xi CHAPTER 1: INTRODUCTION AND LITERATURE REVIEW ................................ ............... 1 1.1 Introduction ................................ ................................ ................................ ...................... 1 1.2 Motivation ................................ ................................ ................................ ........................ 2 1.3 Significant research proposal ................................ ................................ ........................... 4 Chapter 2: Carbon Fiber/Epoxy Matrix Composite Interphases Modified with Cellulose Nanocrystals ................................ ................................ ................................ ............ 4 Chapter 3: Hybrid Cellulose - Inorganic Reinforcement Polypropylene Composites: Lightweight Materials for Automotive Applications ................................ .............................. 5 Chapter 4: Flexible Ultraviolet Sensor based on Carbon Nanotubes ........................... 6 Chapter 5: Surface Modification of Bamboo in Epoxy Composites ............................ 7 Literature review ................................ ................................ ................................ .............. 8 Composite materials ................................ ................................ ................................ ..... 8 Cellulose fiber ................................ ................................ ................................ ............. 10 Natural fiber treatments ................................ ................................ ....................... 13 Cellulose nanocrystals (CNCs) ................................ ................................ ................... 15 Epoxy ................................ ................................ ................................ .......................... 17 m - Phenylenediamine (mPDA) ................................ ................................ ................... 18 Epoxy/amine curing ................................ ................................ ............................ 19 Carbon fibers (CFs) ................................ ................................ ................................ .... 21 Carbon fiber reinforced epoxy composites ................................ ......................... 22 Fiber surface treatments ................................ ................................ ...................... 23 Interphase tailoring via sizing or coating ................................ ............................ 23 Functional interphase: effects of adding nanoparticles in the sizing .................. 25 Polypropylene (PP) ................................ ................................ ................................ ..... 26 Carbon Nanotubes (CNTs) ................................ ................................ ......................... 27 REFERENCES ................................ ................................ ................................ ............................. 30 CHAPTER 2: CARBON FIBER/EPOXY MATRIX COMPOSITE INTERPHASES MODIFIED WITH CELLULOSE NANOCRYSTALS ................................ ................................ ................... 37 2.1 Abstract ................................ ................................ ................................ .......................... 37 2.2 Introduction ................................ ................................ ................................ .................... 38 2.3 Experimental ................................ ................................ ................................ .................. 41 Materials ................................ ................................ ................................ ..................... 41 Methods ................................ ................................ ................................ ...................... 42 Surface treatment of CNCs ................................ ................................ ................. 42 Sizing of CFs with APTES - CNC ................................ ................................ ........ 43 Preparation of SFFT coupon ................................ ................................ ............... 45 Characterization methods ................................ ................................ ........................... 45 Fourier transform infrared (FTIR) spectroscopy ................................ ................. 45 viii X - ray photoelectron spectroscopy (XPS) ................................ ............................ 46 Scanning Electron Microscopy (SEM) ................................ ............................... 46 Single Fiber Fragmentation Test (SFFT) ................................ ............................ 46 Results and discussion ................................ ................................ ................................ .... 48 CNC surface modification with APTES ................................ ................................ ..... 48 Characterization of APTES - CNC sized CFs ................................ .............................. 53 Evaluation of the IFSS by adding APTES - CNCs at the composite interphase .......... 56 Interfacial failure mode analysis ................................ ................................ ................. 60 2.5 Conclusions ................................ ................................ ................................ .................... 61 REFERENCES ................................ ................................ ................................ ............................. 63 CHAPTER 3: HYBRID CELLULOSE - INORGANIC REINFORCEMENT POLYPROPYLENE COMPOSITES: LIGHTWEIGHT MATERIALS FOR AUTOMOTIVE APPLICATIONS ...... 67 Abstract ................................ ................................ ................................ .......................... 67 3.2 Introduction ................................ ................................ ................................ .................... 68 3.3 Experimental ................................ ................................ ................................ .................. 69 Materials ................................ ................................ ................................ ..................... 69 Processing ................................ ................................ ................................ ................... 70 Testing procedure and characterization ................................ ................................ ...... 72 Mechanical test ................................ ................................ ................................ .... 72 Scanning electron microscopy (SEM) ................................ ................................ . 73 Thermal characterization ................................ ................................ ..................... 74 Results and discussion ................................ ................................ ................................ .... 75 Mechanical properties ................................ ................................ ................................ . 75 Tensile properties ................................ ................................ ................................ 75 Flexural properties ................................ ................................ ............................... 79 Impact properties ................................ ................................ ................................ . 81 Morphological properties ................................ ................................ ........................... 82 Thermal properties ................................ ................................ ................................ ...... 88 Thermal Gravimetric Analysis (TGA) ................................ ................................ 88 Differential Scanning Calorimetry (DSC) ................................ ........................... 93 3.5 Conclusions ................................ ................................ ................................ .................... 95 REFERENCES ................................ ................................ ................................ ............................. 97 CHAPTER 4: FLEXIBLE ULTRAVIOLET SENSOR BASED ON CARBON NANOTUBES ................................ ................................ ................................ ................................ ..................... 100 4.1 Abstract ................................ ................................ ................................ ........................ 100 4.2 Introduction ................................ ................................ ................................ .................. 101 4.3 Experimental section ................................ ................................ ................................ .... 103 Materials ................................ ................................ ................................ ................... 103 Methods ................................ ................................ ................................ .................... 104 4.4 Results and discussion ................................ ................................ ................................ .. 105 CNT distribution on sensor substrate ................................ ................................ ....... 105 Sensor response to UV light ................................ ................................ ..................... 108 Sensor mechanical robustness ................................ ................................ .................. 113 Sensor response under natural sunlight ................................ ................................ .... 116 ix 4.5 Conclusions ................................ ................................ ................................ .................. 118 4.6 Acknowledgements ................................ ................................ ................................ ...... 119 REFERENCES ................................ ................................ ................................ ........................... 120 CHAPTER 5: SURFACE MODIFICATION OF BAMBOO IN EPOXY COMPOSITES ....... 123 5.1 Abstract ................................ ................................ ................................ ........................ 123 5.2 Introduction ................................ ................................ ................................ .................. 124 5.3 Experimental ................................ ................................ ................................ ................ 128 Materials ................................ ................................ ................................ ................... 128 Methods ................................ ................................ ................................ .................... 129 Surface treatment of BFs with NaOH ................................ ............................... 129 Surface treatment of NaOH modified BFs with GO ................................ ......... 129 Processing of unidirectional composites ................................ ........................... 129 Characterization methods ................................ ................................ ......................... 131 Scanning Electron Microscopy (SEM) ................................ ............................. 131 X - ray photoelectron spectroscopy (XPS) ................................ .......................... 131 Flexural test ................................ ................................ ................................ ....... 132 SBSS test ................................ ................................ ................................ ........... 132 5.4 Results and discussion ................................ ................................ ................................ .. 132 Characterization of surface treated BFs ................................ ................................ .... 132 Mechanical properties of BF reinforced epoxy composites at 22 v%, 40 v% and 50 v% BFs ................................ ................................ ................................ ................................ 138 Mechanical properties of NaOH and NaOH/GO modified BF reinforced epoxy composites at 40 v% BFs ................................ ................................ ................................ ..... 140 SEM observation of composites fracture surface ................................ ..................... 144 Evaluation of the short beam strength ................................ ................................ ...... 147 Conclusions ................................ ................................ ................................ .................. 148 REFERENCES ................................ ................................ ................................ ........................... 150 CHAPTER 6: SUMMARY AND FUTURE WORK ................................ ................................ . 154 6.1 Summary ................................ ................................ ................................ ...................... 154 6.2 Future work ................................ ................................ ................................ .................. 156 Sizing CF with APTES - CNC ................................ ................................ ................... 156 Adding compatibilizer to hybrid composites ................................ ............................ 156 Improving recovery time of UV sensors ................................ ................................ .. 157 Increasing the concentration of GO on BF reinforced epoxy composites ................ 157 x LIST OF TABLES Table 1.1: Comparison between plant and synthetic fibers [12]. ................................ ................... 3 Table 1.2: Density and properties of some plant based fibers [7]. ................................ ............... 13 Table 1.3: EPON 828 properties [40]. ................................ ................................ .......................... 18 Table 1.4: mPDA properties [43 ]. ................................ ................................ ................................ . 19 Table 1.5: Typical CF properties [48]. ................................ ................................ .......................... 22 Table 2.1: O/C ratio and element composition in percent for CNC and APTES - CNC. ............... 51 Table 2.2: Relative amount of C1s components (%) for CNC and APTES - CNC. ....................... 53 Table 2.3: Fiber fragment aspect ratio (l c /d) and IFSS for 12k tow sized CFs. ............................ 59 Table 2.4: Fiber fragment aspect ratio (l c /d) and IFSS for individually sized CFs. ...................... 59 Table 3.1: As received pellets compositions. ................................ ................................ ................ 70 Table 3.2: Sample identification c odes and composite composition for each constituent source. 71 Table 3.3: Enthalpies of crystallization for neat PP and composites. ................................ ........... 94 Table 3.4: Enthalpies of melting for neat PP and composites. ................................ ..................... 94 Table 4.1: Roughness of CNT coated substrates by AFM (nm) on a 5 µm x 5 µm area. .......... 107 Table 4.2: Roughness of CNT coated substrates by AFM (nm) on a 1 µm x 1 µm area. .......... 108 Table 4.3: Resistance change (%) of se nsors for varying radius of curvature. ........................... 114 Table 5.1: Bamboo Fiber Material Specifications. ................................ ................................ ..... 128 Table 5.2: Composites composition and fiber surface treatment. ................................ ............... 131 Table 5.3: Percent e lement composition for BF, and . ................................ ................................ ................................ ................................ ............... 136 Table 5.4: Relative amount of C1s components (%) for BF, NaOH modified BF and NaOH/GO modified BF. ................................ ................................ ................................ ............................... 137 xi LIST OF FIGURES Figure 1.1: Overview of the project discussed on Chapter 3. ................................ ......................... 5 Figure 1.2: Simple scheme for the classification of composite materials [17]. .............................. 9 Figure 1.3: Stress position profiles when fiber length (a) is equal to the critical length, (b) is greater than the critical length, and (c) is less than the critical length [17]. ................................ . 10 Figure 1.4: Molecular chain structure of cellulose [23] ................................ ................................ 11 Figure 1.5: MFA definition in the microstructure of plant cell wall together with cell wall sub - layers [7] ................................ ................................ ................................ ................................ ....... 12 Figure 1.6: Number of publications related with nanocellulose [34]. ................................ .......... 16 Figure 1.7: Diglycidyl ether of bisphenol A (DGEBA) [41]. ................................ ....................... 17 Figure 1.8: Structure of mPDA [43]. ................................ ................................ ............................ 18 Figure 1.9: Three major reactions occurring in epoxy/amine systems [44]. ................................ 20 Figure 1.10: FIB - SEM micrographs of CF from lower to higher magnification. ......................... 22 Figure 1.11: Chemical structure of polypropylene [63]. ................................ ............................... 26 Figure 1.12: Tacticity of polypr opylene [63]. ................................ ................................ ............... 27 Figure 1.13: Schematic diagram of the chiral vector and the chiral angle [69]. ........................... 28 Figure 1.14: Schematic illustrations of the structures of (A) armchair, (B) zigzag, and (C) chiral SWNTs [70]. ................................ ................................ ................................ ................................ . 29 Figure 2.1: Solvent exchange process using a rotary evaporator. ................................ ................. 43 Figure 2.2: Schematic illustration of the sizing process: a) 20s dipping of the 12k filament tow and b) 33 min immersion of metal frame with individual CFs. ................................ .................... 44 Figure 2.3: Schematic of preparation of the SFFT coupon. ................................ .......................... 45 Figure 2.4: Schematic of preparation of the SFFT coupon. ................................ .......................... 47 Figure 2.5: FTIR spectra for CNC, APTES - CNC (dried at RT for 24h) and APTES - CNC (dried at RT for 24h and at 120 °C for 2h). ................................ ................................ ............................. 49 Figure 2.6: XPS survey of (a) CNC and (b) APTES - CNC. ................................ .......................... 50 xii Figure 2.7: Deconvolution of C1s peak for (a) CNC and (b) APTES - CNC. ................................ 52 Figure 2.8: FIB - SEM micrographs of a) unsized CF and b) epoxy - only sized CF. ..................... 54 Figure 2.9: FIB - SEM micrographs for the 12k tow sizing technique of a) 0.6 wt% APTES - CNC sized CF, b) 1.0 wt% APTES - CNC sized CF and c) 2.0 wt% APTES - CNC sized CF. ............... 55 Figure 2.10: FIB - SEM micrographs for the individual sizing technique of a) 0.6 wt% APTES - CNC sized CF, b) 1.0 wt% APTES - CNC sized CF and c) 2.0 wt% APTES - CNC sized CF. ..... 56 Figure 2.11: IFSS for the a) 12k tow sized CFs and b) individually sized CFs. .......................... 57 Figure 2.12: Transmitted light optical micrographs of the birefringence pattern at 20x and 50x for a) unsized CF, b) epoxy - only si zed CF and 12k tow sized CF at c) 0.6 wt% APTES - CNC, d) 1.0 wt% APTES - CNC and e) 2.0 wt% APTES - CNC. ................................ ................................ . 61 Figure 3.1: Schematic illustra tion of materials, processing and characterization. ........................ 72 Figure 3.2: Schematic illustration of SEM analysis. ................................ ................................ ..... 73 Figure 3.3: Tensile stress at maximum load of a) LGF/Cellulose A, b) (SGF/Mica)/Cellulose B, c) Talc/Cellulose B and d) SGF/Cellulose B composites along with neat PP. ............................. 77 Talc/Cellulose B and d) SGF/C ellulose B composites along with neat PP. ................................ . 78 Figure 3.5: Tensile strain at maximum load of a) LGF/Cellulose A, b) (SGF/Mica)/Cellulos e B, c) Talc/Cellulose B and d) SGF/Cellulose B composites along with neat PP. ............................. 79 Figure 3.6: Stress at 5% strain of a) LGF/Cellulos e A, b) (SGF/Mica)/Cellulose B, c) Talc/Cellulose B and d) SGF/Cellulose B composites along with neat PP. *Specimens broke before 5% strain; maximum stress value used. ................................ ................................ ............. 80 Figure 3.7: Flexural modulus of a) LGF/Cellulose A, b) (SGF/Mica)/Cellulose B, c) Talc/Cellulose B and d) SGF/Cellulose B composites along with neat PP. ................................ . 81 Figure 3.8: Impact strength of a) LGF/Cellulose A, b) (SGF/Mica)/Cellulose B, c) Talc/Cellulose B and d) SGF/Cellulose B composites along with neat PP. ................................ ......................... 82 Figure 3.9: SEM micrographs of a) Neat PP X, b) LGF/Cellulose A (30/0), c) LGF /Cellulose A (15/15) and d) LGF/Cellulose A (0/30). ................................ ................................ ....................... 84 Figure 3.10: SEM micrographs of a) Neat PP Y, b) (SGF/Mica)/Cellulose B (30/0), c) (SGF/Mica)/Cellulose B (15/15) and d) (SGF/Mica)/Cell ulose B (0/30). ................................ ... 85 Figure 3.11: SEM micrographs of a) Neat PP Y, b) Talc/Cellulose B (30/0), c) Talc/Cellulose B (15/15) and d) Talc/Cellulose B (0/30). ................................ ................................ ........................ 86 xiii Figure 3.12: SEM micrographs of a) Neat PP Z, b) SGF/Cellulose B (30/0), c) SGF/Cellulose B (15/15) and d) SGF/Cellulose B (0/30). ................................ ................................ ........................ 87 Figure 3.13: SEM micrographs of a) 30wt. % Cellulose A and b) 30wt. % Cellulose B. ............ 88 Figure 3.14: Temperature at the maximum rate of decomposition determined from DTG curves for a) LGF/Cellulose A, b) (SGF/Mica)/Cellulose B, c) Talc/ Cellulose B and d) SGF/Cellulose B composites along with neat PP. ................................ ................................ ................................ .... 89 Figure 3.15: Temperature at 1% weight loss from TGA curves for a) LGF/Cellulose A, b) (SGF/Mica)/Cellulose B, c) Talc/Cellulose B and d) SGF/Cellulose B composites along with neat PP. ................................ ................................ ................................ ................................ ......... 90 Figu re 3.16: Temperature at 10% weight loss from TGA curves for a) LGF/Cellulose A, b) (SGF/Mica)/Cellulose B, c) Talc/Cellulose B and d) SGF/Cellulose B composites along with neat PP. ................................ ................................ ................................ ................................ ......... 91 Figure 3.17: Residual weight percent at 587 °C from TGA curves for a) LGF/Cellulose A, b) (SGF/Mica)/Cellulose B, c) Talc/Cellulose B and d) SGF/Cellulose B composites along with neat PP. ................................ ................................ ................................ ................................ ......... 92 Figure 3.18: Crystallization and melting temperatures for a) LGF/Cellulose A, b) (SGF/Mica)/Cellulose B, c) Talc/Cellulose B and d) SGF/Cellulose B composites. ................... 93 Figure 4.1: Sensor structure and test set up. ................................ ................................ ............... 104 Figure 4.2: SEM micrographs of CNT coated a) PI, b) glass and c) cellulose paper substrate at higher (top) and lower (bottom) magnification. ................................ ................................ ......... 106 Figure 4.3: Height (top) and amplitude (bottom) profile of CNT coated a) PI, b) glass and c) cellulose paper substrate by AFM on a 5 µm x 5 µm area. ................................ ........................ 107 Figure 4.4: Height (top) and amplitude (bottom) profile of CNT coated a) PI, b) glass and c) cellulose paper substrate by AFM on a 1 µm x 1 µm area. ................................ ........................ 108 Figure 4.5: Response of sensors under UV illumination. ................................ ........................... 109 Figure 4.6: Resistance change of sensors under UV On/Off cycling. ................................ ........ 110 Figure 4.7: Response under UV illumination as a function of CNT weight percent on PI substrate. ................................ ................................ ................................ ................................ ..... 112 Figure 4.8: Resistance change as a function of power density under On/Off cycle for PI substrate. ................................ ................................ ................................ ................................ ..................... 113 Figure 4.9: Mechanical bending test with varying radius of curvature. ................................ ..... 114 Figure 4.10: Bending cycle test at a curvature radius of 10 mm (condit ioning step). ................ 115 xiv Figure 4.11: UV response of the conditioned a) PI and b) cellulose paper sensors under flat and 10 mm bending stat e. ................................ ................................ ................................ .................. 116 Figure 4.12: a) Design of electrodes and b) printing on PI substrate for c) potential application on sunglasses and wristband. ................................ ................................ ................................ ........... 117 Figure 4.13: Resistance change of UV sensor on wristband under sunlight. ............................. 118 Figure 5.1: SEM of polished surface of cross section of bamboo fiber bundle embedded in epoxy. ................................ ................................ ................................ ................................ .......... 124 Figure 5.2: (a) , (b) tensile strength and (c) IFSS of untreated, GO - treated, and Graphene flake - coated jute fibers [6]. ................................ ................................ ........................ 127 Figure 5.3: Processing of BF reinforced epoxy composite. ................................ ........................ 130 Figure 5.4: SEM images of the surface of (a) BFs, (b) NaOH modified BFs and (c) NaOH/GO modified BFs, top is lower magnification (377X) and bottom is higher mag nification (9000X). ................................ ................................ ................................ ................................ ..................... 134 Figure 5.5: XPS survey of (a) BF, (b) NaOH modified BF and (c) NaOH/GO modified BF. ... 135 Figure 5.6: Deconvolution of C1s peak for (a) BF, (b) NaOH modified BF and (c) NaOH/GO modified BF. ................................ ................................ ................................ ............................... 137 Figure 5.7: (a) Flexural modulus, (b) flexural strength and (c) flexural strain along the fiber longitudinal direction for neat epoxy and epoxy composites reinforced at 22 v%, 40 v% and 50 v% BF. ................................ ................................ ................................ ................................ ........ 139 Figure 5.8: (a) Flexural modulus, (b) flexural strength and (c) flexural strain along the fiber longitudinal dir ection for epoxy composites reinforced at 40 v% BF, 40 v% NaOH modified BF and 40 v% NaOH/GO modified BF. ................................ ................................ ........................... 143 Figure 5.9: Flexural f racture surface for epoxy composites reinforced at (a) 40 v% BF, (b) 40 v% NaOH modified BF and (c) 40 v% NaOH/GO modified BF, top is lower magnification (177X) and bottom is higher magnification (2500X). ................................ ................................ ............. 144 Figure 5.10: Flexural fracture surface for epoxy composites reinforced at 40 v% BF. .............. 145 Figure 5.11: Flexural fracture surface for epoxy composites reinforced at 40 v% NaOH modified BF. ................................ ................................ ................................ ................................ ............... 146 Figure 5.12: Flexural fracture surface for epoxy composites reinforced at 40 v% NaOH / GO modified BF. ................................ ................................ ................................ ............................... 147 Figure 5.13: Short beam st rength for epoxy composites reinforced at 40 v% BF, 40 v% NaOH modified BF and 40 v% NaOH/GO modified BF. ................................ ................................ ..... 148 1 CHAPTER 1 : INTRODUCTION AND LITERATURE REVIEW Introduction For decades synthetic fibers , such as petroleum - based fibers have been the subject of much research, development and application. Although s ynthetic fibers , like carbon fiber and g lass fiber , can produce composites with excellent mechanical properties, they consume high energy during production, they are abrasive to the processing machinery and they contribute to a large amount of carbon dioxide (CO 2 ) emission during production [1] . In addition, petroleum - based fibe rs are made from materials that will become heavily depleted over time and are non - biodegradable. Composite materials made from t hese s ynthetic fibers are also difficult to recycle since the separation of the constituents a t end of life is not easy , which can result in disposal in landfills or by incineration [2] . In this context, there is a growing urgency to develop biomaterials, such as with plant - based fibers, as a means to decrease the environmental threat and dependence on fossil fuel. Natural fiber s are obtained from plants , animals or minerals [3] and have been used in various applications such as building materials, insulation boards, animal feed, cosmetics, medicine and fine chemicals [4] . The natural fiber s discussed o n this dissertation are plant - based , consist ing mainly of cellulose , hemicellulose and lignin with lesser amounts of other components [5] , an d will be termed in this study as cellulose fibers . There is a variety of plant - based natural fibers of which bamboo, jute, sisal, hemp, cotton, palm , flax and ramie have found practical uses . CO 2 neutrality after burning, abundant availability , s ustainabl e nature and non - abrasiveness to the processing equipment [6] are some of the cellulose fibers promising benefits that make them attractive for a broad range of applications . 2 Plant - based fibers are attracting considerable attention as reinforcement s for polymer composites owing to their low cost, low density and high mechanical pro perties [7] . The ir low density at about 1.15 - 1.50 g/cm 3 versus 2. 5 g/cm 3 f or glass fiber [8] is attractive for automotive , aerospace and sports applications . Reducing economy by 3 to 7 % [9] and contribute to attaining the CAFE standards. Ther efore, the usage of plant - based fibers in the automotive industry is a central strategy for meeting light - weighting and fuel economy standards. The idea of using bio - based materials in vehicle parts wa s first introduced by Ford Motor Company in early 1930s [10] and nowadays natural fibers are being considered as replacements for synthetic fiber s in different components , such as door and instrument panels, armrests, headrests and seat shells [4] . They also potentially could exhibit advantages in terms of cost. Natural fiber reinforced composites are viable replacement s for glass reinforced composite s in automotive applications since their use red uces the use of expensive glass fibers ($0.50/kg versus $3.25/kg ) [8] and they have attractive properties such as better impact resistance that is essential for vehicles [11] . Motivation Cellulose fibers are increasingl y and successfully being used in a large range of applications. The global natural fiber composite market reached $2.1 billion in 2010 [7] and m any natural fiber composite products have been developed . H owever , d espite the ir advantages and usage in load - bearing components, the industrial uptake of natural fiber reinforced polymers in high - level structural applications has been limited [12] . Effort s need to be taken to promote t heir usage as the primary structural components of structures such as in aerospace and maritime applications [13] . According to Shah [12] , over 95% of natural fiber composites in Europe are applied in vehicles as non - structural components. One reason is the higher mechanical properties 3 of synthetic fibers compar ed to natural fibers , as shown on Table 1 . 1 , which makes the synthetic fibers preferable in some applications. Table 1 . 1 : Comparison between plant and synthetic fibers [12] . Properties Plant fiber G lass fiber Carbon fiber Tensile stiffness ( GPa ) Moderate (~30 80) Moderate (70 85) High (150 500) Tensile strength ( GPa ) Low (~0.4 1.5) Moderate (2.0 3.7) High (1.3 6.3) Considering the potential and beneficial properties of cellulose , m o re research needs to be directed in this path, so the real potential of natural fibers can be exploited . This study shows that cellulose fibers can effective ly reinforce polymer composites as a potential ecofriendly alternative to synthetic fiber reinforce d composites and be used in combination with synthetic fibers . This work investigates ways how cellulose fiber s can be used with or instead of synthetic fibers in polymer composite s. In this sense we investigate cellulose fiber at its macro and nano scale and how they can reinforce different polymer matrices such as thermosetting ( epoxy ) and thermoplastic (polypropylene). T he contribution of this project is to investigate cellulose fiber usage with dif ferent materials targeting different application s , as detailed in the section s below . 4 Significant r esearch p roposal The research proposal s hereafter presented is as follows: Chapter 2 : C arbon Fiber/Epoxy Matrix Composite Interphases Modified w ith Cellulose Nanocrystals A c omposite is a combination of two or more components to produce a new material with improved properties in comparison to its constituents . It is usually formed by a fiber that is responsible for the reinforcement properties and a matrix that protects and holds the fiber. C arbon fiber reinforced polymer (CFRP) composites have gained attention because of their high strength - to - weight ratio. H owever, one drawback of CFRP, specifically epoxy - based CFRP, is its brittle fracture mechanism. Brittle fracture is not only dependent on the mechanical properties of the fiber and matrix, but also dependent on the effectiveness of the interfacial adhesion between the components [14] . Therefore, a well - designed interphase is required to achieve improvements in the mechanic al properties of the composites. In this work, a process to modify the composite interphase by coating carbon fiber ( CF ) with functionalized c ellulose n anocrystals (CNCs) as a nano - reinforcement to improve the adhesion between the CF and an epoxy matrix wa s investigated. CNCs were surface treated with 3 - aminopropyl triethoxysilane (APTES) a nd then incorporated in a sizing applied to the fiber. The project goal was to investigate how CNCs could be used to simultaneously strengthen and toughen composite mater ials. CNCs have attracted considerable attention due to their high mechanical properties, high specific surface area, and the well - known advantages of cellulose fibers: low density, availability, and sustainable nature . An o ptimum CNC concentration in the sizing was 5 identified, single fiber adhesion tests to determine the interfacial shear strength (IFSS) were conducted, and appropriate characterization and analyses were completed. The results demonstrate that sizing CFs with APTES - CNCs is an effective method to increase the interfacial properties in CF reinforced epoxy composites, and the technique can p otentially be implemented for interfacial optimization in petroleum - based as well as bio - based natural fiber composites. Chapter 3: Hybrid Cellulose - Inorganic Reinforcement Polypropylene Composites: Li ghtweight Materials f or Automotive Applications Cellul ose fibers are attracting considerable attention within the transportation industry to reinforce polymer composites . Th is research is focused on the development of hybrid composites combining cellulose fiber s with glass fiber s (long and short) or talc in a polypropylene (PP) matrix , and on evaluating the mechanical and thermal properties of the resulting composites for automotive - the - and body interior applications. T he fibers were combined with PP in the form of master - batch pellets and Figure 1 . 1 illustrates the research rationale and advantages. Figure 1 . 1 : O verview of the p roject discussed on Chapter 3 . 6 The common belief is that natural fibers are limited to processing with low melting temperature thermoplastics, however Ozen et al. [15] succeed in melt blending natural fiber blends with nylon 6. The composites with a high content of natural fiber blend displayed enhanced tensile and flexural properties in comparison with neat polymer, which verified that the processing conditions did no t affect the final properties. In th e present w ork cellulose fiber were processed with PP at a maximum temperature of 193°C (380 F) without noticeable degradation . The effects of combining cellulose and glass f ibers or talc is investigated , looking for the ideal concentration of each constituent, and a lso qualifying the fiber - matrix interphase. Results show that in general the mechanical properties (tensile, flexural and impact strength of notched Izod specimens ) decreased with increasing cellulose content . H owever, hybrid composites with an optimum concentration of cellulose fiber is a viable a pproach leading to weight and cost savings and can r educe or replace the use of inorganic fibers in many automotive applications. Chapter 4 : Flexible Ultraviolet Sensor based on Carbon Nanotubes F lexib le substrates and cellulose paper electronics are attracting considerable attention since they offer new capabilities for devices that are not possible with the conventional rigid substrates. Electronics made of cellulose paper are being investigated as low - cost and disposable alternative s i n applications such as sensors and super capacitors . In this work, a sensor to detect Ultraviolet (UV) radiation was developed in which cellulose fibers in the form of a paper was investigated as a sensor substrate . T wo flexible substrates (Cellulose p aper and Polyimide f ilm (PI)) and one rigid substrate ( G lass) were chosen . Cellulose paper was chosen since it is a low cost, lightweight, ecofriendly and disposable material . 7 UV sensors can h elp in the detection of UV radiation and prevention of health problems related to th e intense exposure to this radiation, especially cancer . UV sensors are also widely used in many applications, including space communication and climate change. Carbon Nano tubes (CNTs) as the sensing material and at d ifferent concentration s of CNT solution w ere drop - cast onto the active region between the electrodes of the sensor on each substrate. Multiple test s were conducted including the response of the sensors under UV On/Off illumination cycles and mechanical bending test at varying radius of curvature . All the sensors respond immediately to UV O n /O ff cycles with a change in resistance due to the ability of the CNTs to adsorb and desorb oxygen on their surface . Although the PI substrate yields the sensor with the highest response , the results demonstrated that the cellulose paper has the potential to be u tilized in this electronics application. Chapter 5 : Surface Modification of Bamboo in Epoxy Composites B am boo has fast growing up to 21 cm per day and abundant availability , which make s their fibers attractive compared to other natu ral fibers [16] . 43 GPa and strength varying from 341 to 860 MPa [16] , bamboo fibers (BFs) are excellent material to reinforce polymer composites. However, the intrinsic hydrophilicity of BFs can weak the bonding into the matrix and lower the composite mechanical pr operties. To achieve improved performance , many fiber surface treatments have been done to achieve good fiber - matrix interaction in BF reinforced epoxy composites. In this work , BFs were surface treated with a solution of NaOH and then coated with Graphen e Oxide (GO) . E poxy composites reinforced at a fiber volume content of 40% were processed to assess the im pacts o n flexural properties and short beam shear strength (SBSS). 40v% 8 NaOH/GO modified BF composites show a considerable increase of 43% and 29% for flexural modulus and flexural strength, respectively, compared to 40v% untreated BF composites . A slightly increase of 6% was observed for the SBSS. This investigation shows that the modification of the BF - epoxy matrix interphase with NaOH/GO has the potential to reduce utilization of synthetic fibers in composites and contribute to the global sustainable development. Literature review The following sections review the chemistry, properties and cha racteristics of the materials used and the state of art of natural fiber surface treatment s , mechanism of epoxy curing and recent advances for engineering the composite interphase for improving the adhesion between CF and an epoxy matrix . Composite m aterials Composite is a combination of two or more components to produce a new material with improved properties in comparison to its constituents . One component is the matrix, which is continuous and surrounds the dispersed phase [17] . In general , composites are classified in particle - reinforced, fiber - reinforced, and structural composites [17] as shown on Figure 1 . 2 . 9 Figure 1 . 2 : Simple scheme for the classification of composite materials [17] . F iber reinforced polymer composites are formed by a fiber that is responsible for the reinforcement properties and a polymer matrix that pr otects and holds the fibers. They are ideal for aerospace and automotive industry applications that require lightweight materials with good mechanical properties. Many polymer composites also offer the advantage of being molded in complex shapes. One important parameter in fiber reinforced polymer composites is the fiber c ritical aspect ratio (l c /d ) . Fiber critical length (l c ) is the segment capable of maintaining its integrity at a n applied strain [18] and Figure 1 . 3 shows the stress position profile for a composite subjected to a tensile stress equal to the fiber tensile strength f * . I f the fiber equals or exceeds the required l c /d the maximum fiber loa d is achieved , and if the fiber is shorter than the l c /d it will n ot achieve the maximum load [17] . An effective reinforcement is achieved when the fiber aspect ratio is greater than the critical aspect ratio . 10 Figure 1 . 3 : Stress position profiles when fiber length (a) is equal to the critical length , (b) is greater than the critical length , and (c) is less than the critical length [17] . Cellulose fiber C ellulose is the main load bearing component of plant cell walls and [19] is a stable polymer formed by repeating units of D - anhydroglucose (C 6 H 11 O 5 - 1,4 - glucosidic bond at C 1 and C 4 position [20] a s shown on Figure 1 . 4 . The degree of polymerization ( DP ) of cellulose ( C 6 H 1 0 O 5 ) n is defined by the number of n repeating unit s , and varies depending on the cellulose source. In native cellulose n is up to 1 0,000 and since the length of the anhydroglucose unit is 0.515 nm, the total length of a native cellulose molecule is about 5 µ m [21] . The hydroxyl groups (OH ) of the glucose unit can interact throug h hydrogen bond s with its own chains to create fibrils or with adjacent chains to create microfibrils [22] . The diameter of the elementary fibril is approximately 2 to 4 nm , and when arranged in parallel they form the microfiber with diameter about 10 to 25 nm [23] . Microfibrils entwine into a network to form a fibril with diameter reported at about 500 nm [23] . Within the microfibrils, c ellulose possess crystalline regions where the molecules are bonded by strong hydrogen bonds and arranged in an orderly manner [21,23] . The n o n - crystalline regions are less orde red and amorphous . 11 Figure 1 . 4 : Molecular chain structure of cellulose [23] A single fiber of a plant - based natural fiber consist s of several cells , which ar e formed out of cellulose microfibrils along with l ignin and hemicellulose [3] . The ratio between cellulose and lignin/hemicellulose var ies among natural fibers , and c otton fiber has the highest cellulose content of 95 97 % and 70 % of crystallinity [23] . Figure 1 . 5 shows that the plant cell wall consists of a primary cell wall (P) and three layers of secondary walls ( S 1 , S 2 and S 3 ) whe re the microfibrils are oriented at distinct angles to the axis of the fiber, along with a cell cavity in the center called lumen [3,24] . The mechanical properties of a natural fi ber are generally correlated to its cellulose content and the microfibrillar angle (MFA) of the fibrils [3] . Since the S 2 is the thickest layer, its MFA angle will dictate the elastic properties of the fiber and i n general, the lower the spiral angle, the higher the modulus. Ramie fiber has a spiral angle of 7.5° [3] that is among the lowest for natural fi bers. 12 Figure 1 . 5 : MFA definition in the microstructure of plant cell wall together with cell wall sub - layers [7] . Cellulose is the most abundant renewable resource [23] and c ellulose fibers have been widely investigated due to the increasing environmental awareness and the reduction of nonrenewable resources [6] . A variety of natural fibers are available, and their properties are listed on Table 1 . 2 . 13 Table 1 . 2 : Density and properties of some plant based fibers [7] . Fiber Density (Kg/m 3 ) Tensile strength (MPa) Elongation at break (%) modulus (GPa) Flax 1380 343 1035 1.2 3 27.6 Ramie 1440 400 938 2 4 61.4 128 Sisal 1200 507 885 1.9 3 9.4 22 Banana 1350 529 914 3 10 8 32 Bamboo 800 1400 391 1000 2 11 30 Natural fiber treatments One of t he greatest challenges of working with natural fiber reinforced polymer composites is related to the interfacial adhesion since natural fibers are polar and hydrophilic and most of polymers are hydrophobic. T he hydroxyl groups on the fiber surface or in the non - crystalline regions may attract and react with water molecules by hydrogen bonding [24] . This reduce s the interfacial interaction with the matrix and may lead to composites with low mechanical properties [25] . Improvement s on the interfacial shear strength and adhesion can be performed through chemical and physical treatments of either the fiber or the m atrix. Alkali treatment is the most common chemical treatment of natural fibers where sodium hydroxide (NaOH) removes a certain amount of lignin and wax in the cell wall of the fiber , which increases the number of reaction sites by expos ing the crystallites [26] . T he treatment can also break hydrogen bond s , which can increas e the fiber surface roughness and promot e better mechanical interlocking [26] . However, there is an optimum concentration of NaOH, since the 14 excess cause s excessive delignification and damage to the fiber. Lu et al . [27] used this technique to treat hemp fiber. They extruded untreated and treated hemp with recycled high - density polyethylene (rHDPE) and the scanning electron microscope (SEM) shows t hat the fracture surface of the untreated fiber composite exhibits fiber pullout due to the poor interfacial adhesion. O n other hand, the treated hemp composite shows fiber breakage without pullout that confirms th e improvement on the interfacial bonding. An increase in the mechanical properties of the composite was also observed due to the alkali treatment. C oupling agent s are effective in enhancing the interfacial compatibility between natural fiber s and the polymer matrix , and silan e is widely used [28] . Silane has a generic chemical formula of X 3 Si - R with bifunctional groups that react with the fiber by one side and the polymer by the other side, establishing a bridge between them [29] . In the presence of water, the alkoxy group of silane (X) will originate silanol that reacts with the OH groups of the fiber, establishing strong covalent bonds. However, careful attention should be taken with chemical treatments to avoid fiber degradation during the process and/or the decrease of other properties . An alternative to treat the fibers are by physical treatments tha t modify the surface and structural characteristics of the fibers [30] and improve the mechanical bonding of the fiber into the matrix [31] . A common physical treatment is the cold plasma done through an electrical discharge on the fiber surface that improves the bond ing characteristics by changing the surface energy slightly [32] . Valá et al . [33] u sed oxygen plasma to treat coconut coir, banana and sisal fib ers at a discharge power of 350 W for 30 s to evaluate the influence of the treatment on the interfacial interaction of the fibers with epoxy resin. The treatment increased the number of new oxygen 15 functional groups on the surface of the fibers , which optimiz ed the interfacial interaction with the epoxy resin as observed by SEM micrographs. After adequate chemical or physical treatment cellulose fiber can yield composites with even better mechanical properties . Overall, natural fiber reinforced composites are emerging as potential substitute s to metal and ceramic based materials in applications such as automotive, aerospace and electronics industries [4] . Many efforts have been done to overcome their weakness and new achievements are promising. Cellulose nanocrystals (CNCs) Recently, researchers have used nano - scale cellulose fibers to take advantage of their high specific surface area and aspect ratio. CN Cs are extracted from cellulose fibers usually by an acid hydrolysis process that degrades the amorphous regions of the fiber rel easing the crystalline nanoparticles [34] . They are rod - like nanocrystals, with average width generally in the order of a few nanometer s [34] a nd with length generally between 100 and 1000 nm [35 ] , depending on their origin and production conditions. Their modulus and strength were estimated around 130 GPa and 7 GPa, respectively [36] . Around 25 years ago , CNC was investigated as a reinforce ment for pol ymer s by Favier et al. [37] and since then, the publications on nanocellulose has grown considerably, as shown in Figure 1 . 6 . This increasing tendency shows the high interest in this area, and explains the huge number of manufacturing facilities that are being built, which will increase production to upwards of multiple tons per day [34] . 16 Figure 1 . 6 : Number of publications related with nanocellulose [34] . The main challenge of CNC s in nanocomposite applications are associated with their high hydrophilicity. M any researchers have modified the cellulose surface by non - covalent modification, or by covalent modification such as esterification, polymer grafting and silylation [19] . Surfactants are a simple and fast way for modifying the CNC surface [19] , while coupling agents are an effective way for promoting a covalent bond between the CNC and matrix. Silanes are the most effective and widely used coupling agent and Kargar zadeh et al . [38] proved that CNCs surface treated with silane could be successfully well dispersed in unsaturated polyester resin (UPR) composites toughened with liquid natural rubber, which contributed to good impact resistance of the composites. Sheltami et al . [39] also observed that the adhesion between CNC in a poly(vinyl chloride) (PVC) matrix also improved upon surface modification of the nanocrystals with silane. 17 Epoxy E poxy is a commonly used thermosetting that exhibits good mechanical, adhesive and electrical insulating properties, as well as high chemical and corrosion resistance [40] . The diglycidyl ether of bisphenol A (DGEBA), which is the intermediate for epoxy resins, is synthesized from the bisphenol A and epichlohydrin [41] as shown i n Figure 1 . 7 . Figure 1 . 7 : Diglycidyl ether of bisphenol A ( DGEBA ) [41] . The number - average degree of polymerization (n) depends on the stoichiometry of the reactants and t ypical values for DGEBA are in the range of 0.03 10 [42] . F or n values close to zero the monomers are crystalline solids , for n values up to 0.5 the monomers are liquids, and for greater values of n the monomers are amorphous solids [42] . The epoxy grade used in this research was EPON TM resin 828 that is a liquid epoxy resin and its properties are shown on Table 1 . 3 . Its molecular weight (MW) is approximately 380 g/mol and it is suitable for many fabrication techniques, such as pultrusion, filament winding, casting, molding and vacuum bag laminates [40] . A low MW resin was selected since it has a low viscosity, does not require the addition of a solvent and serves as a suit able model for other thermoset systems 18 from a processing viewpoint. The present resin is bifunctional since its molecule contains two epoxide groups (C 2 H 3 O ) and the w eight per e poxide ( EEW ) shown on Table 1 . 3 is the ratio of the molecular weight of the epoxy resin to the n umber of epoxy groups . Table 1 . 3 : EPON 828 properties [40] . Properties Weight per Epoxide (EEW) 185 192 g/eq Viscosity at 25 °C 110 150 P Density 1.16 g/cm 3 Melting point 40 44 °C m - Phenylenediamine (mPDA) The amine - based curing agent selected to cure the epoxy is mPDA from ACROS Organics , which is an aromatic diamine . I ts chemical structure and properties are shown in Figure 1 . 8 a n d Table 1 . 4 , respectively. Figure 1 . 8 : Structure of mPDA [43] . 19 Table 1 . 4 : mPDA properties [43] . Properties Physical state Solid Molecular Weight 108.14 g/mol Melting point 63 65 °C Boiling point 282 284 °C Molecular formula C 6 H 8 N 2 Epoxy /amine curing Figure 1 . 9 shows the basic steps of the reaction between epoxy resins and di amine s . It starts by epoxide ring opening mechanism where t he oxygen from the epoxide ring will react with the hydrogen from the primary amine causing the formation of a hydroxyl group and reducing the primary amine into a secondary amine ( Figure 1 . 9 (1)) [44] . The secondary amine can further react with an epox ide group to form a tertiary amine ( Figure 1 . 9 (2)) [44] . Another possible reaction is an ether linkage where the epoxide group reacts with the hydroxy l group ( Figure 1 . 9 (3)) , in the presence of a catalyst or at high temperature [45] . 20 Figure 1 . 9 : Three major reactions occurring in epoxy/amine systems [44] . For the present diamine, since the number of active hydrogen per molecule is four , the e quivalent weight of amine is calculated by Equation 1 [46] : ( 1 ) where the MW of mPDA is 108.14 g/mol , resulting in 27.04 g/eq that represents the grams of hardener containing one equivalent of N - H group. The present epoxy contains two epoxide groups, and the phr that corresponds to the weight amount of hardener needed per 100 g of epoxy, is calculated by Equation 2 [46] : ( 2 ) 21 where the approximate EEW of epoxy is 185 g/eq , leading to approximately 14.5 phr . In this study, EPON Resin 828 cured with 14.5 phr mPDA exhibited tensile strength at break of 89.6 ± 4.7 MPa , tensile modulus of 3.6 ± 0.4 GPa and tensile elongation at break of 5 .0 ± 1 .0 % . Carbon fibers (CFs) CF contains at least 92 wt % carbon and they have high tensile properties, low density and high thermal and chemical stabilities in the absence of oxidizing agents [47] . C Fs are not totally crystalline, but are composed of both graphitic (stable form of crystalline carbon at ambient condition s) and noncrystalline regions [17] . D ifferent organic precursor materials are used for producing CFs including rayon, polyacrylonitrile (PAN) and pitch , which will yield fibers with different properties . In general, the first step of production is the stabilization where the p recursor fibers experience an oxidization process at about 200 400 °C [47] . Then the carbonizati on in an inert atmosphere at high temperature ( around 1,000 °C ) will drive off non - carbon elements of the stabilized fibers . Further heat treatment of the carbonized fibers at 3,000 °C yield s fibers with higher carbon content and [47] . After carbonization, s urface treatment of the CF s is performed to i mprove the ir adhesion into the polymer matrix. The properties of the resultant CF depends on the crystallinity, crystalline distributi on, molecular orientation, carbon content, defects and other parameter s [47] . Figure 1 . 10 shows t he continuous CF used in this project ( PAN - based CF supplied by Hexcel Co . ) and its properties is shown on Table 1 . 5 . 22 Figure 1 . 10 : FIB - SEM micrographs of CF from lower to higher magnification . Table 1 . 5 : Typical CF p roperties [48] . P roperties Tensile Strength 4,413 MPa Tensile Modulus (Chord 6000 - 1000) 231 GPa Ultimate Elongation at Failure 1.7% Density 1.79 g/cm 3 Fi ber Diameter 7.1 Carbon fiber reinforced epoxy composites Many strategies have been used to increase the interfacial bonding between CFs and epoxy matrix , including fiber surface treatments, sizing or coating, and addition of nanoparticles at the 23 interphase. This section provides a background about interphase mec hanisms and a literature review about recent development on engineering the composite interphase. Fiber surface treatments Surface treatment processes are frequently applied to CFs to remove the weak outer layer and add functional chemical groups in the su rface to improve the wettability to the matrix [49] . Some techniques such as dry and wet oxidation steps and electro - discharge have been studied [50] . The Composite Materials and Structures Center (CMSC) at MSU has developed a process where CFs are surface treated with high intensity ultraviolet light in the presence of ozone ( UVO treatment) to increase the surface oxygen concentration. This is beneficial to the fiber wetting by the matrix and details are provided elsewhere [51,52] . Another effective method to enhance fiber matrix adhesion is through plasma treatment process that generates free radicals that contact the solid surface causing surface cleaning, etching, crosslinking and activation [50] . This method only affects few superficial layers and does not change the fiber bul k properties. Interphase tailoring via sizing or coating F iber sizing consists of coating the fiber surface with a thin coating containing polymeric components [53] . The sizing improves wettability and can create an interphase in the composite by dissolution or inter - diffusion [54] , which improves the stress transfer and mechani cal properties such as a stiffness of the interphase modulus that can lead to an improvement of the IFSS. Many efforts have been done to predict the IFSS in terms of the fiber, matrix and interphase properties. One of the first approaches was developed by Cox [55,56] , who considered an elastic 24 fiber embedded in an elastic matrix with a perfect bonding. The model led to Eq uation 3 that assumes no lo equat ad transfer through the ends of the fiber [57] : (3) where E f m is the strain in the matrix, G m is the matrix shear modulus, If the specimen geometry is fixed and same fiber with fixed properties is used, the IFSS will have a direct dependence on the product of the matrix strain to failure and the square root of the matrix shear modulus [57,58] . Drzal and Madhukar [59] investigat ed the differences on CFs surface treated with an electrochemical oxidation step only ( fibers are called AS4), and CFs coated with a 100 - 200 nm layer of epoxy applied on the AS4 fibers ( fibers are called AS4C). The IFSS of AS4 was 68.3 MPa, which increased to 81.4 MPa for AS4C because of the interaction of the coating layer with the bulk matrix, which caus ed a local change in properties in the fiber - matrix interphase. The properties of this layer were imparted to the interphase and controlled adhes ion, proving the effectiveness of the sizing. Recently, Liu et al . [60] used a thermoplastic polymer solution to size CF reinforced epoxy , which has led to an increase of 15.5% for the IFSS compared to the baseline CF/epoxy composite. It was a result of the increased surface energy at the interphase, which means that the fiber surface contains more polar groups that can e stablish strong int eractions with the resin. The sizing has also 25 led to 56.1% improvement in interfacial fracture toughness, which shows that the sizing not only improved the IFSS, but also preserved and increased the fracture toughness. Functional interphase: effects of add ing nanoparticles in the sizing One novel approach for engineering the composite interphase is the incorporation of nanoparticles into the fiber surface . One advantage of adding nanoparticles in the sizing is not only improving the fiber - matrix adhesion b ut also increasing the toughness of the composite by presenting resistance to crack propagation [61] . The presence of nanoparticles in the sizing can also locally increase the shear modulus in the interphase, which can further generate an improvement in the IFSS . Qin et. al [54] investigated the quality of CF - epoxy interface by adding funct ionalized silicon dioxide (SiO 2 ) nanoparticles. The CFs were immersed in a suspension of SiO 2 nanoparticles functionalized with APTES. The composites exhibited 44% improvement on the IFSS for CF s sized with 1.3 wt% relative concentration of SiO 2 , compared to non - coated CFs. This improvement was the result of an increase of the shear modulus of the matrix in the interphase region caused by the presence of the nanoparticles. As shown in the literature review, CNCs have attracted considerable attention due to their low density, low cost, sustainable nature and high mechanical properties. With all these advantages, CNCs are potential candidate to be added as part of a sizing to modify the CF - epoxy interphase as will be shown on Chapter 2 . 26 Polypropylene ( PP ) PP ((C 3 H 6 ) n ) is a semi - crystalline polymer [62] obtained from the p olymerization of the monomer p ropylene ( CH 2 =CHCH 3 ) [63] . The chemical structure of PP is show n on Figure 1 . 11 . Figure 1 . 11 : Chemical structure of polypropylene [63] . W hen polymerized, the methyl groups can be oriented in different spatial arra n ge ment s in relation to the carbon backbone , leading to isotactic PP ( methyl groups are oriented on one side of the carbon backbone , with high degree of crystallinity), syndiotactic PP ( methyl groups are on alternate sides of the chain , with less c rystalline ) and atactic PP ( random arrangem ent of the methyl groups along the carbon backbone , leading to an amorphous polymer ) [63,64] . 27 Figure 1 . 12 : Tacticity of polypropylene [63] . The majority of PP is produced as a homopolymer and its melting temperature is at about 160 ° C [64] . PP can also be polymerized with ethylene to produce a PP copolymer. Re garding the processing technique this thermoplastic can be molded or extruded and the m ost common shaping technique is injection molding . Mechanical properties of neat PP and reinforced PP will vary depending on the structure and morphology of the PP , f iller and reinforcement , p rocessing and t esting conditions [65] . Carbon Nanotubes ( CNT s) CNTs consist of one or several graph ene planes rolled in a cylindrical shape with diameters of 1 to several dozens of nano meters and lengths of up to several microns [66] . They can be single - walled carbon nanotubes (SWCNTs) with a diameter of 1 to 2 nm consist ing of a single graphite 28 sheet rolled into a cylindrical tube , they can be d ouble - walled carbon nanotubes (DWCNTs) made of two concentric nanotubes or they can be multiwalled carbon nanotubes (MWCNTs) made by multiple concentric nanotubes with diameters ranging from 2 to 50 nm depending on the number of tubes [67] . They have high aspect ratio, high electrical conductivity and good thermal and mechanical properties [68] . to 63 GPa w ere reported [67] . In t his study SW CNTs were used and they can be classified in terms of chiral vector ( h ) as shown on Figure 1 . 13 [69] . The chiral vector that indicates the rolling up direction of CNT can be expressed in terms of base vectors ( 1 ) and ( 2 ) and a pair of integer (n, m) according to E quation 4 [69] . The n and m are the chiral indices [68] , and d epending on the ir values CNT can be classified as zigzag, armchair and chiral as shown on Figure 1 . 14 . ( 4 ) Figure 1 . 13 : Schematic diagram of the chiral vector and the chiral angle [69] . 29 If n and m are equal the CNT is armchair (two edges of each hexagon are perpendicular to the cylinder axis ( Figure 1 . 14 ( A ) ) , if n or m is equal to 0 the CNT is zigzag (two edges of each hexagon are parallel to the cylinder axis ( Figure 1 . 14 (B ) ) and f or other values of indices, CNTs are c hiral ( Figure 1 . 14 (C ) ) [66,68,70] . A rmchair CNTs exhibit metallic conductivity type , w hereas zigzag CNTs are either metallic or semiconducting [68] . Figure 1 . 14 : Schematic illustrations of the structures of (A) armchair, (B) zigzag, and (C) chiral SWNTs [70] . 30 REFERENCES 31 REFERENCES [1] M.A.S. Spinacé, K.K.G. Fermoseli, M. - A. De Paoli, Recycled polypropylene reinforced with curaua fibers by extrusion, J. Appl. Polym. Sci. 112 (2009) 3686 3694. doi:10.1002/app.29683. [2] M. Ramesh, Hemp, jute, banana, kenaf, ramie, sisal fibers, in: Handb. Prop. Text. Tech. Fibres, Elsevier, 2018: pp. 301 325. doi:10.1016/B978 - 0 - 08 - 101272 - 7.00009 - 2. [3] A.K. Bledzki, J. Gassan, Composites reinforced with cellulos e based fibres, Prog. Polym. Sci. (1999). doi:10.1016/S0079 - 6700(98)00018 - 5. [4] M.R. Sanjay, G.R. Arpitha, L.L. Naik, K. Gopalakrishna, B. Yogesha, Applications of Natural Fibers and Its Composites: An Overview, Nat. Resour. 07 (2016) 108 114. doi:10.4236 /nr.2016.73011. [5] J.S. Han, J.S. Rowell, Chemical Composition of Fibers, in: R.M. Rowell, R.A. Young, J.K. Rowell (Eds.), Pap. Compos. from Agro - Based Resour., CRC/Lewis Publishers, Boca Raton, 1997: pp. 83 134. [6] G. Francucci, E. Rodriguez, Processing of plant fiber composites by liquid molding techniques: An overview, Polym. Compos. 37 (2016) 718 733. doi:10.1002/pc.23229. [7] S.R. Djafari Petroudy, Physical and mechanical properties of natural fibers, in: Adv. High Strength Nat. Fibre Compos. Constr. , Elsevier, 2017: pp. 59 83. doi:10.1016/B978 - 0 - 08 - 100411 - 1.00003 - 0. [8] T. Westman, MP; Fifield, LS; Simmons, KL; Laddha, SG; Kafentzis, Natural Fiber Composites: A Review, Washington, 2010. https://www.pnnl.gov/main/publications/external/technical_report s/PNNL - 19220.pdf. [9] A. Langhorst, A. Kiziltas, D. Mielewski, E. Lee, Selective dispersion and compatibilizing effect of cellulose filler in recycled PA6/ PP blends, in: 15th Annu. Soc. Plast. Eng. Automot. Compos. Conf. Exhib., Troy (Detroit), 2015. [10] O. Akampumuza, P.M. Wambua, A. Ahmed, W. Li, X. - H. Qin, Review of the applications of biocomposites in the automotive industry, Polym. Compos. 38 (2017) 2553 2569. doi:10.1002/pc.23847. [11] M. Fogorasi, I. Barbu, The potential of natural fibres for autom otive sector - review, IOP Conf. Ser. Mater. Sci. Eng. 252 (2017) 012044. doi:10.1088/1757 - 899X/252/1/012044. [12] D.U. Shah, Developing plant fibre composites for structural applications by optimising composite parameters: a critical review, J. Mater. Sci. 48 (2013) 6083 6107. doi:10.1007/s10853 - 013 - 7458 - 7. 32 [13] K. Lau, P. Hung, M. - H. Zhu, D. Hui, Properties of natural fibre composites for structural engineering applications, Compos. Part B Eng. 136 (2018) 222 233. doi:10.1016/j.compositesb.2017. 10.038. [14] J. Yang, J. Xiao, J. Zeng, L. Bian, C. Peng, F. Yang, Matrix modification with silane coupling agent for carbon fiber reinforced epoxy composites, Fibers Polym. 14 (2013) 759 766. doi:10.1007/s12221 - 013 - 0759 - 2. [15] E. Ozen, A. Kiziltas, E.E. Kiziltas, D.J. Gardner, Natural fiber blends - filled engineering thermoplastic composites for the automobile industry, in: 12th Annu. Automot. Compos. Conf. Exhib. 2012 (ACCE 2012) Unleashing Power Des., Troy (Detroit), 2012. [16] D.E. Depuydt, N. Sweygers, L. Appels, J. Ivens, A.W. van Vuure, Bamboo fibres sourced from three global locations: A microstructural, mechanical and chemical composition study, J. Reinf. Plast. Compos. (2019) 1 16. doi:10.1177/0731684419828532. [17] W.D. Callister, D.G. Rethwisch, Materials science and engineering: An introduction (eight edition), 2009. doi:10.1016/0261 - 3069(91)90101 - 9. [18] T. Lacroix, B. Tilmans, R. Keunings, M. Desaeger, I. Verpoest, Modelling of critical fibre length and interfacial debonding in the fragmentatio n testing of polymer composites, Compos. Sci. Technol. 43 (1992) 379 387. doi:10.1016/0266 - 3538(92)90061 - 7. [19] F. Ansari, M. Salajková, Q. Zhou, L.A. Berglund, Strong Surface Treatment Effects on Reinforcement Efficiency in Biocomposites Based on Cellulo se Nanocrystals in Poly(vinyl acetate) Matrix, Biomacromolecules. 16 (2015) 3916 3924. doi:10.1021/acs.biomac.5b01245. [20] R. Kumar, S. Obrai, A. Sharma, Chemical modifications of natural fiber for composite material, Pelagia Res. Libr. (2011). [21] L. Fa n, M.M. Gharpuray, Y. - H. Lee, Cellulose Hydrolysis, Springer Berlin Heidelberg, Berlin, Heidelberg, 1987. doi:10.1007/978 - 3 - 642 - 72575 - 3. [22] A. Thygesen, Properties of hemp fibre polymer composites - An optimisation of fibre properties using novel defibrati on methods and fibre characterisation, R. Agric. Vet. Univ. (2006). doi:(Risø - PhD; No. 11(EN)). [23] H. Chen, Chemical Composition and Structure of Natural Lignocellulose, in: Biotechnol. Lignocellul., Springer Netherlands, Dordrecht, 2014: pp. 25 71. doi: 10.1007/978 - 94 - 007 - 6898 - 7_2. [24] L. Hua, P. Zadorecki, P. Flodin, Cellulose fiber - polyester composites with reduced water sensitivity (1) chemical treatment and mechanical properties, Polym. Compos. 8 (1987) 199 202. doi:10.1002/pc.750080308. [25] M.M. Ka bir, H. Wang, T. Aravinthan, F. Cardona, K. - T. Lau, Effects of natural fibre surface on composite properties: a review, in: 1st Int. Postgrad. Conf. Eng. Des. Dev. 33 Built Environ. Sustain. Wellbeing, Brisbane, 2011: pp. 94 99. [26] C. Mohan D., M. B., Chara cterization of Kenaf fibre reinforced composites, J. Chem. Pharm. Res. 6 (2014) 626 628. [27] N. Lu, R.H. Swan, I. Ferguson, Composition, structure, and mechanical properties of hemp fiber reinforced composite with recycled high - density polyethylene matrix , J. Compos. Mater. 46 (2012) 1915 1924. doi:10.1177/0021998311427778. [28] S. Kalia, A. Dufresne, B.M. Cherian, B.S. Kaith, L. Avérous, J. Njuguna, E. Nassiopoulos, Cellulose - Based Bio - and Nanocomposites: A Review, Int. J. Polym. Sci. 2011 (2011) 1 35. d oi:10.1155/2011/837875. [29] Y. Xie, C.A.S. Hill, Z. Xiao, H. Militz, C. Mai, Silane coupling agents used for natural fiber/polymer composites: A review, Compos. Part A Appl. Sci. Manuf. 41 (2010) 806 819. doi:10.1016/j.compositesa.2010.03.005. [30] L.A. P othan, A.S. Luyt, S. Thomas, Polyolefin/Natural Fiber Composites, in: Polyolefin Compos., John Wiley & Sons, Inc., Hoboken, NJ, USA, n.d.: pp. 44 86. doi:10.1002/9780470199039.ch3. [31] A.K. Bledzk, V.E. Sperber, O. Faruk, Natural and Wood Fibre Reinforcem ent in Polymers, 2002. [32] O. Faruk, A.K. Bledzki, H. - P. Fink, M. Sain, Biocomposites reinforced with natural fibers: 2000 2010, Prog. Polym. Sci. 37 (2012) 1552 1596. doi:10.1016/j.progpolymsci.2012.04.003. [33] e of Plasma Treatment on Mechanical Properties of Cellulose - based Fibres and Their Interfacial Interaction in Composite Systems, BioResources. 12 (2017) 5449 5461. doi:10.15376/biores.12.3.5449 - 5461. [34] M. Mariano, N. El Kissi, A. Dufresne, Cellulose nan ocrystals and related nanocomposites: Review of some properties and challenges, J. Polym. Sci. Part B Polym. Phys. 52 (2014) 791 806. doi:10.1002/polb.23490. [35] H. Kargarzadeh, R.M. Sheltami, I. Ahmad, I. Abdullah, A. Dufresne, Cellulose nanocrystal: A p romising toughening agent for unsaturated polyester nanocomposite, Polym. (United Kingdom). 56 (2015) 346 357. doi:10.1016/j.polymer.2014.11.054. [36] J. Lu, P. Askeland, L.T. Drzal, Surface modification of microfibrillated cellulose for epoxy composite ap plications, Polymer (Guildf). 49 (2008) 1285 1296. doi:10.1016/j.polymer.2008.01.028. [37] V. Favier, H. Chanzy, J.Y. Cavaille, Polymer Nanocomposites Reinforced by Cellulose Whiskers, Macromolecules. 28 (1995) 6365 6367. doi:10.1021/ma00122a053. [38] H. Kargarzadeh, R.M. Sheltami, I. Ahmad, I. Abdullah, A. Dufresne, Cellulose 34 nanocrystal reinforced liquid natural rubber toughened unsaturated polyester: Effects of filler content and surface treatment on its morphological, thermal, mechanical, and viscoe lastic properties, Polym. (United Kingdom). 71 (2015) 51 59. doi:10.1016/j.polymer.2015.06.045. [39] R.M. Sheltami, H. Kargarzadeh, I. Abdullah, Effects of silane surface treatment of cellulose nanocrystals on the tensile properties of cellulose - polyvinyl chloride nanocomposite, Sains Malaysiana. 44 (2015) 801 810. [40] Hexion, EPON TM Resin 828 Technical Data Sheet, (2005). https://www.hexion.com/CustomServices/PDFDownloader.aspx?type=tds&pid=1fb05b42 - 5814 - 6fe3 - ae8a - ff0300fcd525 (accessed February 10, 2019). [41] J.I. Yang, Part I: Synthesis of Aromatic Polyketones Via Soluble Precurso rs Derived from Bis(A - Amininitrile)S; Part Ii: Modifications of Epoxy Resins with Functional Hyperbranched Poly(Arylene Ester)s, Virginia Polytechnic Institute and State University, 1998. [42] J. - P. Pascault, R.J.J. Williams, General Concepts about Epoxy P olymers, in: Epoxy Polym., Wiley - VCH Verlag GmbH & Co. KGaA, Weinheim, Germany, 2010: pp. 1 12. doi:10.1002/9783527628704.ch1. [43] ThermoFisher Scientific, m - Phenylenediamine Safety Data Sheet, (2018). https://www.acros.com/DesktopModules/Acros_Search_Res ults/Acros_Search_Results.as px?search_type=CatalogSearch&SearchString=130560010 (accessed February 10, 2019). [44] H. Liu, A. Uhlherr, R.J. Varley, M.K. Bannister, Influence of substituents on the kinetics of epoxy/aromatic diamine resin systems, J. Polym. Sci. Part A Polym. Chem. 42 (2004) 3143 3156. doi:10.1002/pola.20169. [45] Y. Zhang, S. Vyazovkin, Effect of Substituents in Aromatic Amines on the Activation 7104. doi:10.1021/jp071001h. [ 46] L.M. Meng, Y.C. Yuan, M.Z. Rong, M.Q. Zhang, A dual mechanism single - component self - healing strategy for polymers, J. Mater. Chem. 20 (2010) 6030. doi:10.1039/c0jm00268b. [47] X. Huang, Fabrication and Properties of Carbon Fibers, Materials (Basel). 2 (2009) 2369 2403. doi:10.3390/ma2042369. [48] Hexcel Corporation, HexTow® AS4 Carbon Fiber Datasheet, (2018). https://www.hexcel.com/user_area/content_media/raw/AS4_HexTow_DataSheet.pdf (accessed February 10, 2019). [49] L.T. Drzal, M.J. Rich, P.F. Lloyd, Adhesion of Graphite Fibers to Epoxy Matrices: I. The Role of Fiber Surface Treatment, J. Adhes. 16 (1983) 1 30. doi:10.1080/00218468308074901. 35 [50] L. Tang, J.L. Kardos, A review of methods for improving the interfacial adhesion between carbon fiber and p olymer matrix, Polym. Compos. 18 (1997) 100 113. doi:10.1002/pc.10265. [51] M.J. Rich, L.T. Drzal, B.P. Rook, P. Askeland, E.K. Drown, Novel carbon fiber surface treatment with ultravilet light in ozone to promote composite mechanical properties, ICCM Int. Conf. Compos. Mater. (2009). [52] M.J. Rich, E.K. Drown, P. Askeland, L.T. Drzal, Surface Treatment of Carbon Fibers By Int. Conf. Compos. Mater., 2013. [53] Z. Dai, F. Shi, B. Zhang, M. Li, Z. Zhang, Effect of sizing on carbon fiber surface properties and fibers/epoxy interfacial adhesion, Appl. Surf. Sci. 257 (2011) 6980 6985. doi:10.1016/j.apsusc.2011.03.047. [54] W. Qin, F. Vautard, P. Askeland, J. Yu, L. Drzal, Modif ying the carbon fiber epoxy matrix interphase with silicon dioxide nanoparticles, RSC Adv. 5 (2015) 2457 2465. doi:10.1002/pc.23325. [55] H.L. Cox, The elasticity and strength of paper and other fibrous materials, Br. J. Appl. Phys. 3 (1952) 72 79. doi:10. 1088/0508 - 3443/3/3/302. [56] T.F. Cooke, High Performance Fiber Composites with Special Emphasis on the Interface: A Review of the Literature, J. Polym. Eng. 7 (1987). [57] V. Rao, L.T. Drzal, The dependence of interfacial shear strength on matrix and inte rphase properties, Polym. Compos. 12 (1991) 48 56. doi:10.1002/pc.750120108. [58] W. Qin, F. Vautard, L.T. Drzal, J. Yu, Mechanical and electrical properties of carbon fiber composites with incorporation of graphene nanoplatelets at the fiber - matrix interp hase, Compos. Part B Eng. 69 (2015) 335 341. doi:10.1016/j.compositesb.2014.10.014. [59] L.T. Drzal, M. Madhukar, Fibre - matrix adhesion and its relationship to composite mechanical properties, J. Mater. Sci. 28 (1993) 569 610. doi:10.1007/BF01151234. [60] R. Liu, WenBo; Zhang, Shu; Li, Bichen; Yang, Fan; Jiao, WeiCheng; Hao, LiFeng; Wang, Improvement in Interfacial Shear Strength and Fracture Toughness for Carbon Fiber Reinforced Epoxy Composite by Fiber Sizing, Polym. Compos. 35 (2013) 482 488. [61] J. Kar ger - Kocsis, H. Mahmood, A. Pegoretti, Recent advances in fiber/matrix interphase engineering for polymer composites, Prog. Mater. Sci. 73 (2015) 1 43. doi:10.1016/j.pmatsci.2015.02.003. [62] B. Pukánszky, Particulate filled polypropylene: structure and pro perties, in: Polypropyl. Struct. Blends Compos., Springer Netherlands, Dordrecht, 1995: pp. 1 70. doi:10.1007/978 - 94 - 011 - 0523 - 1_1. 36 [63] Y. Takashima, Polypropylene, Encycl. Polym. Nanomater. (2014) 1 6. doi:10.1007/978 - 3 - 642 - 36199 - 9_254 - 1. [64] H. Karian, Handbook of Polypropylene and Polypropylene Composites, Revised and Expanded, CRC Press, 2003. doi:10.1201/9780203911808. [65] J. Karger - Kocsis, Microstructural aspects of fracture in polypropylene and in its filled, chopped fiber and fiber mat rei nforced composites, in: Polypropyl. Struct. Blends Compos., Springer Netherlands, Dordrecht, 1995: pp. 142 201. doi:10.1007/978 - 94 - 011 - 0523 - 1_4. [66] I. V. Zaporotskova, N.P. Boroznina, Y.N. Parkhomenko, L. V. Kozhitov, Carbon nanotubes: Sensor properties. A review, Mod. Electron. Mater. (2016). doi:10.1016/j.moem.2017.02.002. [67] K.S. Ibrahim, Carbon nanotubes - properties and applications: a review, Carbon Lett. 14 (2013) 131 144. doi:10.5714/CL.2013.14.3.131. [68] B.K. Kaushik, M.K. Majumder, Carbon Nanot ube: Properties and Applications, in: Carbon Nanotub. Based VLSI Interconnects, 2015: pp. 17 37. doi:10.1007/978 - 81 - 322 - 2047 - 3_2. [69] L. Boumia, M. Zidour, A. Benzair, A. Tounsi, A Timoshenko beam model for vibration analysis of chiral single - walled carbo n nanotubes, Phys. E Low - Dimensional Syst. Nanostructures. 59 (2014) 186 191. doi:10.1016/j.physe.2014.01.020. [70] R.H. Baughman, Carbon Nanotubes -- the Route Toward Applications, Science (80 - . ). 297 (2002) 787 792. doi:10.1126/science.1060928. 37 CHAPTER 2: CARBON FIBER/EPOXY MATRIX COMPOSITE INTERPHASES MODIFIED WITH CELLULOSE NANOCRYSTALS Portions of this chapter were published in the Composites Science and Technology Journal with M.D. Reale Batista, L.T. Drzal, Carbon fiber/epoxy mat rix composite interphases modified with cellulose nanocrystals, Compos. Sci. Technol. 164 (2018) 274 281. doi:10.1016/j.compscitech.2018.05.010. Abstract The incorporation of nanoparticles at the composite interphase has attracted considerable attention d ue to their contribution to the functional and mechanical properties of the composite. Cellulose nanocrystals (CNCs) were used to modify the interphase between carbon fiber (CF) and an epoxy matrix to simultaneously strengthen and toughen the CF composite. CNCs were functionalized with 3 - aminopropyltriethoxysilane (APTES) and surface modification was confirmed by Fourier transform infrared (FTIR) spectroscopy and X - ray ph otoelectron spectroscopy (XPS), which revealed the presence of the new chemical species . F unctionalized CNCs ( APTES - CNCs ) were applied as part of a sizing to coat CFs and an optimum concentration was identified. The APTES - CNCs dispersion on the surface of CFs was characterized by scanning electron microscopy (SEM) and the interfacial adhesio n was assessed by measuring the interfacial shear strength (IFSS) through the single fiber fragmentation test (SFFT). IFSS indicated stronger adhesion between the CFs and the epoxy matrix for the fibers sized with APTES - CNCs, compared to unsized CFs and epoxy - only sized CFs. CFs sized with APTES - CNCs at a concentration of 1.0 wt% resulted in an 81% increase in IFSS compared to unsized C Fs, and the birefri ngen t stress pattern seen during the SFFT supports the assumption that adding APTES - CNCs at the composite 38 interphase promotes an improvement in the failure mode. These results demonstrate that sizing CFs with APTES - CNCs is an effective m ethod to increase the interfacial properties in CF reinforced epoxy composites, and a potential approach for the development of ecofriendly and lightweight composite materials for aerospace and automotive applications . Keywords: Interphase; Fiber/matrix b ond; Polymer - matrix composites; Carbon fibers; Cellulose nanocrystal; Introduction Lightweight, high - strength and high - stiffness are often identified as desirable properties for parts used in aerospace and automotive fields. The U.S. Environmental Protection Agency t by model year 2025 [1] . To achieve these engineering needs and meet the growing requirement for fuel economy, carbon fiber reinforced polymer (CFRP) composites have gained atte ntion because of their high strength - to - weight ratio. However, one drawback of CFRP, specifically epoxy - based CFRP, is its brittle fracture mechanism. Brittle fracture is not only dependent on the mechanical properties of the fiber and matrix, but also dep endent on the effectiveness of the interfacial adhesion between the components [2] . Therefore, a well - designed interphase is required to achieve improvements in the mechanical properties of the composites. According to Karger - Kocsis et al. [3] the term interphase has been defined as a finite interlayer with distinct physico - chemical properties between the fiber and matrix. The interphase comprises a three dimensional region where the local properties vary from the properties of the bulk matrix and bulk fiber [4] . The interfacial adhesion between the fiber and matrix an d the effectiveness of the stress transfer at the interphase are essential elements for composite 39 performance. A n interphase engineered to increase stress transfer from the matrix to the reinforcing fiber will result in a composite with greater strength an d structural integrity [5] . M any str ategies have been used to increase the interfacial bonding between the fiber and matrix, including fiber surface treatments, sizing or coating, and the addition of nanoparticles in the interphase. Surface treatment processes such as dry and wet oxidat ion steps, plasma treatment, electrodischarge [6] and ultraviolet light with ozone (UVO treatment) [7] are frequently applied to CFs to remove the weak outer layer and add functional chemical groups in the surface to improve the wettability to the matrix [4] and to chemically react with the polymer matrix . In addition to surface treatments, the application of a fiber sizing, which consists of a thin coating on the fiber surf ace containing polymeric components [8] is commonly used . The sizing improves wettability , provides fiber protection during handling and weaving and can create an interphase in the composite by dissolution or inter - diffusion [5] into the polymer matrix , which improves the stress transfer and mechanical properties such as stiffness of the interphase modulus that can l ead to an improvement of the - lag model [9,10] shows that the IFSS will have a direct dependence on the product of the matrix strain to failure and the square root of the matrix shear modulus [11,12] . Much work [13,14] has been done o n sizing CFs to improve the interfacial adhesion and mechanical properties of the composites. A recent development has been the incorporation of nanoparticles into the fiber matrix interphase through the sizing. The addition of nanoparticles has the potent ial for not only improving the fiber - matrix adhesion but also for increasing the toughness of the composite by providing resistance to crack propagation [3] . The presence of nanoparticles in the sizing can also 40 locally increase the shear modulus in the interphase , which can further generate an improvement in the IFSS. An additional bene fit to adding nanoparticles in the interphase is imparting other properties beyond mechanical to produce a multifunctional material. Qin et al. [12] showed that incorporating graphene nanoplatelets (GnP) in the CF/epoxy interphase not only improved the flexural and interlaminar shear strength, but also created a conductive path between the fibers that improved electrical conductivity. Several metho ds have been proposed to incorporate nanoparticles on the fiber surface: electrophoretic deposition (EPD), chemical vapor deposition (CVD ), spray coating [3] and dipping [15,16] . It has been reported that the addition of graphite nanoplatelets [17] , GnP [12] and functionalized silicon dioxide nanoparticles [5,18] to the sizing increases the adhesion at the composite interphase by mechanical interlocking, establishment of covalent bonding as well as stiffening of the interphase modulus. CNCs , extracted from cellulose fibers [19] have attracted considerable attention due to their high mechanical properties, high specific surface area, low cost, low density, availability, and sustainable nature . CNCs are rod - like cellulose nanocrystals, with average width gen erally in the order of a few nanometers [19] and with length gene rally between 100 and 1000 nm [20] . Their modulus and strength are estimated around 130 GPa and 7 GPa , respectively [21] , making them an attractive nanoparticle to be added to an engineered sizing to modify the CF - epoxy interphase. The main challenge in utilizing CNC s in composite applications is their high hydrophilicity, which makes them incompatible with hydrophobic polymers. To overcome this drawback, researchers have modified the cellulose surface by non - covalent modification , or by 41 covalent modif ication such as esterification, polymer grafting and silylation [22] . Silane coupling agents are widely used to surface treat CNCs [23] . In this work, a process to modify the composite interphase by coating a CF wit h functionalized CNCs as a nano - reinforcement to improve the adhesion between the CF and an epoxy matrix was investigated. CNCs were surface treated with 3 - aminopropyl triethoxysilane (APTES) a nd then incorporated in a sizing applied to the fiber. The proj ect goal was to investigate how CNCs could be used to simultaneously strengthen and toughen composite materials. This work utilizes the dipping sizing technique for coating single fibers and multiple fiber tows. An o ptimum CNC concentration in the sizing w as identified, single fiber adhesion tests were conducted, and appropriate characterization and analyses were completed. Experimental Materials A CNC slurry was purchased from University of Maine (Process Development Center) with 12.2 wt% CNC in water. APTES was chosen to functionalize the surface of CNCs and was obtained from Aldrich (99%). Polyacrylonitrile (PAN) - based CFs AS4 (12k filament count tow) the UVO treatment developed in our laboratories [7] . The epoxy used was EPON TM resin 828 (viscosity of 110 150 P at 25°C) supplied by Hexion Inc. and the curing agent was m - Phenylenediamine (mPDA) supplied by ACROS Organics. Acetone and ethanol were chosen as solvents for the solvent exchange proce ss of the CNC slurry. 42 Methods Surface treatment of CNCs A solvent exchange process was utilized t o disperse CNCs in an adequate solvent for further chemical modification of their surface . T he CNC slurry was diluted with water and solvent exchanged with ethanol and acetone and then surface treated with APTES based on a method applied to micro fibrillated cellulose ( MFC ) as described elsewhere [21] . First, the CNC slurry was diluted with water to obtain CNCs at 0.5 wt%. Then it was sonicated for 6 min (10s On / 5s Off) using a Cole - Parmer 750 - Watt Ultrasonic processor, and stirred for 72 h. This aqueous suspension was solvent exchanged with ethanol three times and then with acetone three times using a rotary evaporator (BUCHI R - 114) as shown on Figur e 2 . 1 . After each successive mixing with the solvent, 5 min sonication (10s On / 5s Off) was applied to ensure good dispersion of the nanoparticles. The content of CNCs in acetone was adjusted to be 0.6 wt%. After the solvent exchange process, APTES was added into the CNCs acetone suspension to yield a 0.3 wt% final concentration an d the suspension was stirred for 40 h. This APTES - CNC acetone suspensi on was used as the baseline for the sizing preparati on described in Section 2.3.2.2 . 43 Figur e 2 . 1 : Solvent exchange process using a rota ry evaporator. To confirm effectiveness of the CNC surface treatment, an APTES - CNC acetone suspension was filtered and the sediments were dried at room temperature (RT) for 24 h for further analysis. Some specimens received additional treatment at 120 °C for 2 h for comparison [21] . CNCs that were treated with APTES are abbreviated as APTES - CNCs. Sizing of CFs with APTES - CNC Fig ure 2 . 2 represents the process used for sizing CFs with APTES - CNCs. The epoxy was dispersed in the APTES - CNC acetone suspension by sonication for 10 min (10s On / 5s Off) and the sizing was continuous ly stirred for 5 h. Then the curing agent mPDA was added and the sizing was stirred for 1 - 2 h. Evaporation of acetone occurred simultaneously during these 6 - 7 h of stirring to adjust the sizing concentration as required. Final sizing stoichiometry was kept constant at 1.4 44 wt% of (epoxy resin + 9 phr mPDA) in acetone and the CNC concentrations were set at 0.6 wt%, 1.0 wt% and 2.0 wt% to the total sizing solution. These CNC concentrations were calculated based on the CNC weight only . CFs were sized by two dif ferent techniques for comparison purposes. The f irst technique ( Fig ure 2 . 2 (a)) consists of dipping the 12k filament tow into the sizing solution for 20 s econds and slowly pulling the tow out. The second technique ( Fig ure 2 . 2 ( b)) consists of aligning and mounting individual CFs onto a square - shaped metal frame and immersing them into the sizing solution for approximately 32 min (sequence of 5 min without stir / 3 min with stir) and in the last minute slowly pulling the metal frame out. For both techniques , the final step is drying the sized CFs in an oven at 60°C for 3 h. CFs without sizing and CFs sized with epoxy - only were prepared as control samples. For CFs sized with epoxy - only, 1.4 wt% of (epoxy resin + 9 phr mPDA) was dissolved in acetone and then CFs wer e sized accordingly. Fig ure 2 . 2 : Schematic illustration of the sizing process: a) 20s dipping of the 12k filament tow and b) 33 min immersion of metal frame with individual CFs. 45 Preparation of SFFT coupon A single CF was aligned axially in the cavity of a dogbone coupon in a silicone mold and kept taut with rubber cement in the end sprues as shown in Figure 2 . 3 . The epoxy and curing agent were heated at 75 °C , then mixed at a concentration of 14.5 phr mPDA and degassed in a vacuum oven. The silicone mold containing the single CFs was also degassed to remove trapped air in the mold and yield void - free samp les. Then the resin system was carefully added into the silicone mold covering the CFs and coupons were cured at 75°C for 2 h and postcured at 125°C for 2 h. Cured coupons were polished to obtain high fidelity specimens to conduct the fragmentation test . Figure 2 . 3 : Schematic of preparation of the SFFT coupon. Characterization methods Fourier transform infrared (FTIR) spectroscopy FTIR spectroscopy with a diamond attenuated total reflectance (ATR) window was used to investigate the changes in the functional groups as a result of the surface treatment of CNCs. Both untreated CNCs and APTES - CNCs were characterized with a PerkinElmer Spectr um One system at a resolution of 4 cm - 1 , 16 scans, from 4000 - 650 cm - 1 . 46 X - ray photoelectron spectroscopy (XPS) Both untreated CNCs and APTES - CNCs were characterized by XPS using a Physical Electronics 5400 ESCA. Prior to XPS analysis, a Soxhlet extraction process with acetone was given to the APTES - CNCs to remove any physically bonded silane [21] . Survey spectra were collected at 187.85 eV pass energy and higher resolution spectra were collected with 29.35 eV pass energy. Scanning Electron Microscopy (SEM) A Carl Zeiss Auriga FIB scanning electron microscope with accelerating voltage of 5 keV was used to observe the homogeneity of the APTES - CNC distribution on the surface of CFs. Unsized and sized CFs were mounted on the SEM sample holder on top of carbon tape and sputter - coated with tungsten to prevent surface charging. S ingle F iber F ra gmentation T est (SFFT) The SFFT [13,24] was used to determine the level of adhesion between the CFs and the epoxy system. Figure 2 . 4 shows a coupon containing a single CF mounted in a tensile jig fitted onto a transmitted light optical microscope ( Olympus BH - 2). 47 Figure 2 . 4 : Schematic of preparation of the SFFT coupon. The applied tensile stress ( ) on the specimen is transferred to the fiber through an interfacial shear stress mechanism [24] . When the fiber tensile strength is reached, the encapsulated fiber fractures inside the matrix. With increasing loading, the fiber will repeated ly break until the fragment length is too short to allow sufficient stress transfer to exceed the fiber breaking strength [13] . When this saturation point was reached, the number of fractures within a fixed length was determined with the optical micrometer system. A shear - lag analysis was conducted to calculate IFSS, according to Equation 1 [13,24] : (1) 48 c f is the fiber tensile strength at the critical length. The fiber critical length is necessary to maintain the integrity of the composite material and is calculated using Equation 2 [24] : (2) where is the aver age fiber fragmentation length once saturation is reached. At least 5 coupons were tested. As shown in the upp er portion of Figure 2 . 4 using polarized light a birefringent stress pattern can be observed at the fiber - matrix interphase around the fiber breaks, which is a qualitative indicator of the level of adhesion. Results and discussion CNC surface modification with APTES Fig ure 2 . 5 displays th e FTIR spectra for the CNC and APTES - CNC. The CNC spectrum shows bands that are consistent with the spectrum of cellulose: the peaks around 3700 - 3000 cm - 1 correspond to the hydrogen - bonded OH stretching and the peak around 2900 cm - 1 is attributed to the CH stretching. The OH bending of adsorbed water is observed around 1640 cm - 1 , the bending of CH 2 is displayed at 1426 cm - 1 - 1 [20,25] . The surface modification of cellulose fibers by APTES initiates when the triethoxysilane is hydrolyzed [26] . These silanol groups interact by strong hydrogen bonds by a condensation reaction when heated above 100 ° C [26] . In this study, the hydrolysis of triethoxysilane was possible due to the presence of the residual water fr om the solvent exchange process, as observed by Lu et al . [21] , and the FTIR spec tr a for APTES - CNC s show bands that 49 confirm the effectiveness of the surface treatment. The spectrum for APTES - CNC dried at RT for 24h is displayed in Fig ure 2 . 5 and shows peaks characteristics of amine groups at 1565 and 1482 cm - 1 , indicating that there is hydrogen bonding between th e amine groups and the hydroxyl groups of both silanol and cellulose [25] . U n - reacted silanol groups can be noticed in a weak band at 99 9 cm - 1 . It is difficult to identify the band since it overlaps with the at 1030cm - 1 . Other authors report an increase of this peak after silane treatment indicating an evidence of silane adsorption [20,25] . Fig ure 2 . 5 : FTIR spectra for CNC, APTES - CNC (dried at RT for 24h) and APTES - CNC (dried at RT for 24h and at 120 °C for 2h) . The APTES - CNC that received additional h eat treatment at 120 °C for 2 h exhibited some differences in the spect rum, as shown in Fig ure 2 . 5 . The band at 1565 cm - 1 was shifted to a higher frequency at around 1580 cm - 1 . This result agrees with other authors that ob served that the amino groups could react with the carbonyl groups on cellulose (existing and produced during the heat 50 treatment) to form imines [25 ,27] . The differences observed on APTES - CNC spectra compared to CNC spectrum confirm that the silane treatment successfully introduced functional groups onto the CNC surface. XPS analysis was conducted to obtain quantitative and chemical information on the surface composition of the CNC and APTES - CNC. Fig ure 2 . 6 presents for both samples the wide - scan survey spectra with elemental assignments . The sp ectrum for treated CNC detected peaks assigned to silicon (Si2p, Si2s ) and nitrogen (N1s) elements, which are characteristic of APTES. Fig ure 2 . 6 : XPS survey of (a) CNC and (b) APTES - CNC . 51 Table 2 . 1 provides the elemental atomic composition and the oxygen to carbon ratio (O/C) of CNC and APTES trea ted sample. After treatment, silicon content increased from 0 to 3.3% and nitrogen content increased from 0 to 3.1%. Even after the Soxhlet extraction process, there are still significant amounts of silicon and nitrogen present on the cellulose surface , which indicates that APTES reacted chemically with the CNCs. As a result of the silane treatment, the O/C ratio decrease d from 0.78 for CNC to 0.65 for APTES - CNC due the bonding of APTES on the surface of the CNCs. Table 2 . 1 : O/C ratio and element composition in percent for CNC and APTES - CNC . Sample O/C C1s N1s O1s Si2p CNC 0.78 55.5 0 43.2 0 APTES - CNC 0.65 56.9 3.1 36.7 3.3 The deconvolution of C1s peak for CNC and APTES - CNC are given in Fig ure 2 . 7 . Table 2 . 2 summarizes the relative amount of C1s components, which includes C1: C - C, C2: C - O and C3: O - C - O. Surface modification changed the relative amount of C1s components. The C1 relative amount increased from 11.9% for CNC up to 32.0% for the APTES - CNC. The increase in C1 can be assig ned to the propyl groups on APTES that was attached to CNC during the treatment [21,25] . The intensity of C2 peak decreased and C3 slightly increased. These changes on the relative amount of C1s components clearly show that chemical bonds between APTES and CNCs were created. The XPS result s confirm chemical functionalization of the CNC by the APTES treatment. 52 Fig ure 2 . 7 : Deconvolution of C1s peak for (a) CNC and (b) APTES - CNC. 53 Table 2 . 2 : Relati ve amount of C1s components (%) for CNC and APTES - CNC . Characterization of APTES - CNC sized CFs Fig ure 2 . 8 shows SEM images of un sized CF and epoxy - only sized CF, and both exhibit similar surface s with grooves and ridges that are common on CFs produced by spinning of the PAN precursor. W hen APTES - CNCs were added into the sizing, the micrographs ( Fig ure 2 . 9 and Fig ure 2 . 10 ) show that the nanoparticles covered the CF s surface and exhibited a specific topography that resembles a web - like structure . Fig ure 2 . 9 shows the surface of the 12k tow sized CFs at different APTES - CNC concentrations and at different magnifications. In a typical hand dipping sizing process, the outside of the tow exhibited higher coverage while fibers on the in terior of the tow exhibited lower coverage. This may be a result of the 12k carbon fibers being close together in the tow where the sizing was not able to completely diffuse into the tow interior. This could be addressed by using a continuous fiber sizing tower system that constantly spreads the tow of CFs as much as possible using rollers while being sized [12] . The micrographs were taken from fibers harvested from intermediate regions of the tow and CFs with CNC - free portions still could be observed. Sample C1 C2 C3 CNC 11.9 72.3 14.5 APTES - CNC 32.0 50.1 15.3 54 Fig ure 2 . 8 : FIB - SEM micrographs of a) unsized CF and b) epoxy - only sized CF . Fig ure 2 . 9 (a) shows that a sizing was formed on the surface of the CFs sized at 0.6 wt% APTES - CNC and Fig ure 2 . 9 ( b ) shows that th e 1.0 wt% APTES - CNC sizing exhibited some regions where a considerable epoxy coating, which also contains APTES - CNCs, could be observed ( identified by white arrow). It can be clearly noted that the epoxy resin permeates the CNCs, suggesting strong interaction between the resin and the functionalized nanoparticles. This configuration is correlated with a large improvement in the IFSS as will be discussed later in Section 2.4.3 . At 2.0 wt% APTES - CNC ( Fig ure 2 . 9 ( c ) ), there is an excess of nanoparticles in some regions and more bridging between the single fibers (red arrow). E xcess coverage when using high concentrations of nanoparticles were reported in other work [17] . 55 Fig ure 2 . 9 : FIB - SEM micrographs for the 12k tow sizing technique of a) 0.6 wt% APTES - CNC sized CF, b) 1.0 wt% APTES - CNC sized CF and c) 2.0 wt% APTES - CNC sized CF . At higher magnification in Fig ure 2 . 9 (green arrow), it is possible to identify CNC free ends that are derived from the network substructure, and as reported by others using MFC [28] , these free ends offer physical anchoring sites for the epoxy while also providing broad area for chemical interactions. Overall, the surface of the APTES - CNC sized CFs exhibited a specific topography, with more roughness compared to the unsized CF and epoxy - only sized CF. In the individual sizing technique, the individual CF surface was immersed in the sizing solution for a longer time. Fig ure 2 . 10 shows the individual CFs sized at 0.6 wt%, 1.0 wt% and 56 2.0 wt% APTES - CNC . I t can be noted that this technique avoids bridging between the CFs and also avoids deforming the sizing when the 12k CF tow is opened for analysis, as was observed in the previou s technique. A concentration of 0.6 wt% APTES - CNC was able to form a sizing on the surface of CFs as can be noted in Fig ure 2 . 10 (a). For 1.0 wt% APTES - CNC ( Fig ure 2 . 10 (b)) it is possible to observe a considerable epoxy coating, which also contains APTES - CNCs. For 2.0 wt% APTES - CNC ( Fig ure 2 . 10 (c)), there is a dense coverage, as a result of the higher concentration, lengthy immersion and exposure of the CF surface into the sizing. This dense coating can impact the mechanical properties, as will be discussed in Section 3.3 . Overall, both sizing techniques are effective and simple methods for sizing CF. Fig ure 2 . 10 : FIB - SEM micrographs for the individual sizing technique of a) 0.6 wt% APTES - CNC sized CF, b) 1.0 wt% APTES - CNC sized CF and c) 2.0 wt% APTES - CNC sized CF . Evaluation of the IFSS by adding APTES - CNCs at the composite inter phase Fig ure 2 . 11 presents the IFSS calculated according to Equation 1 and Equation 2 for the 12k tow sized CFs and individually sized CFs . The percent increase of the IFSS was calculated using the un sized CFs as reference . For both techniques, the epoxy - only sized CFs exhibited higher IFSS compared to unsized CFs . This is the result of the epoxy coating ha ving a lower concentration 57 of curing agent than the bulk matrix, which diffuses to the fiber surface from the bulk and forms an interphase with a concentration gradient [5] . It generates a gradual change in the properties and a stiffer interphase for improved shear transfer onto the fiber surfac e. Fig ure 2 . 11 : IFSS for the a) 12k tow sized CFs and b) individually sized CFs. 58 T he incorporation of APTES - CNCs at the interphase has promoted an increase of the IFSS, which indicates the high efficacy of the nanoparticles in promoting good stress transfer from the matrix to the fiber. The effect of adding only 0.6 wt% APTES - CNC showed a considerable increase on the IFSS of 57% and 41% for the 12k tow sized CF and individually sized CF, respectively. The optimum improvement was obtained at a concentration of 1.0 wt% APTES - CNC that greatly increased the IFSS in 77% and 81% for the 12k tow sized CF and individually sized CF, respectively. APTES acted as a coupling agent by improving the interactions with the epoxide groups of the matrix and increasing the fiber - matrix adhesion at the interphase. The presence of the nanoparticles at the interphase locally increased the shear modulus of the m atrix, and it can increase the IFSS since it has a direct dependence on the product of the matrix strain to failure and the square root of the matrix shear modulus . The CFs sized at 2.0 wt% APTES - CNC exhibited an increase of 54% for the 12k tow sized CF a nd a reduction of 8% for the individually sized CF. As shown in Fig ure 2 . 10 ( c ) , the excessive sizing pick up generates multilayers of CNCs that can cause slippage of the nanoparticles with respect to each other and decrease the efficiency of stress transfer at the interphase [29] . Table 2 . 3 and Table 2 . 4 summarize the fiber fragment aspect ratio and IFSS for the 12k tow sized CFs and individually sized CFs , respectively. They show that the lowest fiber fragment aspect ratio and hence highest IFSS was achieved for CFs sized at 1.0 wt% APTES - CNC. Only 2.0 wt% APTES - CNC individually sized CFs exhibited aspect ratio higher than the unsized CFs. 59 Table 2 . 3 : Fiber fragment aspect ratio (l c /d) and IFSS for 12k tow sized CFs. Sample Fiber fragment aspect ratio IFSS (MPa) IFSS relative increase (%) Unsized CF 72.9 ± 8.3 30.6 ± 3.4 - Epoxy - only sized CF 60.1 ± 3.6 36.9 ± 2.1 20 0.6 wt% APTES - CNC sized CF 46.1 ± 2.5 48.0 ± 2.6 57 1.0 wt% APTES - CNC sized CF 40.6 ± 0.7 54.3 ± 0 .9 77 2.0 wt% APTES - CNC sized CF 47.2 ± 4.4 47.1 ± 4.6 54 Table 2 . 4 : Fiber fragment aspect ratio (l c /d) and IFSS for individually sized CFs. Sample Fiber fragment aspect ratio IFSS (MPa) IFSS relative increase (%) Unsized CF 72.9 ± 8.3 30.6 ± 3.4 - Epoxy - only sized CF 60.7 ± 5.9 36.7 ± 3.5 20 0.6 wt% APTES - CNC sized CF 51.0 ± 2.1 43.3 ± 1.7 41 1.0 wt% APTES - CNC sized CF 39.9 ± 0.4 55.3 ± 0.6 81 2.0 wt% APTES - CNC sized CF 79.4 ± 9.6 28.2 ± 3.7 - 8 Overall, CNC surface modification with silane and the addition of functionalized nanoparticles at the interphase proved to be an effective method to enhance the interfacial adhesion of composites. The increase in adhesion could be attributed to the increas e in the interphase 60 modulus resulting from the incorporation of CNCs in the epoxy - CF interphase combined with strong chemical bonding generated by the APTES at the interphase. Interfacial failure mode analysis Fig ure 2 . 12 shows representative micrographs of the birefringent stress patterns at saturation in the SFFT for the 12k tow sized CFs at different magnification. Fig ure 2 . 12 (a) shows that the fracture of un sized CFs occurred mostly by frictional de - bonding. It is characteristic of weak interfaces, and results in low IFSS, as described in Section 3.3. The epoxy sizing has changed the birefringence pattern, as evidenced by the shorter fragment length along the CF and a crack propagation into the matrix perpendicular to the fiber ( Fig ure 2 . 12 (b)). For the APTES - CNC sized CFs there was no obvious de - bonding ( Fig ure 2 . 12 (c - e). Instead, the birefringence patterns for CFs sized with nanoparticles was characteristic of very strong adhesion and a well bonded interphase, evidenced by the extensive crack propagatio n into the matrix , that correlates to the highest level of adhesion at the interphase. This birefringence pattern supports the findings of the highest levels of IFSS being associated with APTES - CNC sized CFs, as presented in Fig ure 2 . 11 (a). 61 Fig ure 2 . 12 : T ransmitted light optical micrographs of the b irefringence pattern at 20x and 50x for a) unsized CF, b) epoxy - only sized CF and 12k tow sized CF at c) 0.6 wt% APTES - CNC, d) 1.0 wt% APTES - CNC and e) 2.0 wt% APTES - CNC . Conclusion s This work has shown that the incorporation of CNCs at a CF - epoxy interphase can increase matrix - fiber adhesion. CNCs were successfully functionalized with APTES and the presence of APTES - CNC at the interphase greatly enhanced the stress transfer from the matrix to the fiber. The increase in the IFSS was achieved by the establishment of covalent bonding and by the increase of the modulus at the composite interphase. An optimal sizing concentration for CFs sized with 1.0 wt% APTES - CNCs in the sizing resulted in a n IFSS increase of 77% and 81% for the 12k 62 tow sized CF and individually sized CF, respectively, a s well as an improvement of the failure mode. In summary, beneficial effects of adding APTES - CNCs as a sizing for CF - epoxy matrix composites were shown . The process developed in this work has been shown to be a simple and effective method to size CFs and i mprove their adhesion in polymer composites. This technique can potentially be implemented for interfacial optimization in petroleum based as well as bio - based natural fiber composites. 63 REFERENCES 64 REFERENCES [1] EPA and NHTSA Set Standards to Reduce Greenhouse Gases and Improve Fuel Economy for Model Years 2017 - 2025 Cars and Light Trucks, 2012. https://nepis.epa.gov/Exe/tiff2png.cgi/P100EZ7C.PNG? - r+75+ - g+7+D%3A%5CZY FILES%5CINDEX DATA%5C11THRU15%5CTIFF%5C00000346%5CP100EZ7C.TIF. [2] J. Yang, J. Xiao, J. Zeng, L. Bian, C. Peng, F. Yang, Matrix modification with silane coupling agent for carbon fiber reinforced epoxy composites, Fibers Polym. 14 (2013) 759 766. doi:10.1 007/s12221 - 013 - 0759 - 2. [3] J. Karger - Kocsis, H. Mahmood, A. Pegoretti, Recent advances in fiber/matrix interphase engineering for polymer composites, Prog. Mater. Sci. 73 (2015) 1 43. doi:10.1016/j.pmatsci.2015.02.003. [4] L.T. Drzal, R.M. J., P.F. Lloyd, Adhesion of Graphite FIbres to Epoxy Matrices: I. The Role of Fibre Surface Treatment, J. Adhes. 16 (1982) 1 30. doi:10.1080/00218468308074901. [5] W. Qin, F. Vautard, P. Askeland, J. Yu, L. Drzal, Modifying the carbon fiber epoxy matrix interphase with silicon dioxide nanoparticles, RSC Adv. 5 (2015) 2457 2465. doi:10.1002/pc.23325. [6] L. Tang, J.L. Kardos, A review of methods for improving the interfacial adhesion between carbon fiber and polymer matrix, Polym. Compos. 18 (1997) 100 113. doi:10.10 02/pc.10265. [7] M.J. Rich, E.K. Drown, P. Askeland, L.T. Drzal, Surface Treatment of Carbon Fibers By Int. Conf. Compos. Mater., 2013. [8] Z. Dai, F. Shi, B. Zhang, M. L i, Z. Zhang, Effect of sizing on carbon fiber surface properties and fibers/epoxy interfacial adhesion, Appl. Surf. Sci. 257 (2011) 6980 6985. doi:10.1016/j.apsusc.2011.03.047. [9] H.L. Cox, The elasticity and strength of paper and other fibrous materials, Br. J. Appl. Phys. 3 (1952) 72 79. doi:10.1088/0508 - 3443/3/3/302. [10] T.F. Cooke, High Performance Fiber Composites with Special Emphasis on the Interface: A Review of the Literature, J. Polym. Eng. 7 (1987). [11] V. Rao, L.T. Drzal, The dependence of in terfacial shear strength on matrix and interphase properties, Polym. Compos. 12 (1991) 48 56. doi:10.1002/pc.750120108. [12] W. Qin, F. Vautard, L.T. Drzal, J. Yu, Mechanical and electrical properties of carbon fiber 65 composites with incorporation of graphe ne nanoplatelets at the fiber - matrix interphase, Compos. Part B Eng. 69 (2015) 335 341. doi:10.1016/j.compositesb.2014.10.014. [13] L.T. Drzal, M. Madhukar, Fibre - matrix adhesion and its relationship to composite mechanical properties, J. Mater. Sci. 28 (1 993) 569 610. doi:10.1007/BF01151234. [14] R. Liu, WenBo; Zhang, Shu; Li, Bichen; Yang, Fan; Jiao, WeiCheng; Hao, LiFeng; Wang, Improvement in Interfacial Shear Strength and Fracture Toughness for Carbon Fiber Reinforced Epoxy Composite by Fiber Sizing, Po lym. Compos. 35 (2013) 482 488. [15] K.Y. Lee, P. Bharadia, J.J. Blaker, A. Bismarck, Short sisal fibre reinforced bacterial cellulose polylactide nanocomposites using hairy sisal fibres as reinforcement, Compos. Part A Appl. Sci. Manuf. 43 (2012) 2065 207 4. doi:10.1016/j.compositesa.2012.06.013. [16] B.E.B. Uribe, A.J.F. Carvalho, J.R. Tarpani, Low - cost, environmentally friendly route to produce glass fiber - reinforced polymer composites with microfibrillated cellulose interphase, J. Appl. Polym. Sci. 133 ( 2016). doi:10.1002/app.44183. [17] W. Qin, F. Vautard, L.T. Drzal, J. Yu, Modifying the carbon fiber - epoxy matrix interphase with graphite nanoplatelets, Polym. Compos. 37 (2016) 1549 1556. doi:10.1002/pc.23325. [18] W. Qin, F. Vautard, P. Askeland, J. Yu, L.T. Drzal, Incorporation of silicon dioxide nanoparticles at the carbon fiber - epoxy matrix interphase and its effect on composite mechanical properties, Polym. Compos. (2015). doi:10.1002/pc.23715. [19] M. Mariano, N. El Kissi, A. Dufresne, Cellulose nan ocrystals and related nanocomposites: Review of some properties and challenges, J. Polym. Sci. Part B Polym. Phys. 52 (2014) 791 806. doi:10.1002/polb.23490. [20] H. Kargarzadeh, R.M. Sheltami, I. Ahmad, I. Abdullah, A. Dufresne, Cellulose nanocrystal: A p romising toughening agent for unsaturated polyester nanocomposite, Polym. (United Kingdom). 56 (2015) 346 357. doi:10.1016/j.polymer.2014.11.054. [21] J. Lu, P. Askeland, L.T. Drzal, Surface modification of microfibrillated cellulose for epoxy composite ap plications, Polymer (Guildf). 49 (2008) 1285 1296. doi:10.1016/j.polymer.2008.01.028. [22] F. Ansari, M. Salajková, Q. Zhou, L.A. Berglund, Strong Surface Treatment Effects on Reinforcement Efficiency in Biocomposites Based on Cellulose Nanocrystals in Pol y(vinyl acetate) Matrix, Biomacromolecules. 16 (2015) 3916 3924. doi:10.1021/acs.biomac.5b01245. [23] R.M. Sheltami, H. Kargarzadeh, I. Abdullah, Effects of silane surface treatment of cellulose nanocrystals on the tensile properties of cellulose - polyvinyl chloride nanocomposite, Sains Malaysiana. 44 (2015) 801 810. [24] D. Tripathi, F.R. Jones, Review Single fibre fragmentation test for assessing adhesion in 66 fibre reinforced composites, J. Mater. Sci. 33 (1998) 1 16. doi:10.1023/A:1004351606897. [25] J. Lu, L.T. Drzal, Microfibrillated cellulose/cellulose acetate composites: Effect of surface treatment, J. Polym. Sci. Part B Polym. Phys. 48 (2010) 153 161. doi:10.1002/polb.21875. [26] A. Gandini, M.N. Belgacem, Modified cellulose fibers as reinforcing fil lers for macromolecular matrices, in: Macromol. Symp., 2005: pp. 257 270. doi:10.1002/masy.200550326. [27] M.U. de la Orden, M.C. Matías, C.G. Sánchez, J.M. Urreaga, Ultraviolet Spectroscopic Study of the Cellulose Functionalization with Silanes, Spectrosc . Lett. 32 (1999) 993 1003. doi:10.1080/00387019909350044. [28] B.E.B. Uribe, E.M.S. Chiromito, A.J.F. Carvalho, J.R. Tarpani, Low - cost, environmentally friendly route for producing CFRP laminates with microfibrillated cellulose interphase, Express Polym. Lett. 11 (2017) 47 59. doi:10.3144/expresspolymlett.2017.6. [29] A. Asadi, M. Miller, R.J. Moon, K. Kalaitzidou, Improving the interfacial and mechanical properties of short glass fiber/epoxy composites by coating the glass fibers with cellulose nanocrysta ls, Express Polym. Lett. 10 (2016) 587 597. doi:10.3144/expresspolymlett.2016.54. 67 CHAPTER 3 : HYBRID CELLULOSE - INORGANIC REINFORCEMENT POLYPROPYLENE COMPOSITES: LIGHTWEIGHT MATERIALS FOR AUTOMOTIVE APPLICATIONS Portions of this chapter were published as a j ournal p aper in Polymer Composites Journal . The link and citation for the paper is Reale Batista MD, Drzal LT, Kiziltas A, Mielewski D. Hybrid cellulose - inorganic reinforcement polypropylene composites: Lightweight mat erials for automotive applications. Polymer Composites. 2019; 1 16 . https://doi.org/10.1002/pc.25439 . Abstract Cellulose fibers are attracting considerable attention within the transportation industry as a class of reinforcing agents for polymer composites owing to their low cost, low density, high mechanical properties, and considerable environmental benefits. The objective of this study was to develop hybrid composites combining cellulose fiber with long glass fiber, short glass fiber or talc in a polypropylene (PP) matrix to optimize the overall composite properties. Tensile, flexural and notched Izod impact tests revealed that in general the mechanical properties decreased with increasing cellulose conte nt, however, adding an optimum concentration of the cellulose fiber is a promising alternative to reduce or replace the utilization of inorganic fibers . For applications in - the - - inor ganic reinforcement composite approach not only leads to superior weight and cost savings, but also environment benefits over the inorganic reinforced composites. 68 Introduction The growing environmental awareness and the demand for the utilization of ren ewable sources to develop sustainable and recycled materials have promoted the incorporation of cellulose fibers as reinforcement for polymer composites [1] . Cellulosic fiber reinforced polymer composites have been used for many applications such as automotive components, aerospace parts, sporting goods and in the construction industry [2] . The interest in using this material is due to its sustainable nature, low cost, acceptable mechanical properties, elimination of abrasive damage to process ing equipment, abundant availability and reduced health concerns [3 7] . Cellulose fiber has a lower density compared to glass fibers and talc fillers, approximately 1.5 g/cm 3 versus 2.5 and 2.8 g/cm 3 , respectively. Therefore, its usage in the automotive industry is a central strategy for meeting light weig weight by 10 % can improve the fuel economy by 3 to 7 % [8] and contribute to attaining the CAFE standards. Despite the attractiveness of natural fiber reinforced polymer composites, they exhibit lower modulus and strength as well as inferior moisture resistance compare d to synthetic fiber reinforced composites, such as glass fiber reinforced polymer composites [9] . Hybridization of cellulose fiber with inorganic fibers is one possibility to improve the mechanical properties of the composites. The advantage of combining two or more fiber types in a single matrix is that the unique properties of one type of fiber could complement what is lacking in the other [10,11] . Hybridization is also a means to c ombine different fiber properties to develop a multifunctional composite. In this sense, hybrid composites made with cellulose fibers possess advantages in weight reduction, sustainability and higher mechanical properties. 69 The objective of this study was the development of hybrid composites, investigating the effects of combining cellulose fiber with long glass fiber (LGF), short glass fiber (SGF) or talc in a polypropylene (PP) matrix. The focus was on increasing the biobased content by reducing the amoun t of inorganic reinforcement. The mechanical, thermal and morphological properties of the resulting composites were evaluated in terms of feasibility for automotive applications to decrease the environmental impact while maintaining the product safety, dur ability, and quality. Experimental Materials PP homopolymer pellets were supplied from three commercial sources (Company X, Company Y and Company Z) listed in Table 3 . 1 . Cellulose was obtained from a commercial source combined with PP in the form of pellets and they are named as Cellulose A and Cellulose B. They are from the same commercial source with the same composition, only with different dispersion techniques. The inorganic reinforcement s were supplied in the form of master - batch pellets and Table 3 . 1 . 70 Table 3 . 1 : As received pellets compositions . Composition Comments Homopolymer PP pellets Kindly obtained from local sources: Neat PP X from Company X (4.0 g/10 min, ASTM D1238) Neat PP Y from Company Y (37 g/10min, ISO 1133) Neat PP Z from Company Z (17 g/10min, ISO 1133) 30 wt. % Cellulose filled PP pellets Kindly provided by local source: Cellulose A and Cellulose B . Cellulose pellets have hexagonal shape, a thickness of 0.1 to 1.5 mm a nd a length of 4.5 to 6.5 mm 30 wt. % LGF filled PP pellets Used in instrumental panel substrates 40 wt. % (SGF / Mica) filled PP pellets Supplied by Company Y : Melt flow: 10 g/10min ( ISO 1133 ) / Density: 1.23 g/cm 3 Used in console substrate 42.5 wt. % Talc filled PP pellets Supplied by Company Y with a minimum of 25% post - consumer recycled content. Used in head lamp housing 33 wt. % SGF filled PP pellets Supplied by Company Z : Melt f low : 4.0 g/10 min ( ISO 1133 ) Used in console substrate Processing The c omposites were prepared by injection molding (Boy Machines Model 80M) with the same processing conditions and the total fiber mass content was fixed at 30 wt. %, with the cellulose concentration varying gradually from 0 wt. % up to 30 wt. %. Table 3 . 2 summarizes the composite compositions and sample identification codes, for each constituent source. To achieve the desired ino rganic reinforcement/cellulose concentration, the as received master - batch pellets from Table 3 . 1 were mixed and diluted with neat PP as needed. 71 Table 3 . 2 : Sample identification codes and composite composition for each constituent source . LGF / Cellulose hybrid composites Sample code LGF (wt. %) Cellulose A (wt. %) PP from Company X (wt. %) Neat PP X 0 0 100 LGF/Cellulose A (30/0) 30 0 70 LGF/Cellulose A (20/10) 20 10 70 LGF/Cellulose A (15/15) 15 15 70 LGF/Cellulose A (10/20) 10 20 70 LGF/Cellulose A (0/30) 0 30 70 (SGF / Mica) / Cellulose hybrid composites Sample code (SGF/Mica) (wt. %) Cellulose B (wt. %) PP from Company Y (wt. %) Neat PP Y 0 0 100 (SGF/Mica)/Cellulose B (30/0) 30 0 70 (SGF/Mica)/Cellulose B (20/10) 20 10 70 (SGF/Mica)/Cellulose B (15/15) 15 15 70 (SGF/Mica)/Cellulose B (10/20) 10 20 70 (SGF/Mica)/Cellulose B (0/30) 0 30 70 Talc / Cellulose hybrid composites Sample code Talc (wt. %) Cellulose B (wt. %) PP from Company Y (wt. %) Neat PP Y 0 0 100 Talc/Cellulose B (30/0) 30 0 70 Talc/Cellulose B (20/10) 20 10 70 Talc/Cellulose B (15/15) 15 15 70 Talc/Cellulose B (10/20) 10 20 70 Talc/Cellulose B (0/30) 0 30 70 SGF / Cellulose hybrid composites Sample code SGF (wt. %) Cellulose B (wt. %) PP from Company Z (wt. %) Neat PP Z 0 0 100 SGF/Cellulose B (30/0) 30 0 70 SGF/Cellulose B (20/10) 20 10 70 SGF/Cellulose B (15/15) 15 15 70 SGF/Cellulose B (10/20) 10 20 70 SGF/Cellulose B (0/30) 0 30 70 72 Prior to injection molding, the pellets were dried at 60°C overnight to reduce moisture content. Then they were mixed and fed into the hopper on the injection molding machine as shown on Figure 3 . 1 . An extrusion process was not used in this research. Therefore, it is worth mentioning that this is a n efficient one - step injection process that can speed up part production. To avoid cellulos e degradation the maximum injection temperature was limited to 193°C (380 F). Composites were molded into ASTM test specimens and conditioned in a room at 23 ± 2°C and 50 ± 5% relative humidity for 7 days before conducting mechanical tests. Figure 3 . 1 : Schematic illustration of materials , processing and characterization . Testing procedure and characterization Mechanical test Tensile and flexural tests were conducted in an Instron 3366 in compliance to ASTM D638 - 10 and ASTM D790 - 10, respectively. The tensile test used a crosshead speed of 5.0 mm/min, a 5 kN loa d cell and a 50 mm extensometer attached to the gauge section of the specimen 73 modulus were determined. At least six specimens were tested for each data set. The flexural tests also used a 5 kN load cell, at a rate of 1.0 mm/min with a support span length of 50 mm. The stress at 5% strain and flexural modulus were determined. The impact strength of notched Izod specimens was measured according to ASTM D256 - 10. Tests were conducted on a Testing Machines Inc. 43 - 02 - 03 model impact test machine with a 2 lb. pendulum, and the results are the average from ten specimens of each data set. All the mechanical tests were run in an environmentally conditioned room at 23 ± 2°C and 50 ± 5% relative humidity . Scanning electron microscopy (SEM) A Carl Zeiss EVO LS 25 scanning electron microscope with accelerating voltage of 15 kV was used to observe the morphology of the impact fracture surface of the samples. The samples were sputter - coated with platinum to pr event surface charging. In the micrographs, the direction of impact is from right to left , as shown on the schematic illustration on Fig ure 3 . 2 . Fig ure 3 . 2 : Schematic illustration of SEM analysis. 74 Thermal characterization Melting and crystallization behavior of the neat polymer matrix and the composites were measured using a TA Instruments Q2000 differential scanning calorimet er . To remove the thermal history, samples were heated from room temperature to 250 °C at a rate of 20 °C/min and held isothermally at 250 °C for 5 minutes. Then, they were cooled to - 50 °C at a rate of 10 °C/min and held at - 50 °C for 5 minutes before reheating to 250 °C at a rate of 10 °C/min. The melting and crystallization behavior were collected from the heat flow versus temperature curves. Melting temperature (T m ) was assigned as the peak minimum of the endothermic melt transition, and crystallization temperature (T c ) as the peak maximum of the m ) and enthalpy of c ) were also measured from these curves. Specimen weight was in the range o f 7 to 9 mg and the results were averaged from three specimens of each sample. The thermal stability of the neat PP and the composites was investigated by thermal gravimetric analysis (TGA) curves using a TA Instruments Q500. The thermograms were obtained under constant air flow rate of 50 mL·min 1 and the samples were heated up to 600 °C at a rate of 10 °C/min. The sample weight was in the range of 8 to 10 mg. Three replicates of each sample were performed. 75 Results and discussion Mechanical properties Tensile properties Fig ure 3 . 3 , Figure 3 . 4 and Fig ure 3 . 5 show the effects of different fiber combinations on the tensile properties of the composites, along with the neat PP. Overall, the addition of inorganic fibers and cellulose led to a considerable increase in the ma ximum tensile stress ( Fig ure 3 . 3 ) and Figure 3 . 4 ) in comparison to the neat PP. The inorganic fibers produced the greatest improvement in these properties. An increase of 232% and 401% for tensile stress and modulus, respectively, was observed for LGF/Cellulose A (30/0) composites compare d to neat PP X ( Fig ure 3 . 3 (a), Figure 3 . 4 (a) ) . As the cellulose content was increased to replace a portion of the LG F, SGF/Mica or SGF, the tensile stress and modulus of the composites decreased. For these composites which contain glass fiber, this trend is expected due to the stronger and stiffer properties of the inorganic fiber compared to cellulose [12] . As reported in the work of Thwe and Liao [9] , the tensile strength and modulus of hybrid composite s of bamboo fiber and glass fiber in PP gradually increased with increasing glass fiber to bamboo fiber ratio. It was observed that even with the incorporation of 30 wt. % cellulose, the tensile stress and modulus of the composites are still higher compar ed to neat PP. The addition of 30 wt. % Cellulose B promotes an increase of 18% and 129% for tensile stress and modulus, respectively, in comparison with neat PP Y ( Fig ure 3 . 3 (b,c), Figure 3 . 4 (b,c)). SGF/Cellulose B (15/15) hybrid composites increased the tensile stress and modulus by 58% and 171%, respectively, in comparison with neat PP Z ( Fig ure 3 . 3 (d), Figure 3 . 4 (d)) and is suitable for body interior (console 76 substrate, wiring harness) and under - the - hood (battery and power distribution box covers) applications. The tensile stress at maximum load for the composites made of talc and cellulose did not follow the previous trend ( Fig ure 3 . 3 (c)). Talc/Cellulose B (10/20) composites exhibited an increase of 15% for the tensile stress in comparison with Talc/Cellulose B (30/0) composites, which represents an advantage of the hybrid co mposite over the composite reinforced with talc only. The tensile strains at maximum load of the composites were markedly lower than neat PP ( Fig ure 3 . 5 ) with no considerable difference in strain as a function of fiber content. Overall, the mechanical properties in tension show that the hybridization of cellulose with inorganic reinforcements produced beneficial enhancements. 77 Fig ure 3 . 3 : Tensile stress at maximum load of a) LGF/Cellulose A, b) (SGF/Mica)/Cellulose B, c) Talc/Cellulose B and d) SGF/Cellulose B composites along with neat PP. 78 Figure 3 . 4 : Talc/Cellulose B and d) SGF/Cellulose B composites along with neat PP . 79 Fig ure 3 . 5 : Tensile strain at maximum load of a) LGF/Cellulose A, b) (SGF/Mica)/Cellulose B, c) Talc/Cellulose B and d) SGF/Cellulose B composites along with neat PP. Flexural properties The Figure 3 . 6 and Fig ure 3 . 7 show the flexural properties of the polymer matrix and the composites. Flexural properties exhibited a similar trend as the tensile results, revealing that in general the mechanical properties decreased as the amount of cellulose increased. The stress at 5% strain achieved a 59% enhancement for LGF/ Cellulose A (0/30) compared to neat PP X ( Figure 3 . 6 (a)). The presence of cellulose increased the stress at 5% strain for Talc/Cellulose B composites ( Figure 3 . 6 (c)) with the highest strength of 56.3 MPa for Talc/Cellulose B (10/20) hybrid 80 composites. It represents an increase of 10% in comparison with Talc/Cellulose B (30/0) composites. Similar to the tensile modulus results, all composites showed higher flexural modulus than neat PP ( Fig ure 3 . 7 ). Figure 3 . 6 : Stress at 5% strain of a) LGF/Cellulose A, b) (SGF/Mica)/Cellulose B, c) Talc/Cellulose B and d) SGF/ Cellulose B composites along with neat PP. *Specimens broke before 5% strain; maximum stress value used. 81 Fig ure 3 . 7 : Flexural modulus of a) LGF/Cellulose A, b) (SGF/Mica)/Cellulose B, c) Talc/Cellulose B and d) SGF/Cellulose B composites along with neat PP. Impact properties The addition of cellulose fibers acted as stress concentrators and reduced the impact strength of the composites, as shown in Fig ure 3 . 8 (a, b and d). A d ecrease in impact strength of PP was also observed when sisal fibers were added to PP by unnotched Izod impact test [13] . Fig ure 3 . 8 (a) shows th a t LGF/Cellulose A (15/15) composites exhibited an impact strength 91% higher than LGF/Cellulose A (0/30). As reported by Panthapulakkal and Sain [14] for hemp/ glass fiber/ PP composites, the impact streng th is enhanced with an increase in glass fiber content due to the improved resistance offered from the glass fibers in the composites. For the hybrid composites made of SGF/Cellulose B ( Fig ure 3 . 8 ( d) ) the impact strength is still preserved 82 in comparison with the neat PP Z. The SGF/Cellulose B (20/10) composites increased the impact strength by 34% and the SGF/Cellulose B (10/20) composites still exhibited 16% increase compared to neat PP Z. No change in impact strength was observed when adding cellulose to talc composites ( Fig ure 3 . 8 (c)). Fig ure 3 . 8 : Impact strength of a) LGF/Cellulose A, b) (SGF/Mica)/Cellulose B, c) Talc/Cellulose B and d) SGF/Cellulose B composites along with neat PP . Morphological properties Fig ure 3 . 9 , Fig ure 3 . 10 , Fig ure 3 . 11 , and Figure 3 . 12 s how the impact fracture surface of LGF/Cellulose A, (SGF/Mica)/Cellulose B, Talc/Cellulose B and SGF/Cellulose B composites, 83 respectively, along with the neat PP. The samples investigated are neat PP and composites reinforced with inorganic reinforcement /cellulose at 30/0, 15/15 and 0/30 (wt.%). It can be observed that the basic deformation mechanism of all unmodified PP is shear yielding ( Fig ure 3 . 9 (a), Fig ure 3 . 10 (a), Fig ure 3 . 11 (a), Figure 3 . 12 (a)). There was no significant difference between the surface morphologies of the Cellulose A and Cellulose B composites ( Fig ure 3 . 9 (d), Fig ure 3 . 10 (d), Fig ure 3 . 11 (d), Figure 3 . 12 (d). Figure 3 . 13 show higher magnification of the composites reinforced at 30wt. % cellulose where the width and thickness of the fiber can be observed . Since the cellulose fiber is hydrophilic , interfacial compatibility is not expected to be high. This is consistent wi th t he micrographs of the fracture surfaces which show little evidence of interfacial interaction between the cellulose and the matrix . This incompatibility may reduce the mechanical properties and is in agreement with the reduced tensile, flexural and im pact properties measured on the cellulose reinforced composites . C ellulose fiber fracture is also clear from Fig ure 3 . 9 (d) and Figure 3 . 13 . a) b ) 84 Fig ure 3 . 9 : SEM micrographs of a) Neat PP X, b) LGF/Cellulose A (30/0), c) LGF /Cellulose A (15/15) and d) LGF/Cellulose A (0/30). The micrographs of specimens reinforced with long or short glass fiber exhibited fiber pull out ( Fig ure 3 . 9 (b), Fig ure 3 . 10 (b), Figure 3 . 12 (b)) , an indicative of poor interfacial adhesion between the glass fiber and matrix. This behavior is in agreement with results reported by Arbelaiz et al. [11] . The distribution of the fibers in the matrix appears to be good however. With the incorporation of 15 wt. % cellulose ( Fig ure 3 . 9 (c), Fig ure 3 . 10 (c), Figure 3 . 12 (c)), holes from the pulled out glass fibers are still observed. In the SEM micrographs glass fiber can 85 be distinguished from cellulose by their difference in diam eter. Good distribution of the hybrid fibers is observed. Fig ure 3 . 10 : SEM micrographs of a) Neat PP Y, b) (SGF/Mica)/Cellulose B (30/0), c) (SGF/Mica)/Cellulose B (15/15) and d) (SGF/Mica)/Cellulose B (0/30). Fig ure 3 . 10 (b) identifies a mica particle and Fig ure 3 . 11 (b) identifies the platelet talc filler. Micrographs show good talc distribution in the matrix with no large aggregates present. 86 Fig ure 3 . 11 : SEM micrographs of a) Neat PP Y, b) Talc/Cellulose B (30/0), c) Talc/Cellulose B (15/15) and d) Talc/Cellulose B (0/30) . 87 Figure 3 . 12 : SEM micrographs of a) Neat PP Z, b) SGF/Cellulose B (30/0), c) SGF/Cellulose B (15/15) and d) SGF/Cellulose B (0/30). 88 Figure 3 . 13 : SEM micrographs of a) 30wt. % Cellulose A and b) 30wt. % Cellulose B. Overall, the composites made with this process show good fiber distribution, confirming that the one step injection process with no compatibilizer successfully blended the hybrid materials in the PP. There is an advantage of using these pre - compounded pell ets to speed up the process production. Thermal properties T hermal Gravimetric Analysis (T GA ) Figure 3 . 14 summarizes the thermal stability of the composites, where T d is assigned as the temperature at the maximum rate of decomposition determined from the derivative thermogravimetric (DTG) curves. All composites are more stable than the neat PP and the T d of the inorganic r einforced composites ma nifested the greatest increases . The incorporation of 30 wt. % cellulose also increased the thermal stability relative to PP but to a lesser degree. The inorganic 89 reinforcement /cellulose composites reinforced at 20/10, 15/15 and 10/20 (wt. %) have similar thermal stability. Figure 3 . 14 : Temperature at the maximum rate of decomposition determined from DTG curves for a) LGF/Cellulose A, b) (SGF/Mica)/Cellulose B, c) Talc/Cellulose B and d) SGF/Cellulose B composites along with neat PP. Thermal stability was also defined by the temperature at which 1% and 10% weight loss occurred, T 1 and T 10 , respectively, as shown in Figure 3 . 15 and F igure 3 . 16 . All hybrid composites are more thermally stable than the neat PP. The composites that contain cellulose possess lower T 1 and T 10 compared to th e composites with inorganic reinforcement s only. However, this behavior is not observed for LGF/Cellulose A composites ( Figure 3 . 15 (a), F igure 3 . 16 (a)). 90 Figure 3 . 15 : Temperature at 1% weight loss from TGA curves for a) LGF/Cellulose A, b) (SGF/Mica)/Cellulose B, c) Talc/Cellulose B and d) SGF/Cellulose B composites along with neat PP. 91 F igure 3 . 16 : Temperature at 10% weight loss from TGA curves for a) LGF/Cellulose A, b) (SGF/Mica)/Cellulose B, c) Talc/Cellulose B and d) SGF/Cellulose B composites along with neat PP. 92 Figure 3 . 17 shows the residual weight percent at 587 °C. The residue decreased with cellulose content at this temperature and the residue is mainly from the inorganic reinforcement s. Figure 3 . 17 : Residual weight percent at 587 °C from TGA curves for a) LGF/Cellulose A, b) (SGF/Mica)/Cellulose B, c) Talc/Cellulose B and d) SGF/Cellulose B composites along with neat PP. These results indicate that hybrid composites made of cellulose can be to be con sidered for challenging conditions on under - the - hood applications. 93 D ifferential Scanning Calorimetry (DS C ) Figure 3 . 18 summarizes the T c and T m of PP and the composites. A n increase in the T c for all composites was detected compared to neat PP, which reveals that the fibers act as nucleating agents leading to a faster crystallization of PP matrix. This is a beneficial development which can increase the rate of part production. The DSC measurements indicate that t he addition of cellulose had minimal effect on the melting temperatures. Figure 3 . 18 : Crystallization and melting temperatures for a) LGF/Cellulose A, b) (SGF/Mica)/Cellulose B, c) Talc/Cellulose B and d) SGF/Cellulose B composites. c m decreased with the incorporation of fibers ( Table 3 . 3 and Table 3 . 4 ), with no significant dependence on fiber loading. This trend is in agreement with the results of 94 Huda et al. [15] , wh ere a reduction of the melting and crystallization enthalpies of the composites was observed with the addition of recycled newspaper cellulose fibers and talc compared to neat PP. Composites reinforced with 30 wt. % cellulose have, marginally, the lowest e nthalpies. It may be that cellulose was hindering polymer chain movement during heat cycling, as observed by Langhorst et al. [8] . Table 3 . 3 : Enthalpies of crystallization for neat PP and composites . Composition (wt. %) LGF/Cellulose A (SGF/Mica)/Cellulose B Talc/Cellulose B SGF/Cellulose B H c (J/g) H c (J/g) H c (J/g) H c (J/g) Neat PP X 89.8 ± 3.3 - - - Neat PP Y - 96.7 ± 0.2 96.7 ± 0.2 - Neat PP Z - - - 104.6 ± 2.9 (30/0) 75.1 ± 0.6 67.8 ± 1.8 70.1 ± 1.4 73.6 ± 1.6 (20/10) 73.5 ± 1.5 68.7 ± 3.1 71.6 ± 1.7 69.4 ± 3.0 (15/15) 74.3 ± 3.8 69.2 ± 2.8 69.2 ± 2.7 70.6 ± 3.8 (10/20) 70.7 ± 3.2 66.9 ± 2.8 70.6 ± 0.7 73.3 ± 1.0 (0/30) 67.6 ± 3.3 62.3 ± 5.0 62.3 ± 5.0 62.3 ± 5.0 Table 3 . 4 : Enthalpies of melting for neat PP and composites . Composition (wt. %) LGF/Cellulose A (SGF/Mica)/Cellulose B Talc/Cellulose B SGF/Cellulose B H m (J/g) H m (J/g) H m (J/g) H m (J/g) Neat PP X 86.4 ± 3.6 - - - Neat PP Y - 95.8 ± 0.3 95.8 ± 0.3 - Neat PP Z - - - 104.0 ± 5.5 (30/0) 75.7 ± 1.6 67.3 ± 0.8 69.9 ± 1.5 72.0 ± 0.2 (20/10) 73.4 ± 3.4 67.8 ± 3.3 68.6 ± 0.3 67.9 ± 0.3 (15/15) 63.6 ± 6.7 66.8 ± 1.4 64.2 ± 2.6 71.3 ± 3.2 (10/20) 68.1 ± 3.1 68.0 ± 2.6 68.0 ± 1.4 71.6 ± 2.1 (0/30) 63.0 ± 0.6 62.5 ± 5.3 62.5 ± 5.3 62.5 ± 5.3 95 Conclusion s In this study, we investigated the hybridization of cellulose fiber with several inorganic fibers to reinforce a PP matrix for automotive applications. Composites were processed by injection molding with total reinforcement content kept at 30 wt. %. Overal l, it was found that the composites made with hybrid fibers incorporating cellulose fibers acted as an effective reinforcement. Good improvements on the mechanical, thermal and morphological properties of the hybrid composites were observed. Results from tensile, flexural and notched Izod impact tests show that in general the mechanical properties decreased with increasing cellulose content. However, composites with an optimum amount of cellulose fiber have sufficient properties which may reduce or replace a portion of the inorganic reinforcements in many applications. LGF/Cellulose A composites exhibited the best mechanical properties. However, when comparing thermal properties, (SGF/Mica)/Cellulose B, Talc/Cellulose B and SGF/Cellulose B composites exhibi ted similar or even superior properties. The T c for all composites increased in comparison to neat PP, revealing the fibers ability to act as nucleating agents and speed part production. Cellulose fiber polymer composite properties can be improved by the hybridization with inorganic reinforcement s. The benefits of direc t injection molding process include producing complex parts with high output rate, which increases profit margins. In addition, the adequate thermal stability achieved on the composites is important because it means their ability to withstand machin ery tem peratures and maintain the ir properties in conditions that would may c ause thermal degradation . All these advantages have positive effect on manufacturability. 96 This work shows that hybridization of cellulose fiber with inorganic reinforcements in polypropy lene composites has the potential to reduce the use of inorganic fibers in different automotive applications, leading to weight and cost savings, and contributing to sustainability of composites. 97 REFERENCES 98 REFERENCES [1] N. Saba, P. Tahir, M. Jawaid, A Review on Potentiality of Nano Filler/Natural Fiber Filled Polymer Hybrid Composites, Polymers (Basel). 6 (2014) 2247 2273. doi:10.3390/polym6082247. [2] M. Jawaid, H.P.S. Abd ul Khalil, Cellulosic/synthetic fibre reinforced polymer hybrid composites: A review, Carbohydr. Polym. 86 (2011) 1 18. doi:10.1016/j.carbpol.2011.04.043. [3] E. Erbas Kiziltas, A. Kiziltas, E.C. Lee, Structure and properties of compatibilized recycled pol ypropylene/recycled polyamide 12 blends with cellulose fibers addition, Polym. Compos. 39 (2018) 3556 3563. doi:10.1002/pc.24376. [4] A. Kiziltas, E.C. Lee, Sustainable Composites Based on Polyamides and Cellulose Fibers, Dearborn, 2014. [5] A. Kiziltas, D .J. Gardner, Y. Han, H. - S. Yang, Mechanical Properties of Microcrystalline Cellulose (MCC) Filled Engineering Thermoplastic Composites, J. Polym. Environ. 22 (2014) 365 372. doi:10.1007/s10924 - 014 - 0676 - 5. [6] A. Birch, C. Dal Castel, A. Kiziltas, D. Mielew ski, L. Simon, Development of cost effective and sustainable polyamide blends for automotive applications, in: 15th Annu. Soc. Plast. Eng. Automot. Compos. Conf. Exhib., Troy (Detroit), 2015. [7] J.H. Zhu, A. Kiziltas, E.C. Lee, D. Mielewski, Bio - based pol yamides reinforced with cellulose nanofibers - processing and characterization, in: 15th Annu. Soc. Plast. Eng. Automot. Compos. Conf. Exhib., Troy (Detroit), 2015. [8] A. Langhorst, A. Kiziltas, D. Mielewski, E. Lee, Selective dispersion and compatibilizing effect of cellulose filler in recycled PA6/ PP blends, in: 15th Annu. Soc. Plast. Eng. Automot. Compos. Conf. Exhib., Troy (Detroit), 2015. [9] M.M. Thwe, K. Liao, Effects of environmental aging on the mechanical properties of bamboo glass fiber reinforce d polymer matrix hybrid composites, Compos. Part A Appl. Sci. Manuf. 33 (2002) 43 52. doi:10.1016/S1359 - 835X(01)00071 - 9. [10] M.M. Thwe, K. Liao, Durability of bamboo - glass fiber reinforced polymer matrix hybrid composites, Compos. Sci. Technol. 63 (2003) 375 387. doi:10.1016/S0266 - 3538(02)00225 - 7. [11] A. Arbelaiz, B. Fernández, G. Cantero, R. Llano - Ponte, A. Valea, I. Mondragon, Mechanical properties of flax fibre/polypropylene composites. Influence of fibre/matrix modification and glass fibre hybr idization, Compos. Part A Appl. Sci. Manuf. 36 (2005) 1637 1644. doi:10.1016/j.compositesa.2005.03.021. 99 [12] D. Romanzini, H.L. Ornaghi Junior, S.C. Amico, A.J. Zattera, Preparation and characterization of ramie - glass fiber reinforced polymer matrix hybrid composites, Mater. Res. 15 (2012) 415 420. doi:10.1590/S1516 - 14392012005000050. [13] K. Jarukumjorn, N. Suppakarn, Effect of glass fiber hybridization on properties of sisal fiber polypropylene composites, Compos. Part B Eng. 40 (2009) 623 627. doi:10.101 6/j.compositesb.2009.04.007. [14] S. Panthapulakkal, M. Sain, Injection - molded short hemp fiber/glass fiber - reinforced polypropylene hybrid composites Mechanical, water absorption and thermal properties, J. Appl. Polym. Sci. 103 (2007) 2432 2441. doi:10.10 02/app.25486. [15] M.S. Huda, L.T. Drzal, A.K. Mohanty, M. Misra, The effect of silane treated - and untreated - talc on the mechanical and physico - mechanical properties of poly(lactic acid)/newspaper fibers/talc hybrid composites, Compos. Part B Eng. 38 (200 7) 367 379. doi:10.1016/j.compositesb.2006.06.010. 100 CHAPTER 4: FLEXIBLE ULTRAVIOLET SENSOR BASED ON CARBON NANOTUBES This chapter is a result of an internship done by Mariana D esireé Reale Batista at NASA Ames Research Center during the NASA International Internship (NASA I 2 ) program under the mentoring of Dr. Sun Jin Kim 1 , Lawrence T. Drzal 2 , Jin - Woo Han 1 and Meyya Meyyappan 1 1 Center for Nanotechnology, NASA Ames Research Center, Moffett Field, Ca lifornia, 94035, United States 2 Department of Chemical Engineering and Materials Science, Michigan State University, East Lansing, Michigan, 48824, United States This project was presented at the NASA Intern Poster Symposium (Dec 2018) . Abstract Carbon Nan otubes (CNTs) were used to d evelop a sensor to detect Ultraviolet (UV) radiation , which is important for space communication , monitoring climate change and human exposure to UV light. F lexible substrates and paper electronics are attracting considerable at tention since they offer new capabilities for devices that are not possible with the conventional rigid substrates. In this study, UV s ensors were made not only on glass substrate s but also on flexible substrates such as Polyimide Film (PI) and Cellulose Paper for comparison purpose s . As received CNT s in aqueous solution were dil uted to final concentrations of 0.01 wt%, 0.004 wt%, 0.002 wt%, 0.001 wt% and 0.0005 wt% and drop - cast onto the active region between the electrodes of the sensor on ea ch substrate . An optimum CNT concentration was identified and t he dispersion and topographies of the nanoparticles on the sensor substrate s w ere investigated by Scanning 101 Electron Microscop y (SEM) and Atomic Force Microscopy (AFM ). All the sensors respond i mmediately to UV O n /O ff cycles with a change in resistance due to the ability of the CNTs to adsorb and desorb oxygen on their surface . Results show that sensors with the lowest concentration of CNT s (0.0005 wt%) exhibit the best response. The PI substrate yields the sensor with the highest response at 71% after 21 min of On/O ff cycle, while g lass and c ellulose p aper substrates exhibit response at 48% and 39% , respectively . After mechanically bending the flexible sensors at a curvature radius of 10 mm for 1000 times their functionality is maintained, which is an advantage for practical applic ations. Therefore, the PI sensor was selected for wearable applications using a simpl e printing technique in which electrodes were printed on the PI substrate and could be tailored for wristband or sunglass use . Outdoor t est ing under natural sunlight show s that the sensor accurate ly detects UV radiation and could be further developed for p otential commercial applications. Keywords: Ultraviolet Sensor; Flexible Sensor; Ultraviolet Radiation; Carbon Nanotubes; Introduction Ultraviolet (UV) radiation consists of photons with wavelengths ranging from 10 nm to 400 nm. Most of the UV light from the s un is absorbed by the ozone layer , however photons wi th wavelength s longer than 280 nm can reach the e arth [1] and the in tense exposure to this radiation can cause damage to human skin, especially cancer. T herefore , it is important to develop sensors capable of detecting this type of radiation which will h elp in th e prevention of health problems related to this exposure . UV sensors are also widely used in many other applications, including space communication and measurement of UV for climate change studies . 102 Many UV sensors and materials for sensing UV radiation ha ve been reported [1,2] . Although the traditional Si based photodetectors are commonly used, they exhibit some limitations with respect to UV detection, such as the use of costly and complex fi lters, and degraded performance with temperature [1,3] . These limitations are overcome with the wide bandgap semiconductors such as AlN , In N, GaN, diamond, and SiC based UV detectors that have not only high selectivity a nd response speed but also the ability to operate in harsh and high temperature conditions [1] . More recently, one - dimensional nanostructured semiconductors such as Ga 2 O 3 nanowires, GaN nanowires, ZnO nanowires or other metal - oxide nanostructures have attracted attention for use as UV detectors due to their wide bandgap and improved photosen sitivity [1] . However there are some challenges integrating the nanost ructures in to UV photodetector s [3] and the high cost related to the low photo response current from the small size of the nanowires as reported by Bai et al. [4] . Although those sensors exhibit high performance, they exhibit some limitations and more work is needed for inves tigating their use on flexible substrate for targeting bending and wearable applications. Flexible substrates and pap er electronics are attracting attention since they offer new possibilities that are not attainable with conventional rigid substrate s [5] . They have applicability in displays , detectors , memory and ener gy storage devices. C ellulose paper [6,7] as well as cotton textile [8] has been used as flexible substrates to fabricate gas and vapor sensors a nd there is an increasing demand for the development of new sensors that can bend . 103 Carbon nanotubes ( CNTs ) consist of one or several graphene planes rolled in a cylindrical shape with diameters of 1 to several dozens of nanometers and lengths of up to several microns [9] . They ha ve been reported as excelle nt sensor material for gas , vapor and UV sensor s [9,10] due to their large specific surface area, hi gh electrical conductivity and solution processability [11,1 2] . CNTs also exhibit good mechanical properties with a high degree of flexibility that are critical for bendable and stretchable substrates [2,5] . In this work , carbon nanotubes were used as the sensing material to detect UV light. Detection occurs since the radiation affects their resistance as a result of oxygen adsorption and desorption mechanism s on their surface [13,14] . An optimum CNT concentration was identified, and device fabrication, testing and evaluation were completed. UV sensors were fabricated on flexible substrates such as PI f ilm and Cellulose Paper, and rigid glass substrate for comparison purposes . PI was selected since it withstands high temperature applicatio ns ( around 400 ° C), which is important for space mission exploration. Cellulose paper was chosen since it is a low cost, lightweight, ecofriendly and disposable materi al , leading to an approach for the development of smart papers. Experimental section Materials As received single walled CNT s ( s emiconducting and metallic , diameter = 1.0 ~ 1.3 nm, length = 5 ~ 50 in aqueous solution (0.1 wt%) from KH Chemicals were further diluted with DI water to final concentrations of 0.01 wt%, 0.004 wt%, 0.002 wt%, 0.001 wt% and 0.0005 wt% . V ortex mixing and sonication were applied to the solutions to avoid CNT agglomeration, which could reduce the overall surface area and therefore impact the sensor sensitivity [12] . The solution 104 was drop - cast onto the active region between the electrodes ( Fig ure 4 . 1 ) and the sensors were allowed to dry overnight at room temperature. The electrodes were made of silver (Ag) conductive epoxy adhesive ( M.G. Chemicals ) and the distance between electrodes was 3.5 mm. Substrates were : glass slide, PI film and cellulose paper. Initi al resistance of the fabricated sensors with 0.0005 wt% CNT Fig ure 4 . 1 : S ensor structure and test set up. Methods T he CNT coated sensor substrate s were characterized by SEM on a Carl Zeiss Auriga FIB microscope and to pographies were measured by Cypher AFM. The silicon AFM probe has resonant frequency 300 kHz , force constant 40 N/m and operated in tapping mode. 105 The s ensors were exposed to a UV light source with 365 nm wavelength (CHANZON, 11V) mounted on a bench and the resistance s were collected every second by a real time monitoring system using a multimeter logging system (BTMETER BT - 90EPC), as shown on Fig ure 4 . 1 . The sensor response was calculated according to Equation 1 as th e ratio of resistance shift over the initial resistance : (1) where R f and R i are the resistance upon UV exposure and initial resistance, respectively [2] . T he sensor response with variable UV light power density was investigated from 0.5 to 2.5 mW/cm 2 by changing the distance to the sensor and the calibrat ion was done b y a commercial optical power meter (Optical Associates Inc. 306). Results and discussion CNT distribution on sensor substrate The mic rographs of CNT coated substrates are shown i n Fig ure 4 . 2 . The PI ( Fig ure 4 . 2 ( a ) ) and glass ( Fig ure 4 . 2 ( b ) ) illustrate that the nanoparticles uniformly covered the surfaces forming an interconnected network structure that provide s a continuous pathway for carrier mobility. For the cellulose p aper ( Fig ure 4 . 2 (c)), the distribution of CNT s is not as clear on the paper surface due to its hydrophilicity and surface roughness related to the cellulose fiber arrangement. During the drop - casting process CNT s may have experienced a capillarity force causing concentration into the regions between cellulose fibers [11] and therefore they may not be totally exposed at the surface. Additionally, some CNT network disruption can be noted on th e higher magnification macrograph on Fig ure 4 . 2 (c) that may reduce the carrier mobility. The adhesion of CNT on paper 106 is due to hydrogen bonding betwee n surface carboxyl and hydroxyl groups of the CNT w it h hydroxyl groups on cellulose [7,8] . Fig ure 4 . 2 : SEM m icrographs of CNT coated a) PI, b) glass and c) cellulose paper substrate at higher (top) and lower (bottom) magnification . The AFM observations support the SEM results. The topographies of CNT coated substrates obtained by AFM are shown on Fig ure 4 . 3 (5 µm x 5 µm area) and Figure 4 . 4 (1 µm x 1 µm area). The presence of CNT on the surface of PI is clearly shown . Other artifacts can also be noticed on the surface . The main difference observed is that overall the cellulose substrate has higher surface roughness ( inherent of the paper making process) than the PI or the glass as summarized on Table 4 . 1 . Due to capill arity forces and surface topographical differences between the macro cellulose fiber and the nanotubes, it will not form a uniform homogeneous distribution of the CNTs on the cellulose surface compared to the PI and glass samples resulting in lower sensor 107 sensitivity. Glass surface also exhibits artifacts on the surface and a roughness that introduces difficulties in the characterization of the CNT surface using the tapping mode of the AFM. Fig ure 4 . 3 : Height (top) and amplitude (bottom) profile of CNT coated a) PI, b) glass and c) cellulose paper substrate by AFM on a 5 µm x 5 µm area . Table 4 . 1 : Roughness of CNT coated substrates by AFM (nm) on a 5 µm x 5 µm area. 4.4 ± 1.7 46.0 ± 32.7 ± 31.4 108 Figure 4 . 4 : Height (top) and amplitude (bottom) profile of CNT coated a) PI, b) glass and c) cellulose paper substrate by AFM on a 1 µm x 1 µm area. Table 4 . 2 : Roughness of CNT coated substrates by AFM (nm) on a 1 µm x 1 µm area. 2.3 ± 0.9 20.7 ± 24.2 19.0 ± 11.7 Sensor response to UV light Fig ure 4 . 5 and Fig ure 4 . 6 show the response of the sensors calculated accor ding to Equation 1 for all substrates with 0.0005 wt% CNT when exposed to On/ Off cycles of UV ligh t with 2.5 mW/cm 2 . In Fig ure 4 . 5 the UV light was initially off for 1 min, then turned on for 2 min and finally turned off for 10 min in order to check the recovery. All the samples respond 109 immediately when the light is turn ed on with an increas e in their resistance. Under UV illumination, the O 2 molecules that normally adsorb on the sidewall of the CNT s, are desorbed from the surface of the nanotubes reducing the hole concentration [13] . Since the CNT has a p - type semiconducting behavior [12] a red uction of hole concentration will increase the resistance , and vice versa. In this cycle the sensors exhibited a maximum response of 38%, 26% and 20% for PI, glass and cellulose paper, respectively. Fig ure 4 . 5 : Response of sensors under UV illumination. As soon as the light is turned off the adsorption of O 2 molecules o nto the CNT surface begins , increasing the hole concentration and leading to a gradual baseline recovery. As note d in Fig ure 4 . 5 , the recovery is slower than the sensor response, since the O 2 adsorption is a natural passive process and the O 2 desorption is induced by UV light [13] . Another drawback is that sensors can also exhibit incomplete recovery to the baseline since other type s of molecule s in the 110 air may occupy the oxygen sites [13] . T o overcome this drawback of s low and incomplete recovery , in previous work Kim et al. [13] proposed a UV sensor encapsulated with a polymer membrane, capable of inhibiting gas exchange between the sensor and the external environment. T his strategy yield ed a sensor with fas t and full recovery to the baseline , due to the constant O 2 trapped in the sensor . Fig ure 4 . 6 show the response of the sensors for a sequence starting with 1 min without UV illumination, then 5 cycles of turning the UV light for 1 min followed by 3 min of UV Off. Fig ure 4 . 6 : Resistance change of sensors under UV On/Off cycling. As illustrated by Fig ure 4 . 6 , the PI film yields the sensor with the best response (71% after 21 min of On/Off cycle) , while g lass and c ellulose p aper substrate s have lower response at 48% and 39%, respectively. One reason for the high sensitivity on PI is due to the uniform distribution 111 of CNT s on the PI surface . Therefore, the large st surface are a of CNT s w ere exposed and available at the top of the substrate to UV detection mechanism . On the c ellu lose paper , during sensor preparation the evaporation of CNT aqueous solution asperities. This would have the effect of concentrating the CNTs in those areas, making a more non - uniform CNT network. CNTs in the regions between cellulose fibers may not be totally exposed and a vailable to participate in the mechanism of UV light detection, resulting in lower sensor sensitivity. Previous work [7] shows that a gas sensor with CNT s lying directly on paper surface has higher sensitivity than a sensor that was made by mixing and filtering CNT with dissolved paper since a fraction of the nanotubes will be inside the cellulose matrix, leading to a smaller active surface area of the CNT. The results show that all the CNT based sensors regardless of the substrate type are capable of effectively detecting UV light. It is worth mentioning that after the recovery occurred these senso rs were tested repeatedly without loss of function. Because of the better performance, the PI substrate was chosen for further analysis and a n optimum CNT concentration was investigated. A s shown in Fig ure 4 . 7 after 2 min of UV illumination , sensors with the lowest concentration of 0.0005 wt% CNT exhibit the best response of 38% while the sample with the highest concentration of 0.01 wt% CNT has the lowe st response of 0.4%. The sensor s with 0.00 1 wt% , 0.00 2 wt% and 0 .00 4 wt% CNT show a response of 26%, 14% and 9%, respectively. Since the photons are mostly absorbed at the exposed CNT surface, the samples that contain a greater concentration of CNTs will have a lower resistance change due to a n 112 the O 2 adsorption as well those on the surface. This investigation was repeated with the cellulose paper sensors and the same trend was observed. Therefore, the sensors with the lowest concentration of 0.0005 wt% CNT were chosen for further analysis. Fig ure 4 . 7 : Response under UV illumination as a function of CNT weight percent on PI substrate. Fig ure 4 . 8 shows t he response measured for PI sensors under different UV power density , with 0.0005 wt% CNT . For 0.5, 1.0, 1.5, 2.0 and 2.5 mW/cm 2 the maximum response after 21 min of UV On/Off cycle wa s 13%, 25%, 40%, 46% and 71%, respectively. As noted in previous work [2] , the response of CNT based UV sensor is proportional to the photon flux, so greater changes in the resistance occur with higher UV light intensity. Even the lowest density power tested on the sample was capable to quantify the effects of UV light, which implies that even in a day that the sun light is not intense the sensor is able to detect the UV radiation, showing the potential for applications requiring UV detection . 113 Fig ure 4 . 8 : Resistance change as a function of power density under On/Off cycle for PI substrate. Sensor mechanical robustness The advantage of the present sensors consists on their flexibility and it is essential that they maintain their functionality when deformed from their planar configuration by bending . Assessment of the sensor robustness was conducted by mechanically bending the sensors ( Fig ure 4 . 9 ) from flat to a 7.5 mm radius of curvature and meas uring the change i n their resistance. As summarized on Table 4 . 3 , a maximum change of 4.1% and 4.9% in res istance for the sensors on the PI and cellulose paper substrates , respectively, was observed at the minimum radius of 7.5 mm. After this observation, a conditioning step was carried out by repeatedly bending the sensor up to 1000 times to a radius of curvature of 10 mm to ensure that the sensors reach a stable value of resistance prior to testing their response under UV light. 114 Fig ure 4 . 9 : Mechanical bending test with varying radius of curvature. Table 4 . 3 : Resistance change (%) of sensors for varying radius of curvature. Radius (mm) PI Resistance Change Cellulose paper Resistance Change Flat 0.0 0.0 30 1.1 1.0 20 1.6 1.5 10 3.4 3.4 7.5 4.1 4.9 115 Fig ure 4 . 10 shows that the resistance change increased at the beginning of the test and then was stabilized at 1.5% and 1.2% change for PI and cellulose paper sensor, respectively. After this conditionin g step the sensors exhibited a stable internal resistance and were tested under UV light. Fig ure 4 . 10 : Bending cycle test at a curvature radius of 10 mm (conditioning step). Fig ure 4 . 11 shows the UV response of the conditioned sensors under the flat condition and bent with a radius of curvature of 10 mm. As can be clearly n oted for PI and cellulose paper sensors the response for both states are nearly the same. The results shown on Fig ure 4 . 11 indicate that the sensors c an operate without losing their functionality even when mechanically bent 1000 times, and emphasize the flexibility of the sensor and possible suitability for wearable applications. 116 Fig ure 4 . 11 : UV response of the conditioned a) PI and b) cellulose paper sensors under flat and 10 mm bending state. Sensor response under natural sunlight In order to develop functional sensors for general applications, it is essential to evaluate performance under outdoor UV light. T he present PI sensor was adapted for wearable applications and tailored for wristband and sunglasses use s, and then tested und er natural sunlight . Using a commercially available printer (Voltera V - one), conductive s ilver ink - based e lectrodes were printed o n the P I substrate ( Fig ure 4 . 12 ) , and subsequently, 0.0005 wt% CNT solution was drop - cast ed onto the active region between the electrodes. 117 Fig ure 4 . 12 : a) Design of electrode s and b) printing on PI substrate for c) potential application on sunglasses and wristband . The sensors were tested outdoors in Mountain View, California, with a sunlight power density measured at 2.2 mW/cm 2 . The sensors were covered with a filter that blo cked UV light for 1 min and then they were exposed to the sunlight for 1 min, during 5 cycles. The results are summarized in Fig ure 4 . 13 and show that the sensors immediately respond to the UV On/Off cycle and clearly reproduce the results obtained in the laboratory. Results under natural sunlight are consistent with laborator y measurements indicating that these sensors have the potential to be mass produced using this simple printing technique . 118 Fig ure 4 . 13 : Resistance change of UV sensor on wristband under sunlight. Conclusion s The results reported here show that CNTs are an excellent candidate for UV sensor development on flexible substrates. An optimum CNT concentration was identified, material characterization was performed and device fabrication, testing and evaluation were i nvestigated. All samples respond to UV On/Off cycles and after the recovery, the sensors were tested repeatedly without loss of function. The PI substrate yields the best response at 71% after 21 min of On/Off cycle exposure. Cellulose paper exhibited a re sponse of 39% but it is still effective and is an approach for the development of smart papers taking advantage of its low cost, lightweight, ecofriendly and disposable characteristic s . Additional research may also lead to improvement s i n recovery time. 119 B ending tests conducted under a varying radius of curvature showing that sensors can operate without losing functionality even when mechanically cycled 1000 times. Sensor s work effectively even when exposed to a low density power of 0.5 mW/cm 2 . Response to UV radiation was measured under natural sunlight , and results are consistent with laboratory measurements, which demonstrates the potential of the sensors for wearable applications such as wristband and sunglasses. This work identifies a s imple process to print electrodes on PI film (or cellulose paper) that can potentially be scaled - up for targeting high temperature applications for space mission exploration as well as for earth based exposure to UV light . Acknowledgements We thank the NA SA International Internship (NASA I 2 ) program , specially Porsche Parker, and the support of NASA Nanotechnology group specially Dongil Lee , Dong - Il Moon , Myeong - Lok Seol and Beomseok Kim . We want to acknowledge Michigan State University, e specially Per Ask eland for the SEM measurements at the Composite Materials and Structures Center, and Eric Goodwin and Reza Loloee for the AFM measurements at the Department of Physics and Astronomy . I also thank my sponsors Coordenação de Aperfeiçoamento de Pessoal de Nív el Superior (CAPES) through the Brazil Scientific Mobility Program [99999.013655/2013 - 02] and Agência Espacial Brasileira (AEB) . 120 REFERENCES 121 REFERENCES [1] L. Sang, M. Liao, M. Sumiya, A comprehensive review of semiconductor ultraviolet photodetectors: From thin film to one - dimensional nanostructures, Sensors (Switzerland). (2013). doi:10.3390/s130810482. [2] S .J. Kim, D. - I. Moon, M. - L. Seol, B. Kim, J. - W. Han, M. Meyyappan, Wearable UV Sensor Based on Carbon Nanotube - Coated Cotton Thread, ACS Appl. Mater. Interfaces. 10 (2018) 40198 40202. doi:10.1021/acsami.8b16153. [3] D. Gedamu, I. Paulowicz, S. Kaps, O. Lup an, S. Wille, G. Haidarschin, Y.K. Mishra, R. Adelung, Rapid fabrication technique for interpenetrated ZnO nanotetrapod networks for fast UV sensors, Adv. Mater. (2014). doi:10.1002/adma.201304363. [4] S. Bai, W. Wu, Y. Qin, N. Cui, D.J. Bayerl, X. Wang, H igh - performance integrated ZnO nanowire UV sensors on rigid and flexible substrates, Adv. Funct. Mater. (2011). doi:10.1002/adfm.201101319. [5] T. Takahashi, K. Takei, A.G. Gillies, R.S. Fearing, A. Javey, Carbon nanotube active - matrix backplanes for confo rmal electronics and sensors, Nano Lett. (2011). doi:10.1021/nl203117h. [6] S. Ammu, V. Dua, S.R. Agnihotra, S.P. Surwade, A. Phulgirkar, S. Patel, S.K. Manohar, Flexible, all - organic chemiresistor for detecting chemically aggressive vapors, J. Am. Chem. S oc. (2012). doi:10.1021/ja300420t. [7] J.W. Han, B. Kim, J. Li, M. Meyyappan, A carbon nanotube based ammonia sensor on cellulose paper, RSC Adv. (2014). doi:10.1039/c3ra46347h. [8] J.W. Han, B. Kim, J. Li, M. Meyyappan, A carbon nanotube based ammonia sen sor on cotton textile, Appl. Phys. Lett. (2013). doi:10.1063/1.4805025. [9] I. V. Zaporotskova, N.P. Boroznina, Y.N. Parkhomenko, L. V. Kozhitov, Carbon nanotubes: Sensor properties. A review, Mod. Electron. Mater. (2016). doi:10.1016/j.moem.2017.02.002. [ 10] D. Wen, Y. Liu, C. Yue, J. Li, W. Cai, H. Liu, X. Li, F. Bai, H. Zhang, L. Lin, A wireless smart UV accumulation patch based on conductive polymer and CNT composites, RSC Adv. (2017). doi:10.1039/c7ra10789g. [11] L.R. Shobin, S. Manivannan, Carbon nano tubes on paper: Flexible and disposable chemiresistors, Sensors Actuators, B Chem. (2015). doi:10.1016/j.snb.2015.06.030. [12] P. Teerapanich, M.T.Z. Myint, C.M. Joseph, G.L. Hornyak, J. Dutta, Development and improvement of carbon nanotube - based ammonia g as sensors using ink - jet printed interdigitated electrodes, IEEE Trans. Nanotechnol. (2013). 122 doi:10.1109/TNANO.2013.2242203. [13] S.J. Kim, J.W. Han, B. Kim, M. Meyyappan, Single Walled Carbon Nanotube Based Air Pocket Encapsulated Ultraviolet Sensor, ACS Sensors. (2017). doi:10.1021/acssensors.7b00585. [14] R.J. Chen, N.R. Franklin, J. Kong, J. Cao, T.W. Tombler, Y. Zhang, H. Dai, Molecular photodesorption from single - walled carbon nanotubes, Appl. Phys. Lett. (2001). doi:10.1063/1.1408274. 123 CHAPTER 5 : SURFACE MODIFICATION OF BAMBOO IN EPOXY COMPOSITES Abstract Bamboo fibers (BFs) have very good mechanical properties and are a candidate reinforcement for epoxy matrix composites. However, to achieve improved performance , good fiber - matrix interaction is required in BF reinforced composites. In this work, unidirect ional long BF reinforced epoxy composites at fiber volume content of 22%, 40% and 50% were made by compression molding. A sodium hydroxide (NaOH) treatment process was used to modify the surface of the BFs in the matrix. F lexural coupons for unidirectional BF/epoxy composites made with 40 v % NaOH modified BF show ed an improvement of 29% and 26% for flexural modulus and strength , respectively, compared to the untreated BF /epoxy composites. Additional improvement in mechanical properties was achieved after the NaOH modified BF was coated with graphene oxide (GO) . 40v% NaOH/GO modified BF composites exhibited a considerable increase of 43% and 29% for flexural modulus and strength, respectively, compared to the 40v% untreated BF composites . Surface modification of the BF after the NaOH and NaOH/GO was confirmed by scanning electron microscop y (SEM) and X - ray photoelectron spectroscopy (XPS). This BF surface modification approach with NaOH/GO has the potential to increase the use of sustaina ble plant fibers as alternatives to synthetic fibers. Keywords: Unidirectional composites; Bamboo fiber; Surface Treatment; Graphene Oxide ; Epoxy Composite; Fiber/matrix adhesion; 124 Introduction Bamboo fiber s compared to other natu ral fibers are an excellen t candidate which can be u sed as reinforcement s in polymer composites [1] because of the widespread availability of bamboo species globally, their rapid growth of up to 21 cm per day, their mechanical properties and durability and their abundant availability [2] . In general, 40% of the bamboo culm accounts for fibers, while 50% is parenchyma ( ligneous matrix ) and 10% is conducting tissue [3,4] . Fiber bundles (technical f ibers) can be extracted from the culm by a mechanical process, chemical process or a combination of both techniques. The fiber bundle is composed of elementary tubular fibers that have hexagonal and pentagonal cross sectional shapes, with a small hole in t he middle called lumen [2] , as shown in Figure 5 . 1 . Elementary fibers can have diameters from 10 to 40 µm and length from 1.0 to 4.3 mm, with lumen sizes from 2 to 20 µm [2] . Figure 5 . 1 : SEM of polished surface of cross section of bamboo fiber bundle embedded in epoxy . varies from 341 to 860 MPa depending on the species, growth conditions, etc. [2] . However, the 125 hy drophilic nature of the BFs may lead to weak bonding between the fibers and the relatively hydrophobic polymer matrices, which can negatively impact the mechanical properties of the composites [5] . In addition , the presence of hemicellulose and lignin , which are noncellulosic materials, can lower the crystallinity and affect the properties of natural fibers [6] . Therefore , many surface modification have been reported to improve the interfacial adhesion between the composite constituents, with alkali treatment being one of the simplest and most effective methods [7] . Alkali treatment using sodium hydroxide (NaOH) solutions is one of the most common methods for treating bamboo fiber bundles [8] and can enhance not only the interfacial adhesion between natural fibers and polymeric matrices but also the mechanical, physical and thermal properties of the fi bers [9,10] . This treatment promotes solubilization of hemicellulose and lignin [8,11] and separates the fibers into fibrils, which increases the available surface area of the fiber to be wet by the polymer matrix enhancin g the interfacial bonding [10] . The treatment is also capable of increas ing the chemical reactivity of the fiber by breaking hydrogen bonds and increasing the number of free hydroxyl groups [10,11] . Bonding in the composite interphase can also occur by mechanical anchoring since the treatment incre ases the roughness of the fiber surface [11] . Graphene is a two dimensional crystal with excellent mechanical, electrical, thermal and optical properties [12] . Graphene nanoplatelets are obtained by intercalation and exfoliation of natural graphite. After strong oxidation, single layers of graphene are reconst ituted as graphene oxide. The surface of graphite oxide possess es many oxygen - containing groups that can be exfoliated in water , yielding individual sheets [13] . Exfoliated sheets contain ing one layer of carbon atoms like graphene are named graphene oxide (GO) [12] . G O has a mean lateral dimension of approximately 1 µ m [14] , with hydroxyl and epoxide groups on the basal plane, and carbonyl 126 and carboxyl groups along the sheet edge [12,15] . GO can be easily dispersed in water due to its hydrophilic nature caused by the presence of the polar oxyge n functional groups [16] which also show good chemical reactivity . This assists in chemical functionaliz ation and dispersion in polymer matrices to produce composites [14,17] . The use of GO and GO - derived graphene materials as a filler in polymer composites have increased the elastic modulus, tensile strength, electrical conductivity, and thermal stability of composites [16] . Defect - strength of 130 GPa, along with large interfacial area and high aspect ratio, there fore a small amount of filler is required to give good improvements [16] (PVA) an increase of 62% in Young's modulus and 76% in tensile strength were achieved due to efficient load transfer between the GO and the matrix [18] . Many studies have been reported on the coating of synthetic fibers with GO to increase the mechanical and interfacial properties [6,19] . GO sheets were added in a sizing for carbon fiber reinforced epoxy composites and improvements on the tensile properties were obtained as well as an increase of 70.8% in the in terfacial shear strength (IFSS ) [6,20] . The covalent grafting of GO sheets onto glass fibers enhance d the s trength and toughness of the composite interfacial region [21] . GO was also added in glass fabric/epoxy composites leading to an increase of 32.7% o f the interlaminar shear strength [22] . In regards to natural fibers , a lkali treated jute fibers (HA) were coated with varying concentration of GO (from 0.25 to 1 wt.%) [6] leading to an increase of their and tensile strength compared to untreated jute fibers (UT) , as shown on Figure 5 . 2 , as well as an increase in the IFSS in epoxy matrix. 127 Figure 5 . 2 : (a) , (b) tensile strength and (c) IFSS of untreated, GO - treated, and Graphene flake - coated jute fibers [6] . In this study, long BFs were surface treated with NaOH to improve their mechanical and interfacial properties for the preparation of unidirectional BF reinforced epoxy composites. To further improve the performance of the composites, alkali treated BF were coated with GO by a simple dipping technique. The effects of modifying the BF surface with NaOH and GO on the morphological, flexural and interfacial properties were investigated. The flexural modulus of the composites reinforced at 40v% BF has increased 29% and 43% after modifying the BF with NaOH and NaOH/GO, respectively. The flexural strength increased 26% and 29% after treating the BF with NaOH and NaOH/GO, respectively. 128 Experimental Materials Lo ng bamboo fibers ( 15 25.4 cm ) were obtained by a combination of a chemical and mechanical extraction technique and were kindly provided by Sunstr and C ompany . T heir properties provided by the supplier are listed on Table 5 . 1 . As received BFs were rinsed multiple time with deionized water (DI) and dried at 60 °C for 3.5 h prior utilization. Table 5 . 1 : Bamboo Fiber Material Specifications . Property Value Density 3 Mean Fiber Diameter ø = 256 µm Mean Ultimate Tension Strength UTS = 451 MPa Mean Modulus of Elasticity E T = 31 GPa Specific Strength S = 322 MPa/(g/cm 3 ) Specific Modulus E T = 22 GPa/(g/cm 3 ) NaOH was purchased from Avantor ( Macron Fine Chemicals ) and was chosen to treat the surface of BFs . GO in water was kindly provided by the University of Tennessee with a concentration of 2.8 mg/mL. The GO flakes possess lateral dimensions of appr oximately 1 µm and average thickness of approximately 1 nm that corresponds to 2 3 GO layers, and detailed information is provided elsewhere [23] . The epoxy matrix used was EPON TM resin 828 (viscosity 129 of 110 150 P at 25°C) supplied by Hexion Inc. and the curing agent was m - Phenylenediamine (mPDA) supplied by ACROS Organics. Methods Surface treatment of BFs with NaOH BFs were soaked in 5 wt.% NaOH solution at room temperature for 5 h. The mass ratio of water and BFs was 30:1. The treated fibers were r inse d with DI water several time until reaching a neutral pH . The treated BFs were dried at room temperature for 20 h , followed by oven drying at 60 °C for 3.5 h . These fibers are identified NaOH modified BFs. Surface treatment of NaOH modified BFs with GO The NaOH modified BFs were immersed in the GO solution for approximately 34 min. The fibers were dried in vertical position in an oven at 80 °C for 35 min followed by drying at 60 °C for 3 h. These fibers are named NaOH/GO modifi ed BFs. Processing of unidirectional composites Unidirectional composites were prepared by aligning BFs on the cavity of a mold and processing by compression mold (Carver Laboratory Press) as shown on Figure 5 . 3 . Before making the composite , BFs were dried to remove moisture. The epoxy and curing agent were heated at 75 °C , then mixed at a concentration of 14.5 phr mPDA and degassed in a vacuum oven. Th e mold containing the long BFs was also pre heated and degassed to remove trapped air in the mold. Then the resin system was carefully added into the mold to cover the BFs. The mold was closed, and the assembly was sealed with vacuum, which caused the exce ss resin to flow out of 130 the mold. The closed mold was transferred to the compression press for curing. Pressure was increased and the displacement and final thickness of the composite specimen was controlled by use of a rigid spacer . The composite plate wa s cured at 75°C for 2 h and postcured at 125°C for 2 h. The v acuum was vented after the first 5 min of compression. Cured composite plates were cut into coupons in the longitudinal direction of the fibers for flexural tests and conditioned in the laborator y atmosphere. Figure 5 . 3 : Processing of BF reinforced epoxy composite. Composites were made with non - treated BF at fiber volume content of 22%, 40% and 50% . Based on the mechanical results, a fiber content of 40 vol.% was selected for the composites made of treated BFs . All the composites composition and the corresponding fiber surface treatment are shown on Table 5 . 2 . Void content was below 6% and its effect among the composites seems negligible since they have similar void content. 131 Table 5 . 2 : Composites comp osition and fiber surface treatment. Composite name Fiber volume content (%) Fiber treatment 22 v% BF 22 None 40 v% BF 40 None 50 v% BF 50 None 40 v% NaOH modified BF 40 NaOH 40 v% NaOH/GO modified BF 40 NaOH and GO Characterization methods Scanning Electron Microscopy (SEM) BFs , NaOH modified BFs and NaOH/GO modified BFs were examined with a Carl Zeiss Auriga FIB scanning electron microscope at an accelerating voltage of 5 keV . Fibers were mounted on the SEM sample holder on top of carbon tape . T he fracture surface of flexural coupons was also observed. All samples were sputter - coated with tungsten to prevent surface charging. X - ray photoelectron spectroscopy (XPS) BFs , NaOH modified BFs and NaOH/GO modified BFs were characterized b y XPS using a Physical Electronics 5400 ESCA. Survey spectra were collected at 187.85 eV pass energy and higher resolution spectra were collected with 29.35 eV pass energy. Prior to XPS investigation, NaOH/GO modified BFs were rinsed multiple time with DI water to remove physically bonded GO. 132 Flexural test Flexural tests were conducted in an United Testing Systems SFM - 20 load frame in general compliance to ASTM D790 - 10. The flexural tests used a 100 lbf or 1000 lbf load cell, a support span - to - depth ratio o f 16:1 and the rate of crosshead motion was calculated per ASTM D790 - 10. The flexural samples had dimensions of approx imately 77 x 12.7 x 2.3 mm. Tests were terminated at 5% strain (or before it if break occurred), and t he flexural strength, flexural modul us and strain were calculated. SBSS test The composites were characterized by testing modified short beam shear specimens in a United Testing Systems SFM - 20 load frame . A 100 0 lbf load cell was used and the s pan - to - measured thickness ratio was set to 4:1. The specimen length was 6 times the thickness and the specimen width was 2 times the thickness as recommended by ASTM D2344/D2344M . Although samples are not multi - layer unidirectional specimens, they consist of unidirectional composites specimen test for SBSS and it is expected that they will experience same stress field. Results and discussion Characterization of surface treated BF s Figure 5 . 4 displays SEM images of the surface morphologies of non - treated BFs, NaOH modified BFs and NaOH/GO modified BFs. Over 90% of the bamboo mass i s cellulose, hemicellulose and lignin, with lesser concentrations of soluble polysaccharides, waxes, ashes and others [3,5] . As noted by the red arrows i n Figure 5 . 4 (a ) , the surface of non - treated BFs has some 133 features, such as the presence of soft cells and soft parenchyma cells [3] , which make the measurement of the fiber diameter more difficult [11] . As shown on Figure 5 . 4 (b), after the alkali treatment with NaOH solution, the surface materials (consisting of hemicelluloses, lignin, pectin , wax, and other impurities [6] ) were part ial ly removed making it easier to identify the single BFs. The surface o f the BF became rougher due to the removal of the surface materials. This roughness may be beneficial to promote mechanical interlocking between the BF and the epoxy matrix and improve the adhesion [7,10] . It has been reported that treatment with excessive concentration of NaOH can remove cellulose and degrade the integrity of the fibers, resulting in decrease of the fiber strength [7] . Therefore, the treatment with 5 wt.% NaOH solution used in this work was effective in cleaning the surface without degrading the fiber mechanical properties. The fibers that were coated with GO exhibited a similar surface morphology. The GO can be identified on the BF surface by the yellow arrows at the higher magnification micrograph in Figure 5 . 4 (c). 134 Figure 5 . 4 : SEM images of the surface of ( a) BFs, ( b) NaOH modified BFs and ( c) NaOH/GO modified BFs , top is l ower magnification (377X) and bottom is higher magnification (9000X) . The surface chemical composition of BFs and the treated BFs was determined by XPS. The wide - scan survey spectra with elemental assignments are shown on Figure 5 . 5 . 135 Figure 5 . 5 : XPS survey of (a) BF, (b) NaOH modified BF and (c) NaOH/GO modified BF. 136 All the spectra exhibited the main BF surface constituent peaks assigned to carbon (C1s) and oxygen (O1s) and also detected a sodium peak ( Na1s) , due to the alkali treatment. Even the non - treated BFs have a small amount of Na (0.9%) resulting from the NaOH to extract the fibers from the bamboo culm at the supplier . The elemental atomic composition and the carbon to oxygen ratio ( C/O ) of the fibers are summarized on Table 5 . 3 . BFs had a carbon content of 65.5% that increased to 69.7 % and 72.5% a fter treatment with NaOH and NaOH/GO, respectively. An increase on carbon content from 41.98% ± 0.13% to 46.7% ± 0.11% was observed on jute fiber after treating the fibers for 24 h with 0.5% NaOH [10] . As a result of the coating of the NaOH modified BFs with GO, the C/O ratio increases from 2.4 to 2.9 due to the presence of the GO (GO C/O ratio is approximately 3:1). Even after rinsing the NaOH/GO modified BFs multiple time with DI wa ter , the change in the C/O ratio was detected , which indicates that GO reacted chemically with the BFs . Table 5 . 3 : Percent e lement composition for BF , and . 137 The deconvolution of C1s peak for BF, NaOH modified BF and NaOH/GO modified BF are given in Fig ure 5 . 6 . Table 5 . 4 summarizes the relative amount of C1s components, which includes C1: C - C, C2: C - O and C3: O - C - O , C=O . The C1s spectrum of BFs and NaOH modified BFs are relative similar, while the s urface modification with GO greatly changed the relative amount of C1s component s. The C1 relative amount increased from 44.9% for BFs and 45.1% for NaOH modified BFs up to 68.7% for the NaOH /GO modified BFs. The increase in C1 can be assigned to the SP3 carbon of the GO . The intensity of C2 and C3 peak s decreased after coating the fibers with GO. Fig ure 5 . 6 : Deconvolution of C1s peak for (a) BF, (b) NaOH modified BF and (c) NaOH/GO modified BF . Table 5 . 4 : Relati ve amount of C1s components (%) for BF, NaOH modified BF and NaOH/GO modified BF . Sample C1 C2 C3 BF 44.9 45.6 9.5 NaOH modified BF 45.1 41.3 9.7 NaOH/GO modified BF 68.7 26.8 4.5 138 Mechanical properties of BF reinforced epoxy composites at 22 v%, 40 v% and 50 v% BFs Figure 5 . 7 shows the f lexura l properties along the fiber longitudinal direction for composites reinforced with un treated BFs at 22 v%, 40 v% and 50 v% compared to neat epoxy. The flexural modulus ( Figure 5 . 7 (a) ) and flexural strength ( Figure 5 . 7 (b) ) of all composites are superior compared to the neat epoxy, and as expected these properties increase with an increase in fiber content. A similar increase was noted by Takagi and Ichihara [24] . They combined short BFs int o a starch - based resin and the flexural and tensile strength of the composite s increase with i ncreas ing fiber content from 10 to 50 wt %. No significant difference for flexural strain ( Figure 5 . 7 (c)) was observed among the composites. 139 Figure 5 . 7 : ( a) Flexural modulus, ( b) flexural strength and ( c) flexural strain along the fiber longitudinal direction for neat epoxy and epoxy composites reinforced at 22 v%, 40 v% and 50 v% BF . 140 Composites with fiber volume greater than 50% w ere not made due to the possi bility of excessive void formation during processing that could lower the mechanical properties, as observed by Thwe and Liao [25] who observed that the compos ite tensile strength can decrease at high BF content in polypropylene due to void s and micro crack formation under loading . Epoxy matrix composite reinforced with BFs at 40v% were chosen for further investigation as this fiber content gave a considerable increase of 439% and 107% for flexural modulus and strength, respectively, compared to neat epoxy, with the lowest standard deviation among the composites. Mechanical properties of NaOH and NaOH/GO modified BF reinforced epoxy composites at 40 v% BFs Figure 5 . 8 shows the flexural properties of the composites reinforced with 40v% BF with different surface treatments. By treating the BFs with a s olution of 5wt% NaOH, an increase of 29% and 26% i n the composite flexural modulus and strength, respectively, was achieved compared to BFs without treatment. As show on the SEM images on Figure 5 . 4 (b), after the alkali treatment the fiber surfaces are relative clean of impurities, with no apparent damage and greater chemical reactivity due to the free hydroxyl groups. These advantages and greater surface roughness can all contribute to the interfacial bonding and mechanical interlocking into the epoxy matrix [5] . Moreover, the exposure of the fibers and greater surface area available after the alkali treatment also facilitate the impregnation of the fibers by the epoxy enhancing the interfacial bonding. The increase in the composite mechanical properties for the NaOH modified BFs can also be attributed to an increase on the mechanical properties of the BF itself due to the alkali treatment. 141 Sarker et al. [6] observed that jute fibers after alkali treatment can easily rearrange themselves along the direction of a tensile force due to the removal of the non - cellulosic components that act as a constraint in the interfibrillar region of the fibers. As a result, when stretched the fibrils can better sustain and share the load between themselves and contribute to higher strength development [10] . This same behavior was observed by Zhang et. al [7] that showed that 2 wt.% NaOH treatment on BF had minimal effect on the tensile properties of BFs since the large amount of surface substances were prevent ing the fibrils from rearranging themselves along the direction of the tensile force during te st. However, when they used h igher concentration of NaOH of 6 wt.%, of the non - cellulosic components and increase of the cellulose crystallinity. A slight re duction in the flexural strain Figure 5 . 8 (c) was observed for the alkali treated BFs reinforced composites. A reduction of extension of jute fibers during tensile testing was observed in other work [6] and it was possibly due to the removal of lignin from the interce llular region of jute fibers. The composites made of fibers that were coated with GO exhibited the highest improvement on the mechanical properties at 43% and 29% for the flexural modulus and strength, respectively, compared to composites made of untreated BFs. The greater performance of NaOH/GO modified BF reinforced composites is related to the increased chemical bonding between the functional groups of GO and the alkali treated BFs, as was observed by SEM on Figure 5 . 4 (c) and the XPS results. The BFs coated with GO possess oxygen functional groups such as have the potential to chemically bond with the groups of the epoxy resin [6] opening polymerization may also occur with the amine curing agent (m PDA ) [6] . In addition, since 142 GO possesses high modulus, its presence will stiffen the BFs [6] and consequently enhance the properties of the composites. Overall, alkali treatment is a simple process to remove non - cellulos ic components and impurities from the BF surface and increase the properties of t he fiber for effective reinforcement of polymer composites. The alkali treatment also reduces the hydrophobicity of the fibers which may contribute to better interphase bonding [10] . The coating of NaOH treated BFs with GO is an alternative to further increase the mechanical properties , and the resulting reinforced epoxy composites showed considerable improvement in flexural modulus and strength . 143 Figure 5 . 8 : ( a) Flexural modulus, ( b) flexural strength and ( c) flexural strain along the fiber longitudinal direction for epoxy composites reinforced at 40 v% BF , 40 v% NaOH modified BF and 40 v% NaOH/GO modified BF. 144 SEM observation of composites fracture surface The fracture surfaces of the composites after flexural test ing were analyzed by SEM revealing the presence of fiber pull out, some fiber breakage, brittle regions and relative good BF distribution as shown on Figure 5 . 9 . Figure 5 . 9 : Flexural fracture surface for epoxy composites reinforced at (a) 40 v% BF , (b) 40 v% NaOH modified BF and (c) 40 v% NaOH/GO modified BF , top is l ower magnification (177X) and bottom is higher magnification (2500X) . Figure 5 . 9 (a) reveals that composites reinforced with n on - treated BFs exhibit little epoxy matrix adhering to the fiber surface, which is an indication of poor interfacial adhesion between the untreated fiber and matrix. Hydrophilic BF materials and impurities on the fiber surface does 145 not facilitate the bondi ng. Gaps, interfacial debonding and p arenchyma cells can be clearly observed on Figure 5 . 10 . Their presence helps explain the lowest mechanical properties for the composites reinforced with untreated fibers. Figure 5 . 10 : Flexural fracture surface for epoxy composites reinforced at 40 v% BF . Figure 5 . 9 (b) shows composites reinforced at 40v% NaOH modified BF where considerable amount of epoxy resin can be observed covering the fiber surface. A lkali treated fibers have increased roughness for mechanical in terlocking of the matrix as well cleaner surface and oxygen functional groups available for chemical reactions with the epoxy (as discussed in 146 section 5.4.3 ) and for improved interfacial bonding [7] . Figure 5 . 11 shows that the alkali treatment favors the exposure of fibrils facilitating the impregnation of the fibers with the epoxy matrix [11] that is correlated to the improvement in the composite mechanical properties compared to the untreated fiber reinforced composites. Figure 5 . 11 : Flexural fracture surface for epoxy composites reinforced at 40 v% NaOH modified BF . Although it is difficult to observe GO on the fracture surface of the 40v% NaOH/GO modified BF composite as shown on Figure 5 . 9 (c), a considerable amount of epoxy resin is noted on the fiber surface, which is an indication of good bonding between the fiber and the matrix. Some GO peeling can be noted on Figure 5 . 12 and Wang et. al [19] also noted peeling and tearing of GO grafted on surface of carbon fiber reinforced epoxy composite during Mode I opening mode test that was bene ficial for crack propagation resistance. By coating carbon fiber with GO and polydopamine (PDA), they observed that the crack propagated through the GO and PDA layers with tearing and peeling of GO sheets from fibers, which improved the load transfer effec tiveness and absorbed more energy. 147 Figure 5 . 12 : Flexural fracture surface for epoxy composites reinforced at 40 v% NaOH / GO modified BF. Overall, the composite fracture surface observations are in agreement with the mechanical properties discussed on section 5.4.3 . Evaluation of the short beam strength The short beam test is an efficient way to determine the shear strength on composites, which dictates the efficiency of the stress transfer from the matrix to the fibers and is correlated to the level of interfacial bonding. Figure 5 . 13 shows the short beam strength results for the composites made on this study. The composites processed with non - treated BFs possess the lowest value of strength at 34.5 MPA due to the non - cellulosic components present on the fiber surfac e as discussed on the previous sections, that prevents a good interfacial bonding into the epoxy matrix. By treating the BFs with NaOH, the composites exhibited an increase of 14% compared to the composites with non - treated BFs. This increase is possibly a ttributed to the mechanical interlocking of the rougher fibers into the matrix. 148 Compared to 40v% BF reinforced composites, the 40v% modified NaOH/GO BF reinforced composites showed a slight increase of 6%. This may be due the low concentration of GO in wat er as low as 2.8 mg/mL, that was not sufficient to provide greater improvements. Sarker et. al [6] measured the I FSS at a jute fiber/epoxy matrix interface by single - fiber microbond pull - out test and showed that IFSS increases with the increase of GO concentration. Figure 5 . 13 : Short beam strength for epoxy composites reinforced at 40 v% BF , 40 v% NaOH modified BF and 40 v% NaOH/GO modified BF. The increase on the short beam shear strength supports the conclusion that the treatment of the BFs with NaOH and GO enhances the interfa cial properties, however , greater concentrations of GO could be more effective. Conclusion s This work presents encouraging results for the treatment of BFs and their application as reinforcement for epoxy composites. BFs were treated with NaOH solution an d then coated with GO. The mechanical properties of the corresponding composites reinforced with fiber volume 149 content of 40% were greatly improved. By treating the BFs with a solution of 5wt% NaOH, an increase of 29% and 26% on the composite flexural modul us and strength, respectively, was achieved compared to composites processed with BFs without the treatment. The composites made of fibers that were coated with GO exhibited the highest improvement on the mechanical properties at 43% and 29% for the flexur al modulus and strength, respectively. The alkali treatmen t removes hemicelluloses, lignin, pectin, wax, and other impurities from the fiber surface, opening up the fiber bundles to facilitate the impregnation with the polymeric resin , increasing the chemical reactivity with the free hydroxyl groups available to es tablish covalent bonding as well as increasing the surface roughness for better mechanical interlocking with the matrix . Consequently, better mechanical properties of the composite were achieved. GO further increased the composite performance due to the st iffening of the BFs and establishment of chemical bonding upon the oxygen functional groups on GO surface that chemically bond with the fibers and the groups of the epoxy resin . Fiber surface treatment was confirmed by XPS, and the SEM observations support the mechanical properties findings. The BF n atural fiber reinforced epoxy composites developed i n this investigation offer a useful approach t o reduce the utilization of synthetic fibers, produce a low density structural composite, made from a renewable r esource, bamboo fibers. These BF composite materials have potential to be used in automotive industry ( d oor panels , s eat cushions, b ackrests , trim parts in dashboards) and household sectors ( f urniture and low cost housing materials) [1] . 150 REFERENCES 151 REFERENCES [1] H. Banga, V.K. Singh, S.K. Choudhary, Fabrication and Study of Mechanical Properties of Bamboo Fibre Reinforced Bio - Composites, Innov. Syst. Des. Eng. 6 (2015) 84 98. http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.823.1104&rep=rep1&type=pdf. [2] D.E. Depuydt, N. Sweygers, L. Appels, J. Ivens, A.W. van Vuure, Bamboo fibres sourced from three global locations: A microstructural, mechanical and chem ical composition study, J. Reinf. Plast. Compos. (2019) 1 16. doi:10.1177/0731684419828532. [3] X. Wang, H. Ren, B. Zhang, B. Fei, I. Burgert, Cell wall structure and formation of maturing fibres of moso bamboo (Phyllostachys pubescens) increase buckling r esistance, J. R. Soc. Interface. 9 (2012) 988 996. doi:10.1098/rsif.2011.0462. [4] L. Nayak, S.P. Mishra, Prospect of bamboo as a renewable textile fiber, historical overview, labeling, controversies and regulation, Fash. Text. 3 (2016) 2. doi:10.1186/s406 91 - 015 - 0054 - 5. [5] H.P.S. Abdul Khalil, I.U.H. Bhat, M. Jawaid, A. Zaidon, D. Hermawan, Y.S. Hadi, Bamboo fibre reinforced biocomposites: A review, Mater. Des. 42 (2012) 353 368. doi:10.1016/j.matdes.2012.06.015. [6] F. Sarker, N. Karim, S. Afroj, V. Konch erry, K.S. Novoselov, P. Potluri, High - Performance Graphene - Based Natural Fiber Composites, ACS Appl. Mater. Interfaces. 10 (2018) 34502 34512. doi:10.1021/acsami.8b13018. [7] K. Zhang, F. Wang, W. Liang, Z. Wang, Z. Duan, B. Yang, Thermal and Mechanical Properties of Bamboo Fiber Reinforced Epoxy Composites, Polymers (Basel). 10 (2018) 608. doi:10.3390/polym10060608. [8] K. V, C. DP, K. S, K. K, Study on the Performance of Bamboo Fibre Modified with Different Concentrations of Sodium Hydroxide and Chlorine Containing Agents, J. Text. Sci. Eng. 08 (2018). doi:10.4172/2165 - 8064.1000362. [9] M.I. Ibrahim, M.Z. Hassan, R. Dolah, M.Z.M. Yusoff, M.S. Salit, Tensile behaviour for mercerization of single kenaf fiber, Malaysian J. Fundam. Appl. Sci. 14 ( 2018) 437 439. doi:10.11113/mjfas.v14n4.1099. [10] A. Roy, S. Chakraborty, S.P. Kundu, R.K. Basak, S. Basu Majumder, B. Adhikari, Improvement in mechanical properties of jute fibres through mild alkali treatment as demonstrated by utilisation of the Weibul l distribution model, Bioresour. Technol. 107 (2012) 222 228. doi:10.1016/j.biortech.2011.11.073. [11] M.M.E. Costa, S.L.S. Melo, J.V.M. Santos, E.A. Araújo, G.P. Cunha, E.P. Deus, N. Schmitt, Influence of physical and chemical treatments on the mechanical properties of bamboo fibers, Procedia Eng. 200 (2017) 457 464. doi:10.1016/j.proeng.2017.07.064. 152 [12] S. Pei, H. - M. Cheng, The reduction of graphene oxide, Carbon N. Y. 50 (2012) 3210 3228. doi:10.1016/j.carbon.2011.11.010. [13] J.I. Paredes, S. Villar - Ro dil, A. Mart nez - Alonso, J.M.D. Tasc n, Graphene Oxide Dispersions in Organic Solvents, Langmuir. 24 (2008) 10560 10564. doi:10.1021/la801744a. [14] D.A. Dikin, S. Stankovich, E.J. Zimney, R.D. Piner, G.H.B. Dommett, G. Evmenenko, S.T. Nguyen, R.S. Ruoff , Preparation and characterization of graphene oxide paper, Nature. 448 (2007) 457 460. doi:10.1038/nature06016. [15] S. Stankovich, D.A. Dikin, R.D. Piner, K.A. Kohlhaas, A. Kleinhammes, Y. Jia, Y. Wu, S.T. Nguyen, R.S. Ruoff, Synthesis of graphene - based nanosheets via chemical reduction of exfoliated graphite oxide, Carbon N. Y. 45 (2007) 1558 1565. doi:10.1016/j.carbon.2007.02.034. [16] Y. Zhu, S. Murali, W. Cai, X. Li, J.W. Suk, J.R. Potts, R.S. Ruoff, Graphene and Graphene Oxide: Synthesis, Properties, and Applications, Adv. Mater. 22 (2010) 3906 3924. doi:10.1002/adma.201001068. [17] D.R. Dreyer, S. Park, C.W. Bielawski, R.S. Ruoff, The chemistry of graphene oxide, Chem. Soc. Rev. 39 (2010) 228 240. doi:10.1039/B917103G. [18] J. Liang, Y. Huang, L. Zha ng, Y. Wang, Y. Ma, T. Guo, Y. Chen, Molecular - Level Dispersion of Graphene into Poly(vinyl alcohol) and Effective Reinforcement of their Nanocomposites, Adv. Funct. Mater. 19 (2009) 2297 2302. doi:10.1002/adfm.200801776. [19] P. Wang, J. Yang, W. Liu, X. - Z. Tang, K. Zhao, X. Lu, S. Xu, Tunable crack propagation behavior in carbon fiber reinforced plastic laminates with polydopamine and graphene oxide treated fibers, Mater. Des. 113 (2017) 68 75. doi:10.1016/j.matdes.2016.10.013. [20] X. Zhang, X. Fan, C. Y an, H. Li, Y. Zhu, X. Li, L. Yu, Interfacial Microstructure and Properties of Carbon Fiber Composites Modified with Graphene Oxide, ACS Appl. Mater. Interfaces. 4 (2012) 1543 1552. doi:10.1021/am201757v. [21] J. Chen, D. Zhao, X. Jin, C. Wang, D. Wang, H. Ge, Modifying glass fibers with graphene oxide: Towards high - performance polymer composites, Compos. Sci. Technol. 97 (2014) 41 45. doi:10.1016/j.compscitech.2014.03.023. [22] X. - J. Shen, L. - X. Meng, Z. - Y. Yan, C. - J. Sun, Y. - H. Ji, H. - M. Xiao, S. - Y. Fu, Im proved cryogenic interlaminar shear strength of glass fabric/epoxy composites by graphene oxide, Compos. Part B Eng. 73 (2015) 126 131. doi:10.1016/j.compositesb.2014.12.023. [23] S. Hu, E.L. Ribeiro, S.A. Davari, M. Tian, D. Mukherjee, B. Khomami, Hybrid nanocomposites of nanostructured Co 3 O 4 interfaced with reduced/nitrogen - doped graphene oxides for selective improvements in electrocatalytic and/or supercapacitive properties, RSC Adv. 7 (2017) 33166 33176. doi:10.1039/C7RA05494G. 153 [24] H. TAKAGI, Y. ICH Composites Using a Starch - Based Resin and Short Bamboo Fibers, JSME Int. J. Ser. A. 47 (2004) 551 555. doi:10.1299/jsmea.47.551. [25] M.M. Thwe, K. Liao, Effects of environmental aging on th e mechanical properties of bamboo glass fiber reinforced polymer matrix hybrid composites, Compos. Part A Appl. Sci. Manuf. 33 (2002) 43 52. doi:10.1016/S1359 - 835X(01)00071 - 9. 154 CHAPTER 6 : SUMMARY AND FUTURE WORK Summary The research presented in this dissertation investigated cellulose fibers in its various forms from the nano - level to the macro - level as a potential ecofriendly alternative to replace synthetic fiber s in reinforced composites . Cellulose fiber was investiga ted at the macro scale as the main reinforcement in a polymer matrix composite and at the nanoscale (CNCs) as a reinforcement at the polymer - composite interphase. Cellulose fibers by themselves made into a cellulose paper were investigated as a substrate f or flexible electronics. This study showed that cellulose in all of its forms, can effectively reinforce different polymer matrices such as thermosetting (epoxy) and thermoplastic (PP) targeting different applications as well as being useful for flexible s ensors. Chapter 2 investigated how functionalized CNCs can be used as a sizing agent to increase the interfacial adhesion between CF and epoxy in composites. An increase in the IFSS was achieved for CFs sized with APTES - CNCs as well as an improvement in th e failure mode as detected by the birefri ngen t stress pattern seen during the SFFT . This supports the assumption that adding the APTES - CNCs at the composite interphase greatly enhance s the stress transfer from the matrix to the fiber . CFs sized with APTES - CNCs at a concentration of 1.0 wt% resulted in the highest increase on IFSS of 77% and 81% for 12k tow sized CF and individually sized CF, respectively, compared to unsized CFs. This beneficial enhancement on IFSS was achieved by the establishment of covalent bonding coupled with an increase of the modulus at the composite interphase. Mechanical, thermal and morphological properties of hybrid composites o f cellulose - inorganic fibers targeting automotive applications were investigated and reported in Chapter 3. In 155 general, inorganic reinforcing fibers led to the greatest improvements in mechanical properties. However, hybridizing an optimum concentration of cellulose fiber with inorganic reinforcement in the composite showed a positive effect and a n opportunity to reduce or even replace a portion of the inorganic fibers in some automotive applications. H ybrid composites with 15 wt% SGF and 15 wt% Cellulose B exhibited a 58% and 171% increase in tensile stress and modulus, respectively, compar ed to neat PP Z , producing a composite material suitable for body interior (console substrate, wiring harness) and under - the - hood (battery and power distribution box cove rs) applications. Hybrid composites are more thermally stable than the neat PP , which may offer an opportunity for hybrid composites made of cellulose to be considered for challenging under - the - hood applications. The T c of these composites also increased c ompared to the neat PP , due to the fibers acting as nucleating agents, which can increase the rate of part production. Chapter 4 investigated CNT based UV sensors that were fabricated on flexible cellulose paper substrates. Sensors fabricated on cellulose paper respond immediately to UV On/Off cycle, and can also operate without losing functionality after being mechanically bent. Results show that cellulose has the potential to be part of electronic devices and p rovide advantages in terms of low cost, light weight, ecofriendly and disposable characteristic s . Unidirectional long b amboo fiber reinforced epoxy composites were fabricated at a 40% fiber volume content and their mechanical properties were investigated in Chapter 5 . The results show that the treatment of BFs with a solution of NaOH greatly improves the composite flexural modulus and flexural strength by 29% and 26%, respectively , compared to composites processed with non - treated BFs . Greater improvement of 43% and 29% for the composite flexural modulus and flexural strength , respectively, is achieved by adding GO at the BFs surface. The fracture surface observation of the composite s by SEM is in agreement with the mechanical properties 156 showing that treated BFs ha ve more resin attached to their surface meaning better impregnation of the epoxy and better interfacial bonding, while non - treated BFs are relative clean of resin which is a n indication of lower adhesion . Therefore, the treatments developed on this work were beneficial for the development of natural fiber reinforced epoxy composites. Future work Sizing CF with APTES - CNC The sizing of CF with APTES - CNC was shown to produce benefical composite interfacial properties. One area of concern is that the increase in the interphase modulus by itself can lower the strain to failure, which would tend to embrittle the material. Howev er, the insertion of nanoparticles at the interphase is a balance to increase the composite modulus and to provide crack deflection which can improve the toughness. Future work should be undertaken to investigate t hese two effects simultaneously . I mprov ing the uniformity of the sizing, investigating the optimum content of the APTES - CNC on surface of coated CFs and relating the IFSS improvement to other composite mechanical properties would be a significant contribution to the composite field. Adding compati bilizer to hybrid composites SEM observation of the fracture surface of hybrid composites showed good fiber distribution, however due to the inherent lack of compatibility between cellulosic fibers and polymeric matrices such as PP, the use of a compatibil izer is recommended as future work to improve the adhesion of the fiber to the PP matrix . This can lead to even greater mechanical and thermal performance of the hybrid composites. Thus, a method to functionalize the cellulose fiber by adding chemical grou ps to its surface or adding a compatibilizer during the processing, such as 157 maleic anhydride , should be developed. With such a development, the potential for facilitating chemical bonding between the cellulosic fiber and the matrix could lead to improved m echanical properties of the composite. Improving recovery time of UV sensors Although the sensors developed in this study proved to be effective for detecting UV light, there still may be opportunities for improvement i n the recovery time of the sensors. A polymer membrane to encapsulate the sensor and inhibit gas exchange with the external environment, could be added to the sensor configuration. An investigation to determine the improvement in the time for full recovery to the baseline sh ould be conducted . Factors that need to be investigated include both the variability of the paper composition and the variability of cellulose fiber arrangement during the paper making process to assess the ir impact on the UV response. Increasing the conc entration of GO on BF reinforced epoxy composites The insertion of GO as a coupling agent between the BF and the polymer matrix showed a slightly increase of 6% for the SBSS in 40v% BF reinforced composites . Future investigations should seek to determine the optimum GO concentration in the coating solution through determination of the effect of the GO concentration and functionalization on the interfacial bonding of the BFs into the matrix as well as on the composite mechanical properties such as the trans verse properties (flexural strength at 0 ° direction) and fracture toughness to assess the level of adhesion between the coated BFs and the epoxy matrix.