Poly(lactic acid), PLA was blended with poly(3-hydroxybutyrate-co-3-hydroxyvalerate), PHBV, based on a crossed mixture-factorial experimental design with three levels of factorial variable of the type of pre-produced maleated PLA, PLAgMA-type, used as the blend compatibilizer, and three components mixture variable which were the contents of PLA, PHBV, and PLAgMA, included in the polymer blends. The mixture model was based on the constrained level of the weight fraction of each mixture component as follows: 0.2 ⁹́Þ PHBV ⁹́Þ 0.7, 0.2 ⁹́Þ PLA ⁹́Þ 0.7, and 0.05 ⁹́Þ PLAgMA ⁹́Þ 0.15. The design of experiment yielded 16 runs of compatibilized blends, with 2 runs of non-compatibilized blend and 2 runs of neat polymers, PLA and PHBV, for comparison. The model of relationship between variables was derived based on the multiplication of a linear relationship of one factorial variable with a quadratic Scheffe model of the mixture ingredients. Multiple formulas of the blend compatibilizer, maleated PLA (PLA-g-MA), were pre-produced by a reactive melt blending method to functionalize maleic anhydride, MA, on the PLA backbone in a twin-screw co-rotating extruder. Dicumyl peroxide, DCP, was used as a free radical initiator in the reactive blending. The formulas were designed using response surface experimental design to determine the effect of the contents of MA and DCP on the amount of grafted MA, MA-grafting yield, and the molecular weight properties, Mn, Mw, IV, and dispersity of PLA-g-MA. The model regression indicated a significant effect of DCP with increasing DCP tending to reduce the MA-grafting yield, Mn, Mw, and IV, and increase the dispersity. The optimum point that maximized the desirability of these responses simultaneously was with the content of DCP = 0.1 wt. % and MA = 3.94 wt. % (PLA basis). Blending of PLA and PHBV clearly increased the crystalline fraction of the blends compared to neat PLA, which affects the barrier properties of the materials. Inclusion of PHBV at 25 wt. % in the non-compatiblilized blend and at 45-60 wt. % in compatibilized blends resulted in more than 60% reduction of water and O2 permeability compared to PLA. The compatibilized PLA/PHBV blend with PLA weight fraction of 0.45 achieved 300% increase in the tensile strength compared to the neat PHBV; this level of improvement was equivalent to the non-compatibilized blend containing PLA 75 wt. %. This was attributed to enhanced interfacial adhesion that was evidently supported by increased miscibility between the blend components in compatibilized blends which was exhibited through the shifting of Tg of PLA and the decrease of k constants based on the Gordon-Taylor equation of the compatibilized blends. The factorial-mixture model regression suggested the validity of the mixture variable of PLA, and PHBV in both tensile and barrier properties; the PLAgMA had a significant effect only on the tensile performance of the polymer blends. The overlapped contour plots as well as the desirability functions could be used to optimize the mixture of the PLA/PHBV blend components that provide desirable tensile and barrier properties. A biodegradation study was conducted on neat PLA, PHBV, non-compatibilized blend of 75:25 PLA/PHBV, and compatibilized blend of 65:15:20 PLA/PLA-g-MA/PHBV. PLA/PLA-g-MA/PHBV was the fastest to reach 100% mineralization, followed by PLA and PLA/PHBV samples, according to the CO2 evolution and % mineralization, whereas PHBV reached only 81% mineralization at the end of the test of 180 days. The facilitation of anhydride present in PLA-g-MA on the hydrolysis of PLA was a major cause of the fast biodegradation of PLA/PLA-g-MA/PHBV. A sharp increase in enthalpy of fusion, Î₄Hf, as well as a rapid reduction of the molecular weight of PLA/PLA-g-MA/PHBV compared to PLA and PLA/PHBV support the occurrence of an elevated rate of hydrolysis. The PHBV sample showed the biodegradation was barely affected by abiotic hydrolytic degradation as the thermal properties did not show any shifting of the melting transition and the Î₄Hf remained stable until 30 days of the test; the main mechanism was the enzymatic microbial degradation causing an erosion at the surface rather than affecting the bulk properties such as the molecular weight. The scanning electron micrographs also revealed the biodegradation of PHBV that initially occurred was from the surface and later showed the degradation of the crystalline structure. The PLA crystals formed during the biodegradation of PLA/PHBV and PLA/PLA-g-MA/PHBV samples could be seen from SEM photos.
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