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USING NANOPARTICLES TO INHIBIT DEWETI'ING IN THIN
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CHEMICAL SENSORS

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USING NANOPARTICLES TO INHIBIT DEWE'I'I'ING IN THIN POLYMER FILMS
FOR POTENTIAL APPLICATIONS IN CHEMICAL SENSORS

By

Melissa Ann Holmes

A DISSERTATION
Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of
DOCTOR OF PHILOSOPHY
Department of Chemical Engineering and Materials Science

2007

ABSTRACT

USING NANOPARTICLES TO INHIBIT DEWETTING IN THIN POLYMER FILMS
FOR POTENTIAL APPLICATIONS IN CHEMICAL SENSORS

By

Melissa Ann Holmes

Thin polymeric films are used in many new and emerging technologies, including
nanolithography resist layers, fuel cells, dielectric coatings and chemical and
biological sensors. Successful use of these materials requires that they be
continuous, of uniform thickness and remain stable on a variety of organic and
inorganic substrates. However, polymer films above their glass transition
temperature or plasticized through solvent absorption may dewet from a
substrate. The stabilization of these films by addition of various nanoparticles
such as fullerenes (060) and polystyrene nanoparticles was investigated in this
study. Previous researchers have shown that addition of fullerenes, dendrimers
and polystyrene nanoparticles to thin polystyrene films (<50 nm) inhibits
dewetting upon high temperature annealing. In this work the dewetting behavior
of nanoparticle filled polymer films was investigated upon solvent annealing,
similar to that seen in the manufacture and use of chemical sensors. It was found
that under certain conditions the thin films were stabilized by the addition of a
small amount of nanoparticles upon exposure to a solvent atmosphere just as
they are stabilized during thermal annealing. This stabilization is due to the
nanoparticles segregating to the substrate surface upon manufacture of the

polymer film, forming a jammed state or gel-like network which makes the system

stable. In addition, the effect of surface energy on the ability of nanoparticles to
inhibit dewetting was investigated. In final studies, nanoparticle-filled polymer

films were used in live sensors where the reliability and robustness were studied.

This dissertation is dedicated to my family and friends who have supported me

over the years.

iv

ACKNOWLEDGEMENTS

I would like to thank my family for all of their love and support. I would like
to thank Allan for his love and his patience with me during the stressful times,
and for always being willing to let me vent to him about my frustrations. I would
like to thank my mom for teaching me to love learning as a young child and for
bestowing in me that I could achieve anything I put my mind to. I would like to
thank my dad for going above and beyond to make sure I had everything I could
every need or want, and for always helping me out whenever I asked. I would like
to thank my sister, Michelle, for being such a good friend and roommate to me
and helping me survive grad school.

I would like to thank Dr. Mackay for his support and guidance over the
past 5 years. He has given me a wealth of knowledge of which I am extremely
grateful for. Also, I would like to thank Rachel Giunta for being a mentor to me
and for helping me survive outside of the walls of academia.

Finally, I would like to thank the Mackay Group members for all their help
in the lab. I would like to especially thank Tiffany and Dave who have literally
been at my sides during the entire journey that was grad school. Thank you to
Tiffany for running with me to give me an excuse to get out of the lab. Thank you

to Dave for always answering my numerous questions.

Table of Contents

 

List of Tables ................................................................................................... viii

List of Figures ....... - .................................................................... x

Key to Symbols or Abbreviations ................................................................... xv

Chapter 1 Introduction ....................................................................................... 1

1.1. Motivation ........................................................................................... 1

1.2. Background ......................................................................................... 3

Chapter 2 Dewetting of Polymer Films under Solvent Annealing .................. 6

2.1. introduction ......................................................................................... 6

2.2. Experimental Method .......................................................................... 9

2. 3. Results and Discussion .................................................................... 11

2.4. Conclusion ........................................................................................ 28

Chapter 3 Dewetting Inhibition of Polymer Films Using Nanoparticles ...... 29

3.1. Introduction ....................................................................................... 29

3.2. Experimental ..................................................................................... 31

3.3. Results and Discussion .................................................................... 35

3.3.1. Polystyrene/Fullerene Systems .................................................... 35

3.3.2. Polystyrene/Polystyrene Nanoparticle Systems ........................... 47

3.4. Conclusion ........................................................................................ 53
Chapter 4 Influence of Substrate Surface Energy on the Dewetting of

Polymer/Nanoparticle Films ............................................................................ 54

4.1. Introduction ....................................................................................... 54

4.2. Experimental ..................................................................................... 56

4.3. Results and Discussion .................................................................... 60

4.3.1. Polystyrene/Polystyrene Nanoparticle Systems ........................... 60

4.3.2. Polystyrene/Fullerene Systems .................................................... 68

4.4. Conclusion ........................................................................................ 76

Chapter 5 Application of Technology to Chemical Sensors ......................... 77

5.1. introduction ....................................................................................... 77

5.2. Experimental ..................................................................................... 79

5.3. Results and Discussion .................................................................... 81

5.3.1. Dewetting Inhibition ..................................................................... 81

5.3.2. Sensor Performance .................................................................... 85

5.4. Conclusion ........................................................................................ 89

Chapter 6 Summary and Conclusions ............................................................ 90

vi

 

Appendices ............................................................... 94

Appendix A Principles of Neutron Reflectivity .............................................. 95
Appendix B Neutron Reflectile Data Interpretation .................................. 102
Appendix C Additional Data .................................................................... 105

vii

List of Tables

Table 2.1. Viscosities based upon dewetting data and rheological

characterization for the different molecular weights tested. ................................ 17
Table 2.2. Cross model parameters for the different molecular weight PS
samples used. .................................................................................................... 23
Table A.1. Scattering length densities (SLDs) for the materials used in the
reflectivity studies. ............................................................................................ 101
Table 8.1. Known parameters used in all reflectivity models. .......................... 104 .
Table C.1. Fitted values for the model parameters used in Figure 0.1. ........... 107
Table 0.2. Fitted values for the model parameters used in FigureC.2. ............ 109
Table C.3. Fitted values for the model parameters used for the piranha
cleaned sample in Figure 0.3. .......................................................................... 111
Table C.4. Fitted values for the model parameters used for the
Sigmacote® treated sample in Figure 0.3 ........................................................ 111
Table C.5. Fitted values for the model parameters used in Figure 0.4. ........... 113

Table 0.6. Fitted values for the model parameters used for the before
annealed sample in Figure 0.5 ......................................................................... 115

Table 0.7. Fitted values for the model parameters used for the after
annealed sample in Figure 0.5 ......................................................................... 115

Table 0.8. Fitted values for the model parameters used for the normal film
before annealing in Figure 3.2. ......................................................................... 116

Table 0.9. Fitted values for the model parameters used for the normal film
after annealing in Figure 3.2. ............................................................................ 116

Table 0.10. Fitted values for the model parameters used for the flipped
film before annealing in Figure 3.2. .................................................................. 116

Table (3.11. Fitted values for the model parameters used for the film
before annealing in Figure 3.4. ......................................................................... 117

Table C.12. Fitted values for the model parameters used for the film after
annealing in Figure 3.4. .................................................................................... 117

viii

Table C.13. Fitted values for the model parameters used for the film
during annealing in Figure 3.5. ......................................................................... 118

Table C.14. Fitted values for the model parameters used for the before
annealed sample in Figure 3.7. ........................................................................ 119

Table C.15. Fitted values for the model parameters used for the before
annealed sample in Figure 3.7. ........................................................................ 119

Table C.16. Fitted values for the model parameters used for the film
before annealing in Figure 4.8. ......................................................................... 120

Table C.17. Fitted values for the model parameters used for the film after
annealing in Figure 4.8. .................................................................................... 120

Table C.18. Fitted values for the model parameters used for the flipped
film before annealing in Figure 4.9. .................................................................. 121

Table 0.29. Fitted values for the model parameters used for the flipped
film after annealing in Figure 4.9. ..................................................................... 121

ix

List of Figures

Figure 2.1. Optical micrographs showing the progression of dewetting for
various molecular weight PS films arranged in columns on silicon
substrates under solvent annealing with toluene over time. (a) 5 k0 PS,
(b) 19 kD PS, (c) 75 kD PS. Films are all ~36 nm in thickness. From the
images one can see the instability of the rims increase with increasing
molecular weight. Scale bars are 100 um and the scale is the same for all
images. ............................................................................................................... 12

Figure 2.2. Hole growth data for 5 RD, 19 kD and 75 kD PS samples over
time. All films are ~36 nm thick. The plot shows that the holes grow at a
constant velocity and the rate is dependent on the molecular weight of the
film ...................................................................................................................... 13

Figure 2.3. Dynamic frequency sweeps of different molecular weight
saturated PS samples with their Cross model fits. ............................................. 16

Figure 2.4. (a) Hole growth data for 5 k0 PS samples of various
thicknesses over time. Film thicknesses are given in the legend. (b) Plot of
the dewetting rates versus film thickness for the 5 kD PS films .......................... 20

Figure 2.5. (a) Hole growth data for 19 kD PS samples of various
thicknesses over time. Film thicknesses are given in the legend. (b) Plot of
the dewetting rates versus film thickness for the 19 kD PS films ........................ 21

Figure 2.6. (a) Hole growth data for 75 kD PS samples of various
thicknesses over time. Film thicknesses are give in the legend. (b) Plot of
the dewetting rates versus film thickness for the 75 kD PS films ........................ 22

Figure 2.7. Optical micrographs of the evolution of a dewetting hole for a
75 kD PS film. The fingering instabilities of the rim are apparent as the
hole grows over time from 5 minutes (left) to 10 minutes (middle) to 20

minutes (right). Scale bars are 50 um. ............................................................... 26

Figure 3.1. a) Pure PS film after annealing with a saturated toluene
atmosphere for 1 hour. b) PS with 3% C60 by mass after annealing with a
saturated toluene atmosphere for 3 hours. Films are ~30 nm thick. ................... 35

Figure 3.2. a) Neutron reflectivity data and models for polystyrene films
with 3 wt% C60. Normal films refer to films that were spin-coated directly
onto silicon substrates while the flipped film was spin-coated onto mica
and then flipped onto a silicon substrate. b) Scattering length density
profiles for the models used to fit the data. Reflectivity data were shifted to
allow for easier viewing. ..................................................................................... 37

Figure 3.3. Optical micrographs of spin-cast films (~30 nm) of PS with
various concentrations of 060 after exposure to a saturated toluene
atmosphere. a) Pure PS after 1 hour. b) PS + 0.1 wt% Cso after 5 hours. c)
PS + 3 wt% Ceo after 3 hours. d) PS + 10 wt% 060 after 1 hour. TEM
micrographs of e) PS film with 3 wt% Cso and f) PS film with 10 wt% Ceo
(inset shows diffraction pattern for a cluster of fullerenes). ................................ 40

Figure 3.4. a) Neutron reflectivity data and models and b) scattering
length density profiles for a polystyrene film with 7 wt% C50. The film was
measured before (a) and after (0) annealing under a saturated toluene
atmosphere. The film was ~18 nm thick. After annealed reflectivity data
were shifted up a decade to allow for easier viewing. ........................................ 43

Figure 3.5. a) Neutron reflectivity data and model and b) scatting length
density profile for the same film as Figure 3.4 during exposure to a
saturated toluene atmosphere. The horizontal lines correspond to the SLD
of pure d-Toluene and PS. ................................................................................. 44

Figure 3.6. PS + 10 wt% 25 kD PSNP film after annealing with a saturated
toluene atmosphere for 5 hours. Inset shows film before annealing. Film is
~30 nm thick. ...................................................................................................... 47

Figure 3.7. a) Neutron reflectivity data and models for two polystyrene
films with 10 wt% 25 kD PSNP. One film is immediately after spin-coating
(o) and the other has been exposed to a saturated toluene atmosphere for
5 hours (0). b) Scattering length density profile of the models used to fit
the data. The SLD for pure dPS and PSNP are shown on the graph. For
the after annealing sample, it is clearly seen from the SLD profile that the
nanoparticles migrate to the substrate, as shown. ............................................. 48

Figure 3.8. Optical micrographs of PS films with varying concentrations of
25 kD PSNP annealed under a saturated toluene atmosphere for 1 hour.
a) Pure PS film. b) PS + 5% 25 kD PSNP film. 0) PS + 15% 25 kD PSNP
film. d) PS + 50% 25 kD PSNP film. All films are ~30 nm thick. ........................ 50

Figure 4.1. Optical micrographs of PS + PSNP films (~30 nm thick) on
piranha cleaned wafers annealed for 5 hours under a saturated toluene
atmosphere. a) PS + 17% 60 kD PSNP (0:1.2). b) PS + 23% 140 kD
PSNP (0:12). C) A schematic of how the monolayer of PSNP stabilizes
the film against the adverse van der Waals forces. The arrows represent
the extent of the van der Waals force interaction. .............................................. 62

Figure 4.2. Optical micrographs of a pure PS film (~30 nm thick) on a
silanized water after annealing under a saturated toluene atmosphere for
15 minutes. Inset shows film prior to annealing. ................................................. 63

xi

Figure 4.3. Optical micrographs of PS + 25 kD PSNP films on a silanized
wafer after annealing under a saturated toluene atmosphere. a) PS 4» 5%
25 kD PSNP after annealing for 15 minutes (0:05). b) PS + 10% 25 kD
PSNP after annealing for 15 minutes (0:1.0). 0) PS + 15% 25 kD PSNP
after annealing for 5 hours (9:1.4). d) PS + 26% 25 kD PSNP after
annealing for 5 hours (0:25). All films are ~30 nm thick .................................... 64

Figure 4.4. Optical micrographs of PS + 60 kD PSNP films on silanized
wafers annealed under a saturated toluene atmosphere. a) PS + 12% 60
kD PSNP after annealing 30 minutes (0:09). b) PS + 20% 140 kD PSNP
after annealing 1 day (0:1.4). Films are ~30 nm thick ........................................ 65

Figure 4.5. Optical micrographs of PS + 140 kD PSNP films on silanized
wafers annealed under a saturated toluene atmosphere. a) PS + 16% 60
kD PSNP after annealing 30 minutes (0:09). b) PS + 23% 140 kD PSNP
after annealing 20 hours (0:12). Films are ~30 nm thick. ................................. 66

Figure 4.6. Optical micrographs of PS + 3 wt% C60 films (~30 nm thick)
spin-coated onto mica and floated onto a) a piranha cleaned wafer and
annealed for 5 hours and b) a silanized wafer and annealed for 15
minutes. Annealing was performed by exposing the films to a saturated
toluene atmosphere. Insets show films prior to annealing. ................................. 68

Figure 4.7. Optical micrographs of PS + 3 wt% C60 films (~30 nm thick)
spin-coated onto mica and flipped onto a) a piranha cleaned wafer and
annealed for 1 hour and b) a silanized wafer and annealed for 15 minutes.
Annealing was performed by exposing the films to a saturated toluene
atmosphere. Insets show films prior to annealing ............................................... 70

Figure 4.8. a) Neutron reflectivity data and models for a PS + 60 kD
PSNP + 3 wt% Cso flipped film before and after annealing under at
saturated toluene atmosphere for 2.5 hours. b) The scattering length
density profiles for the models used in a). The film was formulated to
ensure that the PSNP are at sufficient concentration to form a monolayer. ....... 72

Figure 4.9. a) Neutron reflectivity data and models for a PS + 140 kD
PSNP + 3 wt% C60 film before and after annealing under a saturated
toluene atmosphere for 5 hours. b) The scattering length density profiles
for the models used in a). The film was formulated to ensure that the
PSNP are at sufficient concentration to form a monolayer. ................................ 73

Figure 5.1. Optical micrographs of spin-cast (a) pure PECH and (b) PECH
+ 1 wt% Ceo films after exposure to a saturated methanol atmosphere for
20 hours. Insets show films prior to annealing ................................................... 81

xii

Figure 5.2. Optical micrographs of spray-coated films after exposure to a
saturated methanol atmosphere. (a) Pure PECH after 42 hours and (b)
PECH + 10 wt% C50 after 47 hours of exposure. Insets show films prior to
annealing. ........................................................................................................... 83

Figure 5.3. Optical micrographs of spray-coated (a) pure DKAP and (b)
DKAP + 10 wt% C50 films after exposure to a saturated methanol
atmosphere for 17 hours. Insets show films prior to annealing ........................... 84

Figure 5.4. Optical micrograph of a live SAW sensor. The left side (Line
1) is coated with PECH + 10 wt% Cso. The right side (Line 2) is coated
with pure PECH. ................................................................................................. 85

Figure 5.5. Response curves for a live SAW sensor. (a) Response to
challenges for line 2 coated with pure PECH (A) immediately after spray-
coating and (B) after sitting for 1 day. (b) Response to challenges for Line
1 coated with PECH with 10 wt% 060 (C) immediately after spray-coating,
(D) after sitting for 3 days and (E) after spraying more solution 4 days after

the initial deposition. ........................................................................................... 87
Figure A.1. Schematic of the POSY2 reflectometer at Argonne National
Laboratory. ......................................................................................................... 98
Figure A.2. Schematic of SPEAR at Los Alamos National Laboratory. ............. 98
Figure A.3. Schematic of the reflection and refraction of a beam of
neutrons through a thin film (1) on a substrate (2) in air (0) ................................ 99
Figure 3.1. Illustration of a reflectivity model for a film having m layers,
each with a distinct thickness (d) and neutron refractive index (n). .................. 102
Figure 8.2. Image of the reflectivity panel window in Motofit. .......................... 104

Figure 6.1. a) Neutron reflectivity data and model for a polystyrene film on
a piranha cleaned silicon wafer. b) Scattering length density profile of the
model used to fit the data. ................................................................................ 106

Figure C.2. a) Neutron reflectivity data and model for a deuterated
polystyrene film on a piranha cleaned silicon wafer. b) Scattering length
density profile of the model used to fit the data. ............................................... 108

Figure C.3. a) Neutron reflectivity data and models for deuterated
polystyrene film. One was spin-coated onto a piranha cleaned silicon
water while the other was floated onto a Sigmacote® treated wafer. b)
Scattering length density profiles of the models used to fit the data. ................ 110

xiii

Figure C.4. a) Neutron reflectivity data and models for polystyrene with 3
wt% 060 films on piranha cleaned silicon wafers. One was tested without
annealing and the other was first annealed with a saturated toluene vapor.
b) Scattering length density profiles of the models used to fit the data ............. 112

Figure C.5. a) Neutron reflectivity data and models for polystyrene with 3
wt% C60 films on piranha cleaned silicon wafers. One was tested without
annealing and the other was first annealed with a saturated toluene vapor.
b) Scattering length density profiles of the models used to fit the data ............. 114

xiv

ACS

i’sr.

Key to Symbols or Abbreviations

Nanoparticle radius

Degrees of freedom gained by releasing a monomer unit from
substrate constraints

Hamaker constant for a 3 layer assembly
American Chemical Society

Coherent scattering length

Background added to the reflectivity fit
Concentration

Parameter used to determine the goodness of a fit
Fullerenes

Film thickness

Film thickness

Diffusivity

Characteristic diameter

Diameter of a growing hole

A poly(dimethylsiloxane) derivative
Dimethyl methyl phosphonate

Deuterated polystyrene

Monomeric interaction energy

Volume fraction of i

Surface tension

Liquid surface tension

Solid surface energy

Interfacial energy between a solid and liquid

Shear rate

Film thickness

Characteristic film thickness defined as the absolute value of the
spreading parameter divided by the plateau modulus
Initial film thickness

Packing density

Viscosity

Complex viscosity

Terminal viscosity

Specific viscosity

Intrinsic viscosity

Hydrofluoric acid

Dimensionless constant

Huggins constant

Boltzmann constant

Constant equal to 3.2x10'8 ° '3

Kilodalton

Coefficient dependent on n, kand h,-

XV

I], j+1
rpm
.02

0'0
SAW
SLD

Neutron wavelength

Modulated differential scanning calorimetry

Methanol

Methyl salicylate

Weight average molecular weight

Power law index

Atomic number density

Neutron refractive index of medium j

Polydispersity index (weight average molecular weight/number
average molecular weight)

Polyepichlorohydrin

Polystyrene

Polystyrene nanoparticles

Polytetrafluoroethylene (commonly referred to as Teflon“)
Surface area coverage

Angle of incidence

Critical angle of incidence below which total external reflection
occurs

Equilibrium contact angle

Angle of incidence

Neutron momentum transfer

Critical value of qz below which total external reflection occurs
Neutron momentum transfer perpendicular to the surface
Reflectivity

Fresnel reflection coefficient

Revolutions per minute

Scattering length density at depth 2

Kuhn or monomer length

Roughness

Plateau modulus

Surface acoustic wave

Scattering length density

Time

Relaxation time

Crossover time, below which Dd scales linearly with t
Equilibration time

Temperature

Glass transition temperature

Polymer glass transition temperature

Solvent glass transition temperature

Transmission electron microscopy

Rate of dewetting

Weight fraction

Weight percent

Coefficient dependent on n ranging from 1 to 00

Depth in the direction perpendicular to the surface

xvi

Chapter 1

Introduction

1.1. Motivation

In the aftermath of the attacks on September 11, 2001, the level of
concern and awareness of a terrorist attack has significantly risen. Specifically,
concerns over the contamination of the nation’s water supply as well as harmful
agents being released into the atmosphere have been discussed. These types of
chemical and biological warfare have the possibility to kill millions of people and
thus, the need for a way to detect and protect against such possible attacks has
surfaced. Numerous types of chemical and biological sensors are currently being
developed. Many of these sensors employ the use of thin polymer films to aid in
the detection of harmful pathogens.

At the forefront of this effort is Sandia National Laboratories, which has
developed a chemical sensor it calls the pChemLabTM. This sensor is a portable
chemical analysis system consisting of three different stages. The first stage of
the device collects and concentrates the vapor sample. Next, a micro-gas
chromatograph separates the sample components based on size. Finally, the
sample is fed to the sensing stage. The sensor used isva surface acoustic wave
(SAW) device which uses a thin polymer film deposited on the quartz as the
detector. The polymer film is designed to selectively absorb a specific analyte
and is deposited onto the substrate along the wave’s propagation path. When the

analyte is present, it is absorbed by the polymer film causing a minute increase in

the film’s mass that can then be measured by a shift in the SAW operating

frequency relative to an uncoated reference device.

1.2. Background

Thin polymeric films (<100 nm) are used in many of today’s new and
emerging technologies. These include nanolithography resist layers, fuel cells,
dielectric coatings and chemical and biological sensors to name a few." 2 For the
majority of the applications, successful use of these materials requires that they
be continuous, of uniform thickness and remain stable on a variety of organic and
inorganic substrates. However, for films of this thickness, the film may be
unstable or metastable, leading to the dewetting (or beading up) of the polymer
on the substrate under the appropriate conditions.

Dewetting can occur either as a spinodal process3 or by nucleation and
hole growth.4 In the first, thermal fluctuations at the polymer-air interface increase
in amplitude until they reach the substrate and create a hole. In the latter, a
nucleation site is formed due to either defects or impurities in the film. In both
cases, once the holes are formed, they continue to grow in size until they
coalesce to form a honeycomb of polymer fibrils, eventually decaying into
spherical droplets through Rayleigh instabilities.5

In order for a polymer film to dewet, it must be in a liquid state, and thus
above its glass transition temperature, or T9. One method to induce dewetting is
to thermally anneal the polymer film well above its T9, thereby allowing it to flow.
In this case, long-range van der Waals forces lead to the polymer film instability
and its eventual breakup.6' 7 Another method to induce dewetting is to lower the
effective T9 of the polymer film by vapor annealing. In this case, solvent vapor is

introduced into the system effectively plasticizing the film, leading to a depression

in the Tg as well as decreasing the viscosity.8 Vapor annealing differs from
thermal annealing by the addition of a third component to the system, increasing
the possible surface and bulk interactions, and to date has been given very little
aflenfion.

In the present work, we look at the issues associated with polymer film
dewetting that occur upon vapor annealing, similar to the situation for sensor
applications. The polymer films are exposed to a saturated solvent atmosphere
which plasticizes the polymer and reduces its T9, allowing dewetting to occur.
This type of dewetting is seen in the use of chemical sensors,9 such as surface
acoustic wave (SAW) sensors, which are devices with an input and output
transducer patterned on a piezoelectric substrate. When the input transducer is
excited, it generates a surface acoustic wave that propagates across the
substrate creating an output signal at the receiving transducer. To make the
device a sensor, a polymer coating designed to selectively absorb a specific
analyte is deposited onto the substrate along the propagation path. When the
analyte is present, it is absorbed by the polymer coating causing a minute
increase in the mass of the film that can then be measured by a shift in the SAW
operating frequency relative to an uncoated reference device. When dewetting
occurs in a sensor, the output frequency can become erratic or stop altogether
due to the reduced analyte detection area, leading to a faulty sensor.9

The typical approaches for stabilizing thin polymer films often involve
modification of the polymer, the substrate or the interaction between the two.

These may include crosslinking of the polymer,10 sulfonation and metal

1 surface treatment of the substrate to alter its

complexation of the polymer,1
surface energy and thus promote wetting12 or grafting of the polymer chains
directly to the substrate.13 However, for some applications it is not feasible to
utilize these methods as they may impact the desired properties of the film or
hinder the ability for rapid manufacture of devices. Consequently, an alternate
method for eliminating dewetting is desired.

Recently it has been shown that addition of fullerenes,14 dendrimers15 and
other nanoparticles‘a'18 to polymer films inhibits dewetting during thermal
annealing. These studies suggest that the nanosized fillers segregate to the
substrate surface, which leads to stabilization of the film. To support this, recent
simulations have shown that the distribution and mobility of the nanoparticles as
well as the polymer-filler attraction are important factors in the inhibition of
dewetting.19 This stabilization effect is hypothesized to be the result of contact
line pinning by fractal-like structures formed at the substrate as well as due to
modification of the polymer-substrate interaction as a result of an increased
surface (nano)roughness and the concomitant surface energy modification due to
a fractal-like polymer molecule conformation.“ In the work presented here, this
novel strategy of using nanoparticles to inhibit dewetting is adopted for use in
vapor annealed polymer films such as those in chemical sensors. This technique
will be used with a standard, well characterized polymer (polystyrene) as well as
polymers associated with sensor applications to determine if the above

hypothesis is valid. In addition, sensor performance will be tested with and

without nanoparticles to determine their influence.

Chapter 2

Dewetting of Polymer Films under Solvent Annealing

2.1 . Introduction

The stability of thin polymer films (<100 nm) has been studied at length
due to their numerous applications in many new and emerging technologies such
as dielectric coatings, nanolithography resist layers and chemical and biological
sensors, to name a few." 2 The applications of these films require that they be
continuous, of uniform thickness and remain stable on a variety of substrates.
However, depending on the intermolecular forces present, as well as the
wettability of the liquid on the substrate, the film may dewet.

Currently, there are two separate mechanisms which are believed to
initiate the onset of dewetting. One mechanism involves the influence of
heterogeneous defects or impurities in the film, such as dust particles." 2° These
defects create a dry patch, or nucleation site, where dewetting can begin. The
other mechanism is a spontaneous rupturing of the film, or spinodal dewetting,3'
21'2“ due to the fact that it is comparable to spinodal decomposition seen in fluid
mixtures. Here, modulations at the free surface of the film induce a pressure
gradient resulting from a competition between the Laplace pressure that will level
the film, and the disjoining pressure which thickens and roughens the film.
Depending on the system, these modulations may be amplified exponentially
until they reach the substrate, spontaneously rupturing the film. In either case,
heterogeneous or homogeneous dewetting, once the holes are formed they

continue to grow in size.

Much work has been done investigating the dynamics of growing holes by
considering the balance between the capillary driving forces and the viscous
dissipation forces. For a sample where the viscous dissipation is dominant, one
finds that the growth rate remains constant with time (0.25 On the other hand, if
there is very little viscous dissipation and instead the energy is dissipated by
friction, then the polymer has been seen to slip on the substrate.26' 27 In this case,
the diameter of the hole grows proportional to 12’3 rather than linearly. Some
studies have shown that there is a combination of these two cases, where slip
and viscous dissipation are present in the hole gromrth.28' 29 Still, other work,
involving the growth of holes in free-standing films, found an exponential growth
of the hole size.3°' 3‘

Work has also been performed with regard to the rim stability of the
growing holes. In most instances, as the holes grow, the removed polymer
accumulates at the edges into rims. Eventually the holes will coalesce which
results in the formation of a honeycomb of polymer ribbons. These ribbons then
decay into spherical droplets through Rayleigh instabilities.5 Other studies have
shown that the rims undergo fingering instabilities as they grow, such as those
seen in Hele-Shaw flow patterns?" 32' 33

Although there has been much research in the area of polymer films
dewetting from a silicon substrate, much of it has been done using high-
temperature annealingf” 4' 20' 2" 33'” Little work has been done in the area of

solvent annealed films. This is a current area of interest due to the fact that

polymer films used in sensor applications are exposed to solvents and must

remain stable to ensure satisfactory performance.38 In one previous study, the
instability of the film was attributed to the short-range polar interactions as
opposed to the long-range van der Waals forces.39 Also, they suggest there are
two types of rim instabilities in solvent dewetting depending on the polarity of the
solvent used. Dewetting of block copolymer films induced by solvent vapor has
also been considered as well as the resulting morphologies.” 4‘ However, none
of these studies have specifically focused on the dynamics of the dewetting holes
over time.

In the current work, we investigate the dewetting of thin polystyrene films
on silicon substrates under a saturated toluene atmosphere. Solvent annealed -
dewetting has been seen in the manufacture of sensors, and therefore the aim of
this work is to better understand the stability and dynamics of polymer films
exposed to solvent vapor. The dewetting dynamics of films are investigated for
various molecular weights of polystyrene as well as different initial film
thicknesses. In addition, a look at the instability of the moving rim is studied. This
work differs from many of the other dewetting dynamic studies due to the fact
that a third component is introduced, the solvent, which changes the interfacial

energies of the system as well as the flow properties.

2.2. Experimental Method

Polystyrene (PS) standards were purchased from Scientific Polymer
Products, Inc. with weight average molecular weights (MW) of 5.1, 19.3 and 75.7
kD with respective polydispersity indexes (PDI, weight to number average mass
ratio) of 1.07, 1.07 and 1.17. These molecular weights were chosen to allow for
samples which are unentangled (5.1 kD and 19.3 kD) and entangled (75.7 kD)
since the critical molecular weight for entanglement coupling is 35 kD. The
polymers were used as received. Solutions were made using ACS reagent grade
toluene. Stock solutions were first made and then diluted to the desired
concentration and were filtered using a 0.2 pm PTFE filter before use.

The substrates used for the experiments were silicon wafers with native
oxide on their surface and were purchased from Wafer World Inc. The wafers
were cleaved to the desired size (~1 cm x 1 cm) and then cleaned for 1 hour with
an acid wash consisting of a 70:30 by volume mixture of 96% H2SO4 and 30%
H202, respectively, to remove any residual organic contaminants. After the acid
wash, the wafers were rinsed with copious amounts of Millipore water and dried
under a nitrogen stream. Wafers were used immediately after cleaning.

Thin films were made by spin-coating the polymer solutions onto the
substrates at a speed of 5000 rpm for 40 seconds. Film thickness was controlled
by the total mass concentration of the solution and ranged from 20-50 nm as
determined using ellipsometry (JA Woollam M-44 ellipsometer). Films were found
to be uniform and continuous upon inspection with optical microscopy and were

annealed by exposing them to a saturated toluene environment in a closed

vessel at room temperature to observe the dewetting process. Reflective mode
optical images (bright field) were obtained in situ using an Edmund Industrial
Optics optical microscope with a JAI camera attachment.

Rheology experiments were carried out using a Rheometrics ARES
rheometer using 8 mm parallel plates set at a gap of approximately 0.3 mm.
Dynamic frequency sweeps in the range of 1-100 radians/second were carried
out at room temperature (25°C 11°) under saturated vapor conditions. Bulk P8
was placed on the rheometer plates and a glass chamber filled with toluene
vapor was placed around the sample and allowed to sit for several days to insure
equilibrium had been achieved. Several measurements were taken and then

averaged.

10

2.3. Results and Discussion

In the experiments presented here, annealing of the polymer films was
done by introducing solvent vaporinto the system, effectively plasticizing the film
leading to a depression in the Tg as well as decreasing the viscosity.8 The
evolution of holes for different molecular weight polystyrene films of the same
thickness are shown in Figure 2.1 as they dewet from silicon substrates. After a
certain time, holes spontaneously form and, as can be seen from the images for
all the molecular weights tested, the holes have rims as soon as they are visible
with the optical microscope. Also, one can see that the time for holes to form, as
well as grow, varies significantly with molecular weight.

The growth of hole diameters for the three molecular weight PS films
studied are shown in Figure 2.2 where all films in this case are 36 :1: 1 nm thick.
To insure an accurate value for the diameters of the holes, five measurements
were taken at different orientations around the hole, neglecting any fingering that
occurred, and then averaged to give the reported value. For all molecular weights
a linear growth rate is observed, meaning that viscous forces dominate and there
is no slip. The rate of dewetting is also shown to be dependent on the polymer
molecular weight in agreement with previous studies which suggest that the rate
of dewetting is inversely proportional to the viscosity.42 We of course find that the
higher molecular weight sample, which is expected to have a higher viscosity, as

discussed below, has a smaller dewetting rate.

11

 

Figure 2.1. Optical micrographs showing the progression of dewetting for various
molecular weight PS films arranged in columns on silicon substrates under
solvent annealing with toluene over time. (a) 5 k0 PS, (b) 19 kD PS, (0) 75 kD
PS. Films are all ~36 nm in thickness. From the images one can see the
instability of the rims increase with increasing molecular weight. Scale bars are
100 pm and the scale is the same for all images.

12

 

 

 

 

 

 

 

Hole Diameter (pm)

 

 

 

 

 

 

 

 

16030

Time (seconds)

Figure 2.2. Hole growth data for 5 k0, 19 kD and 75 kD PS

samples over time. All films are ~36 nm thick. The plot shows that

the holes grow at a constant velocity and the rate is dependent on

the molecular weight of the film.

To more fully understand the flow mechanism of the dewetting films,
experiments were performed using bulk samples of polystyrene powders of all
three molecular weights to determine the amount of toluene absorbed by the
polymers. Polystyrene was used in powder form as received, placed in a pan and
weighed both before and after exposure to a saturated toluene atmosphere.
Upon reaching equilibrium, these experiments show that the approximate
equilibrium polymer concentration is 0.73 :I: 0.01 g/ml for all molecular weights

after one week, and is assumed to be equal to the concentration of the

PS/toluene films used for the dewetting experiments.

We expect equilibrium to be achieved in a much shorter time in the thin

film and the equilibration time can be estimated by 43

Dr
A25 >1 . (2.1)

 

where D is the toluene diffusivity in PS (~10'6 cm2/s) 44, A, the film thickness (~30
nm) and 15, the equilibration time. Inserting these values, a time of ~10 us is
found and thus, solvent exposure creates an essentially instantaneously
equilibrated film. The dewetting time scale is on the order of minutes and so it is
expected that dewetting occurs under equilibrium concentration conditions.
Knowing the concentrations of our films, the solution glass transition

temperature can be estimated using the Fox equation“

 

_1_ _ w 1— w
T -—P 3
T8 T8 T8 (2.2)

Here, w is the weight fraction of the polymer in solution (~0.70), and T9,P and T5,S

are the polymer (379 K for 75 kD PS) and solvent (117 K) ‘5 glass transition
temperatures. At this concentration, the T9 is found to be -46°C. The fact that the
T9 of the solution is well below room temperature, it is expected that the viscosity
will be low and rationalizes why rapid dewetting occurs when the polystyrene film
is exposed to a saturated toluene atmosphere even at room temperature.
Further, the solution surface energy is expected to be ~31 mJ/m2 and
similar to that of pure PS (~28.5 mJ/mz) under thermal annealing and so
dewetting is expected.46 Thus, the flow behavior may be different during vapor

exposure compared to thermal annealing since different final patterns are

14

produced, yet, dewetting is expected under surface energy arguments and the
ultimate result is the same with the appearance of gross dewetting.

To help quantify the dewetting time we first estimate the viscosity of the
polymer/solvent mixture using the Martin equation47 for the 75 kD molecular

weight sample

:73 = ek'InIc

477] (2.3,
where 175,, is the specific viscosity {solution viscosity/solvent viscosity - 1, where
the solvent viscosity is taken as 0.56 GP for toluene at 25°C“}, 0, the polymer
concentration (0.73 g/ml), [77], the polymer intrinsic viscosity (calculated to be ~38
ng using the Mark-Houwink-Sakurada equation with the appropriate constants
for the 75 kD molecular weight polystyrene) and k', Huggins constant (~0.33).49
The resulting solution viscosity using this method is 150 Pa-s which is equivalent
to the viscosity of pure 75 kD polystyrene at a temperature of ~210°C.5°' 5‘ For
the two lower molecular weight samples the results obtained using the Martin
equation were much lower with values of 0.2 Pa-s for the 19 kD PS and 7x10'3
Pa-s for the 5 k0 PS.

Viscosity measurements were carried out under saturated vapor
conditions to compare to those calculated using the Martin equation and are
shown in Figure 2.3. We can see that the viscosity of the saturated films is
dependent upon the molecular weight as expected, and in agreement with our

results for the dewetting rates given in Figure 2.2 where higher molecular weights

dewet slower. We were unable to reach the terminal region for the highest

15

,I l 1 lllllll 1 111
' T '

17 I f I U ' U W V V : i

o 75kDPS]

 

 

 

 

 

 

" :, O 19kDPS
2 e ' u - A 5kDPS
u _ ,
4 u ._..,
10 .. n ..
- n
’6; 4?" b n "‘ " ll
(6 I " u Q. U "
9; 2 .'l‘.. Q; " H II
a: 3 . “ " ._‘~\‘\“~.. " II II
c 10 ' \\-[ ._. “ u ”-
- “ \x u j.
5 ‘I “\ n 1
4 . “\..\“fi'
2 2'
102 .
' é éis'éié" 2'» :3 15655”
1 10 100
Frequency (rad/sec)

Figure 2.3. Dynamic frequency sweeps of different molecular

weight saturated PS samples with their Cross model fits.
molecular weight polymer so we can only speculate as to its zero-shear viscosity
by fitting with the Cross model discussed below (see Table 2.1), however, we
estimated the viscosity of the films to be 2800, 6700 and 67 000 Pa-s, in order of
increasing molecular weight. These are much higher than those calculated from
predictions using the Martin equation, which may be explained by the fact that we
have fairly concentrated solutions (~0.73 g/ml) while the Martin equation is valid
for dilute or semi-dilute solutions.

For no-slip conditions, as our experiments show, the rate of dewetting (V)
is expected to be inversely proportional to the viscosity through the following

relation:42

16

_ 3
V —k —7 9.

where r; is the viscosity, y is the surface tension, 8., is the equilibrium contact .
angle and k9 is a constant equal to 3.2x10‘8 degrees'a.52 Using the dewetting
rates calculated from the data in Figure 2.2, a surface tension of 31 mN/m53 and
an equilibrium contact angle of 15° (which was found for a sample of saturated
PS on a silicon water), we can calculate the Viscosities for the three different
molecular weight samples which are given in Table 2.1. From the table we can
see that the Viscosities are less than those found in the rheological
characterization experiments perhaps indicating that the viscosity of thin films
may differ from the bulk“ as has been seen for various other bulk properties (i.e.

glass transition temperature55'57

) or that equation 2.4 is not applicable to our
system. In Chapter 4 we will show through neutron reflectivity experiments that
the concentration of the polymer in thin films upon absorption of toluene is much

less than that calculated from the bulk experiments, which would lead to a lower

Table 2.1. Viscosities based upon dewetting data and rheological
characterization for the different molecular weights tested.

 

 

 

 

 

 

 

 

Molecular Dewetting Dewetting Chiraetgtggzcaetiilon
Weight Rate Data Viscosity Viscosity
75.7 0.015 220 67 000
19.3 0.13 25 6700
5.1 1 .43 2.3 2800

 

l7

 

viscosity compared to that measured by rheological characterization. This would
explain why we would predict a much lower viscosity based on the dewetting
rates.

Consider now the influence of film thickness on the rate of dewetting for
the polymer films. The dewetting hole growth data for 5 kD PS films of various
initial film thicknesses is shown in Figure 2.4a. Holes rupture only a few minutes
after being exposed to the saturated toluene atmosphere and over time grow in
size. At shorter times, an interesting observation is made, the rate of dewetting
appears to follow a nonlinear trend before becoming linear. This may be clue to
the fact that the dewetting occurs so rapidly that it has not become an equilibrium
process at this point, yet, ultimately the rate of hole growth becomes linear. To
characterize the rate of dewetting, the data were fitted to a linear model at longer
times neglecting the initial hole diameters which do not follow this trend. From
this analysis the influence of the initial film thickness on the observed rate of
dewetting is found to be inversely proportional to the initial film thickness under
solvent annealing at odds with the thickness independent prediction of equation
2.4 (see Figure 2.4b).

By comparison, the hole growth data for the higher molecular weight PS
films, 19 kD and 75 kD, shown in Figures 2.5a and 2.6a, respectively, always
show a linear trend. It takes much longer in these cases for the holes to rupture
and once the holes are formed and visible, rims are also immediately observable.
For all film thicknesses we again see a linear growth rate, yet for higher

molecular weight films, we do not observe the brief non-linear dewetting at

18

shorter times as was seen in the 5 k0 PS films. The influence of initial film
thickness on the rate of dewetting is shown in Figures 2.5b and 2.6b where again
a linear trend is seen, similar to that in Figure 2.4b.
Previous studies have considered the influence of film thickness on the rate of
dewetting. In one study liquid-liquid dewetting was performed where the liquid
substrate thickness is shown to significantly influence the rate of dewetting.58 The
other study showed polystyrene films on silicon substrates of varying initial
thicknesses had different rates of dewetting upon thermal annealing59 which was
attributed to a difference in the viscosity of the films based on the film thickness.
A purely Newtonian fluid (equation 2.4) would not give a film thickness
effect, however, our results in Figure 2.3 suggest that the fluids exhibit a shear-
thinning effect, which has been previously considered by Saulnier et al.60 Shear-
thinning was investigated due to the fact that the extremely thin films experience
high shear rates at even moderate dewetting rates. We can determine an
apparent shear rate by dividing the rate of dewetting by the film thickness, which
yields a shear rate on the order of 100 5". According to Saulnier et al., dewetting
holes in thin films which experience shear-thinning behavior should grow linearly
with time, and in addition, the rate of dewetting should be faster for thinner films,
both of which are seen in our data. For shear-thinning, the viscosity of the film

can be approximated using the Cross model:

. n 2.5
1+4”) ( )

 

19

A
m
v

 

500

cli-
. -. ..... ,-
‘-

I . 2
; f i 2

I
..t
I
l
I

I
.

Hole Diameter (um)

 

l

b 0 CI
' 91 ~ ‘4’:
>->->« '

 

 

 

 

 

o
b 200 300 400 500 600
( ) Time (seconds)

 

1.8

 

l
.'

. -........~_
. .. .......q_

I

x l 2
I E C
a . i
1 5 db"........-......-..v.......‘,..............-...........-....;.-.... ............-.....,-...i........-......_.....,..,_....
- ‘ : :

. . .
. . .
. . .
. . .
x z z
1 4 .--- o.-.:-..... ...- ... ..¢ . -
O
. .
l
I I

Rate of Dewetting (urn/s)

 

 

 

dp.-.-........ ...........

1.2
100 200 300 400 500
Film Thickness (A)

 

Figure 2.4. (a) Hole growth data for 5 k0 PS samples of various
thicknesses over time. Film thicknesses are given in the legend. (b)
Plot of the dewetting rates versus film thickness for the 5 RD PS
films.

20

A
m
v

 

500

 

 

 

300

 

 

200

 

 

Hole Diameter (pm)

100 '-

 

 

 

 

 

 

(b) 0 1000 2000 3000 4000
Time (seconds)
028

0.26
024
022
020
0.18
0.16

Rate of Dewetting (pm/s)

0.14

 

0.12
150 200 250 300 350 400

Film Thickness (A)

Figure 2.5. (a) Hole growth data for 19 kD PS samples of various
thicknesses over time. Film thicknesses are given in the legend. (b)
Plot of the dewetting rates versus film thickness for the 19 kD PS
films.

21

A
N
V

 

  
  
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

E 300~~-
.6.
g
a 200 ._ .. ._-
Q a
3 A 135
100- c. 261A
0 329A
. . :1 356A
0 I I . .
b 0 5CDO 10000 15000 20000 25000
( ) Tlrne (seconds)
020 I i : I 4.
a
g .
0.10 : i ..
3 ;
8 \
*6 i \
0 : K
g 0.05 ; . \ . ,,.,,..._L
0.00 r \

 

 

 

 

I l
100 150 200 250 300 350 400
Film Thickness (A)

Figure 2.6. (a) Hole growth data for 75 kD PS samples of various
thicknesses over time. Film thicknesses are give in the legend. (b)
Plot of the dewetting rates versus film thickness for the 75 kD PS
films.

22

Table 2.2. Cross model parameters for the different
molecular weight PS samples used.

 

 

 

 

 

Parameter 5 kD PS 19 kD PS 75 kD PS
0'0 5200 Pa 11 300 Pa 40 000 Pa
7 0.54 s 0.62 s 1.7 s
0.25 0.70 0.49
n 0.86 0.80 0.99

 

 

 

 

 

 

where 00 is taken as the plateau modulus, ris the relaxation time taken as 170/00

(where no is the terminal viscosity that was found from the data, see Table 2.1 ), n

is a power law index found at higher shear rates (7}), which is between 0 and 1
for shear-thinning fluids and k is a dimensionless constant. Applying equation 2.5
to the rheological data (see Figure 2.3) we can determine the parameters of the
Cross model which are given in Table 2.2. For non-Newtonian thin films, the
diameter of the growing hole (0..) should follow the equation:

Ddz001+{(1—-5’l’]

(r! -1)Z (2'6)

where Do is a characteristic diameter, n is the power-law index from the Cross
model, i]! is a coefficient dependent on n and to is a characteristic time, called the
crossover time, in which dewetting follows the linear trend and is proportional to

the initial film thickness and is given by the equation:

(27)

23

with

1
A = k W2 __1 fl: K-n
wl'" +1 h. (2-8)

I

where h* is a characteristic length, defined as the absolute value of the spreading
parameter divided by the plateau modulus, and h,- is the initial thickness of the
film. The crossover times for the systems in order of increasing molecular weight
are on the order of 1 second, 10 seconds, and nearing infinity. We would expect
the crossover times to be larger than the values calculated for the two lower
molecular weight systems since we observe a linear dewetting rate at times
above these values. Applying equation 2.6 to the dewetting data gives
reasonably good fits and realistic values of Do for the 5 RD and 19 kD PS films
(~3um). For the 75 kD PS films the characteristic diameter that must be used is
unrealistically high for the length scales seen in these experiments. The reason
for this is due to the fact that n is very close to unity for the high molecular weight
system and l// diverges to infinity at n=1. However, overall the shear-thinning
model for the dewetting of polymer films is able to closely follow the experimental
results much better than that of a Newtonian fluid since a film thickness effect is
predicted and seen. Yet, agreement between the model and our results is not
quantitative.

Reasons for discrepancies, particularly for the high molecular weight
system, may be due to the fact that under solvent annealing, the conditions of the
films during dewetting are not precisely known. The exact film thickness during

annealing is not known due to the fact that the polymer swells when the toluene

24

is absorbed into it.61

Further one may expect the composition of the film to vary
with film thickness especially near the solid substrate which would allow for a
viscosity gradient. The viscosity gradient would be dependent upon the thickness
of the film, and therefore may explain the results in Figures 2.4-2.6 where the
different dewetting rates are seen for different film thicknesses since the rate of
dewetting is dependent upon the viscosity of the film. Also, it is possible that
there may be a condensed layer of pure toluene on top of the film and/or a layer
of toluene at the substrate which forms during the annealing process.62

Another interesting observation is that the 75 kD PS film, which would be
an entangled film in pure form, does not exhibit slip on the substrate. de Gennes
predicted that entangled polymers slip on smooth surfaces, such as silicon
substrates,63 as has been seen by us“ 65 and others.66 Slip has been apparent in

numerous thermally annealed film dewetting studies”: 27' 67

and if the polymer
film were to slip on the substrate, the growth rate should follow a 12’3 power law.
However, this is not seen in our solvent annealed samples, suggesting that
surface frictional forces are not dominant in our system, but instead the energy is
dissipated through viscous effects perhaps by an extremely thin toluene layer
near the silicon wafer surface.61 However, we believe the shear-thinning is
important due to the magnitude of the shear rate and the fact that the model of
Saulnier et al. gives a reasonable representation of the data.

Rim instability is apparent in all three molecular weight films, however it is

most apparent in the 75 kD sample. The fingering instability of a rim is shown for

25

 

 

 

 

w._ _ v 'w
i. J” I. . "
a- C
. . /'--'.. . . -fh.‘
. fl . o -"
_' -.‘*
If . \. . . 'I"
o. - _ ‘ ~ \
n ‘ II
E ' o
—ij_‘4 q l... Oh

 

 

Figure 2.7. Optical micrographs of the evolution of a

dewetting hole for a 75 kD PS film. The fingering

instabilities of the rim are apparent as the hole grows

over time from 5 minutes (left) to 10 minutes (middle)

to 20 minutes (right). Scale bars are 50 pm.
a dewetting hole for this system in Figure 2.7. As can be seen, the fingers grow
from the unstable rim and are left behind. They eventually break from the rim and
decay into spherical droplets through Rayleigh instabilities, just as is seen in the
final dewetting stage for all polymer films.

The fingering instabilities that are seen mimic those patterns seen in Hele-

Shaw flow that develop when a low-viscosity fluid is injected into a high-viscosity
medium in a confined space.32 Our system is analogous to this where the
polymer film is confined to the substrate and a lower viscosity fluid, in our case
air saturated with toluene, is displacing a higher viscosity fluid, a
polystyrene/toluene solution. The interface between the two is unstable and the
saturated air intrudes into the polymer solution in the form of fingers.68 The
observation that the fingers form more readily in the higher molecular weight
sample can be attributed to the higher elasticity. Numerous studies have
investigated viscous fingering in shear-thinning fluids in Hele-Shaw cells,6971
however, there has been much debate over the exact effect that an increased

elasticity has on fingering. Our observation is consistent with Reiter21 however,

who suggested that higher molecular weight samples are more susceptible to

26

fingering, due to the fact that the higher viscosity and entanglements of the

molecules make the fingers more stable.

27

2.4. Conclusion

An experimental observation into the dynamics and instabilities of polymer
films by solvent annealing has been performed. Solvent annealing adds a new
dimension to the dynamics of polymer film dewetting by the introduction of a third
component which is apparent in both the rate of hole growth, as well as the rim
instabilities. It has been found that the dewetting dynamics of solvent annealed
polystyrene films follows those of non-slipping films for all molecular weights
tested. Also, both the molecular weight and initial film thickness greatly influence
the rate of dewetting where the smaller viscosity of the lower molecular weight
films allows for faster dewetting. Also, the dewetting rates of the saturated films
are inversely proportional to the initial film thickness. In addition, we have found
that solvent annealing of polymer films increases the instability of the growing
rims, and that for higher molecular weight polystyrene, these instabilities lead for
finger formation, such as that seen in Hele-Shaw flows. This is attributed to the

difference in the Viscosities of the liquid and vapor components in our system.

28

Chapter 3

Dewetting Inhibition of Polymer Fllms Using Nanoparticles

3.1 . Introduction

Numerous technological applications, such as fuel cells, dielectric coatings
and nanolithography resist layers, require the use of thin polymer films on a
variety of organic and inorganic substrates." 2 As the film is made thinner and
thinner, the ability to manufacture stable, uniform films lessens. Due to capillary
forces, the polymer films may rupture and dewet from the substrate under certain

27' 33 which is detrimental to the effectiveness of the polymer film and

conditions
may render devices useless.9' 38

Several methods have been employed to stabilize thin polymer films on
substrates. Some of these methods involve chemically modifying the polymer or
the substrate to enhance their interaction with each other.“"13 In recent years, a
novel approach to inhibit dewetting through the addition of nanoparticles has
been studied. Nanoparticles such as fullerenes,“ dendrimers,72 polystyrene
nanoparticles18 and other nano-sized fillers16'17'73‘75 have been shown to inhibit
or eliminate dewetting in thin polymer films under high temperature annealing.
Barnes 91 al.14 first showed that the addition of fullerene nanoparticles to polymer
films eliminates dewetting due to a pinning of the contact line at the polymer-
substrate interface. In addition, simulations have been performed which have
supported the idea of nanoparticles inhibiting the dewetting of thin polymer films

through not only a pinning effect, but also due to local viscosity changes in the

film by the addition of the nanofillers.19 It is suggested that the size, mobility and

29

the interaction of the nanoparticle with the polymer all influence the ability to
inhibit dewetting of polymer films.

We have previously reported that the addition of fullerenes to a variety of
polymer films can also inhibit dewetting under solvent annealing?8 Solvent
annealing differs from high-temperature annealing due to the introduction of a
third component that plasticizes the polymer film rather than heating above the
polymer’s glass transition temperature.76 In the present study, we will extend this
work as well as compare the ability of polystyrene nanoparticles to inhibit
dewetting of thin polystyrene films annealed under a saturated toluene
atmosphere. These nanoparticles differ from fullerenes both in size and chemical
composition. The fullerenes are less than 1 nm in diameter and chemically
dissimilar to linear polystyrene used to form the film, while the polystyrene
nanoparticles are larger in diameter (4-5 nm) and chemically similar to the bulk
film. Since the only difference between linear and nanoparticle polystyrene is
their molecular morphology, this allows for minimal enthalpic interactions
between the polymer and nanoparticle on the monomer-monomer length scale.

Specifically, fullerenes and polystyrene nanoparticles will be added to
polystyrene films at various concentrations to determine their effectiveness for
dewetting inhibition. The films will be exposed to a saturated toluene
atmosphere, plasticizing the film to achieve potential dewetting conditions.
Neutron reflectivity experiments will also be performed to determine the location
of the nanoparticles in the film, to gain insight into the cause for dewetting

elimination.

30

3.2. Experimental

The polystyrene (PS) standard obtained from Scientific Polymer Products,
Inc. (weight average molecular weight (M) = 75.7 kD, polydispersity index (PDI)
= 1.17) was used as received. Fullerenes (060 or buckyballs) were purchased
from Term-USA and were used as received. Polystyrene nanoparticles (PSNP)
were synthesized from a linear precursor (M. = 25 kD) and characterized as
described elsewhere.” 78 The procedure to make the nanoparticle is to drip a
solution of the linear precursor, which contains 20 mol% crosslinking groups, into
hot solvent which activates the crosslinking process to produce a nanoparticle
from a single molecule.

The PSNP were rid of any silicon containing impurities before use. This
was done by first digesting any silicon compounds by stirring the nanoparticles in
a hydrofluoric acid (HF) bath for several hours. Toluene was then added to the
bath to dissolve the nanoparticles. The mixture was then allowed to settle and
separate, then the HF layer was decanted off, leaving only the organic solution.
The solution was then dripped into methanol to precipitate the silicon-free
nanoparticles which were dried under vacuum at 40°C until all the solvent had
been removed. This procedure was performed to ensure that silicon
contaminants did not alter the surface energy of the PSNP which may have led to
false wettability of the nanoparticle filled films. A previous study in our lab
demonstrated some presence of silicon containing compounds in PSNP when x-
ray photon spectroscopy (XPS) was performed necessitating the above

procedure to ensure accurate results.

31

Polystyrene/nanoparticle solutions were made using ACS reagent grade
toluene. Stock solutions of both polystyrene and the nanoparticles were first
made, and then mixed at the correct amounts to get the desired overall mass
concentration as well as the desired mass concentration of nanoparticles relative
to polystyrene (0.01 - 20%). The resulting solutions were filtered using a 0.2 pm
PTFE filter before use. In addition, all solutions were sonicated immediately
before use to ensure the nanoparticles were dispersed.

Solutions were spin-coated onto silicon wafers to produce thin films. The
silicon wafers (Wafer World Inc.) were first cleaned with an acid solution of 70:30
by volume 96% H2804 and 30% H202 for 1 hour to remove any organic
contaminants. The wafers were then rinsed with Millipore water and dried under
nitrogen. Solutions were spin-coated at a speed of 5000 rpm for 40 seconds.
Film thickness was controlled by the overall mass concentration of the solution
and ranged from 20-50 nm as determined using ellipsometry (JA Woollam M-44
ellipsometer). Films were determined to be uniform and continuous upon
inspection with optical microscopy. Annealing of the films was performed by
exposing them to a saturated toluene environment to observe the dewetting
process. Reflective mode optical images (bright and dark field) were obtained in
situ using an Edmund Industrial Optics optical microscope with a JAI camera
attachment.

Samples for neutron reflectivity measurements were prepared on 2 inch
diameter silicon wafers that were 1 mm thick to ensure the substrate would not

warp and give erroneous results. Samples were prepared in the same way as

32

described above. Deuterated polystyrene (dPS) standards were purchased from
Scientific Polymer Products, Inc. and Polymer Source with weight average
molecular weights of 63 kD (PDI = 1.10) and 82 kD (PDI = 1.15), respectively. A
silanizing agent Sigmacote® (Sigma-Aldrich), which is a clear, colorless solution
of a chlorinated organopolysiloxane ((SiCl2C4H9)20, 2.5%) in heptane (97.5%)
was employed to prepare the silanzed surface for the flipped film reflectivity
experiments. The silicon wafer was flooded with the solution for 1 minute and
then the excess was rinsed off and the wafer was dried under nitrogen. The
surface energy of silicon wafer treated with Sigmacote® was found to be 28 :I: 1
mN/m based on contact angle measurements with polar (water) and non-polar
(methylene iodide) liquids. Neutron reflectivity experiments were carried out at
two different facilities. The first was the POSY2 reflectometer at the Intense
Pulsed Neutron Source at Argonne National Laboratory. The intensity of the
instrument is 100 neutrons per pulse with wavelengths (i...) in the range of 2.5 -
16 A. The second was the SPEAR reflectometer at the Los Alamos Neutron
Scattering Center at Los Alamos National Laboratory. Reflectance (R)
measurements were taken at different angles of incidence (0° < e < 3°) and then
spliced together by fitting the data in the partially overlapping wave vector (q)
ranges. The overall q range that could be obtained from the wavelengths and
angles possible was 0 — 0.25 A“, where q = 41tSiI'l9/An. The reduced reflectivity

t.79 To do this, models were

data was analyzed using the program Motofi
approximated which consisted of a series of layers of constant concentrations

with discrete parameters. In addition, layer roughness was included in the

33

modeling using values on the order of 1-10 A. The reflectance of the model was
then calculated and then the model adjusted until the best fit was determined by

minimizing 76" using the Levenberg-Marquardt algorithm.

34

3.3. Results and Discussion

3.3.1. Polystyrene/Fullerene Systems

Dewetting experiments were performed using polystyrene films since it is

a well characterized and readily available polymer. The polymer films were

exposed to a saturated toluene atmosphere to induce dewetting with results for a

pure PS film and a PS film with C60 added shown in Figure 3.1. The pure PS film

has fully dewetted after only one hour of exposure to the toluene vapor while the

film which has only a small amount, 3 wt%, Cso added shows no signs of

dewetting after 3 hours of exposure.

Previous studies have suggested that the location of the nanoparticles in

the film may be the reason that dewetting is inhibited.” 18' ‘9' 38' 8° In order to

confirm this, neutron reflectivity measurements were performed and the results

 

 

 

 

200 um

 

 

Figure 3.1. a) Pure PS film after annealing with a saturated toluene atmosphere
for 1 hour. b) PS with 3% 060 by mass after annealing with a saturated toluene

atmosphere for 3 hours. Films are ~30 nm thick.

35

are presented in Figure 3.2. Three samples were considered; one where the 3
wt% Cso solution was spin-coated onto a wafer (normal film before annealing),
the same composition film after solvent annealing (normal film after annealing)
and a flipped film which will be described below. To allow for easier viewing, the
data for the normal annealed film have been shifted up one decade and the data
for the flipped film has been shifted up two decades for the plot of Rq4 versus q in
Figure 3.2a. The reason for using this plot is that if sharp interfaces are present
then the curves are approximately horizontal. Using this plot and due to the large
difference in the neutron scattering length densities (SLD) of PS (1.41x10'6 AZ)
and Ceo (5.73x10'6 A'Z) we are able to determine the location of the nanoparticles
in the film despite the low concentration.

The first sample was a film which had been spin-coated directly onto a
silicon substrate and had not been exposed to toluene vapor. The data were
fitted using a 2 layer model consisting of a region having high SLD near the
substrate and low elsewhere as demonstrated by the solid lines in the figure.
From the SLD profile in Figure 3.2b, for the normal unannealed film, one can see
that the nanoparticles are located at the substrate in a ~2 nm C60 enriched layer
(~ 29 vol% 060). This can be explained by the fact that the fullerenes are much
less soluble in toluene than polystyrene is. The maximum solubility of fullerenes

in toluene has been reported in the range of 2.90 mg/ml,81

while polystyrene at
the molecular weight used in the current work is more than two orders of
magnitude more soluble. Therefore, the fullerenes are thought to phase separate

to the substrate during the spin-coating process forming the enriched layer. The

36

-e ' A Flipped Film Before Annealing
10 ' " 0 Normal Film After Annealing
- ; Cl Normal Film Before Annealing

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Normal Film Before Annealing
Normal Film After
- - Film Before

0 50 100 150 200 250 300
<--— air h(A) substrate --—>

Figure 3.2. a) Neutron reflectivity data and models for polystyrene
films with 3 wt% Ceo. Normal films refer to films that were spin-
coated directly onto silicon substrates while the flipped film was
spin-coated onto mica and then flipped onto a silicon substrate. b)
Scattering length density profiles for the models used to fit the data.
Reflectivity data were shifted to allow for easier viewing.

37

second sample tested was exposed to toluene vapor for 3 hours (normal film
after annealing). Again, the data were fitted using a 2 layer model and it is seen
from the SLD profile in Figure 3.2b that the nanoparticles are in a C60 enriched
layer at the substrate and remain there after annealing with little change.

The fullerenes’ location in the film and the fact that they do not move
throughout the film when exposed to the saturated toluene atmosphere can be
explained by the effective interfacial potential of the system. Confirming that the
Hamaker constant of the 3 layer assembly is negative will ensure that the system
is stable and will not reorder to achieve a lower energy state.80 One can
determine the sign of the Hamaker constant (A132) for a 3 layer assembly where

component 3 is between component 1 and component 2 using the equation:

A132 "' (”12 "”3 anz —n32) (3.1)

For our system, n1 (air) is 1.0, ng (PS+Cso) is 1.77, and n3 (PS) is 1.59. The value
for the PS+C60 layer was determined through an average developed from the
reflectivity modeling. Using these values the Hamaker constant is calculated to
be less than zero. Due to the negative value, this arrangement of layers will
remain stable. If we were to change the order and have the nanoparticles move
to the air interface upon annealing, the Hamaker constant would become
positive, which would lead to a negative interfacial potential and result in an
unstable film that dewets.82' ‘33

To confirm these results and to make sure that the nanoparticles were at

the substrate immediately after spin-coating, a third sample was characterized.

This sample was spin-coated onto freshly cleaved mica and then “flipped” onto a

38

silanized silicon wafer by placing the mica with the film facing down onto a silicon
wafer and using water to separate the film from the mica thereby transferring it
onto the silicon wafer. By flipping the film, one would expect that the
nanoparticles would now be located at the polymer-air interface, which is what
modeling of the data suggests. Modeling performed on this sample confirms that
there is a 060 enriched layer at the air interface as seen in Figure 3.2b (flipped
film before annealing), as expected. Therefore, the nanoparticles assemble at the
polymer-substrate interface during the spin-coating process and subsequently
remain there through annealing. We were unable to perform neutron reflectivity
on an annealed flipped film to determine if the nanoparticles would separate to
the substrate, since the film was not stable and dewetted, thus strengthening the
argument that the location of the nanoparticles at the polymer-substrate interface
is the cause for dewetting inhibition.” ‘8' ‘9' 38

The concentration of Cso used is important to induce wetting of PS films.
Previously we found that a gel-like layer rich in fullerenes forms near the
substrate shielding adverse van der Waals forces which we to produced a stable
trilayer above.38 Here we discover there is a optimum concentration of fullerenes
to achieve elimination of dewetting for a given system, as can be seen in Figure
3.3. In Figure 3.3a, a pure PS film that has fully dewetted is again shown. Adding
a very small concentration of nanoparticles, such as 0.1 wt% shown in Figure

3.3b, retards the dewetting process. In this case we are below the optimum

concentration to eliminate dewetting, which allows for holes to form

39

 

 

 

 

 

 

 

 

 

Figure 3.3. Optical micrographs of spin-cast films (~30 nm) of P3 with
various concentrations of 060 after exposure to a saturated toluene
atmosphere. a) Pure PS after 1 hour. b) PS + 0.1 wt% Cso after 5 hours. 0)
PS + 3 wt% 060 after 3 hours. (I) PS + 10 wt% 060 after 1 hour. TEM
micrographs of e) PS film with 3 wt% 060 and f) PS film with 10 wt% Ceo
(inset shows diffraction pattern for a cluster of fullerenes).

and grow, but they eventually stop growing even upon further annealing. In
Figure 3.3c we are at the optimum concentration of 3 wt% for this system and
therefore no dewetting is seen upon toluene vapor exposure. We estimate the
optimum concentration is ~3 wt% through extensive experimentation at this film
thickness where below this concentration partial dewetting occurs. Above the
optimum concentration, dewetting can be seen again which is apparent in Figure
3.3d, where a PS film with 10 wt% 060 has fully dewetted after annealing.

It is believed that for smaller concentrations of fullerenes, there are large
gaps present between the nanoparticles at the substrate so there are too few
nanoparticles to form the gel-like layer, and thUs holes are able to form in these
spots. Yet they will only grow until they reach an area with a high enough
concentration to form the gel-like layer as postulated by Barnes et al. for thermal
annealing.“ For large fullerene concentration it is hypothesized that the
fullerenes aggregate84 due to their high concentration in the solution and limited
solubility81 forming larger agglomerates that can act as nucleating sites leading to
dewetting. Note we sonicated all solutions just prior to spin-coating to ensure
maximum dispersion. Microscopy was performed to test this hypothesis as
shown in Figures 3.3e and 3.3f where TEM micrographs of PS films with 3 wt%
and 10 wt% C60 are presented. There are very few agglomerates present in the
PS film with 3 wt% film (fullerene concentration in solution of 0.21 mg/ml) as
shown in Figure 3.3e. In addition, the agglomerates present are much less than
100 nm in size. This is not the case for the PS film with 10 wt% C60 (fullerene

concentration in solution of 0.70 mg/ml) shown in Figure 3.3f. Here, the

41

agglomerates are in the micron size range, and clustered together into fractal-like
networks several micrometers in size. The electron diffraction pattern shown in
the inset demonstrates the crystalline order within an agglomerate.

Since dewetting occurs while the polymer is plasticized with toluene vapor
we measured the reflectivity while exposed to the vapor to determine the solvent
concentration and if it was homogeneous. We know the film will become
saturated when the polymer and solvent are eventually at thermodynamic
equilibrium,""5 yet, since the film is so thin equilibrium is expected to rapidly occur.
Unfortunately, because of this, it is not possible to determine the mass uptake
over time in order to determine a diffusion coefficient as performed in many other
swelling studies of polymers.”87 So, here, we present only equilibrium
measurements.

The films are expected to undergo a large degree of swelling to negatively
affect the reflectivity modeling since many fringes are produced which are close
together in q-space, so, a much thinner film, initially 18 nm thick, was prepared.
To do this, a lower total (PS + C60) solution concentration was used during the
spin-coating operation. The sample was characterized before, during and after
exposure to a saturated deuterated toluene atmosphere, and the results are
presented in Figures 3.4 and 3.5. The reflectivity data for the film before and after
annealing are very similar (Figure 3.4a) and the SLD profiles shown in Figure
3.4b confirm that the C60 are located in a thin layer at the substrate just as seen

in Figure 3.2. Note, the modeling was performed such that there is a constant

42

El BeforeAnnealing
0 After

------ Before Annealing
— After

 

0 50 100 150
<--- air h(A) substrate --->

Figure 3.4. a) Neutron reflectivity data and models and b)
scattering length density profiles for a polystyrene film with 7 wt%
060. The film was measured before (a) and after (0) annealing
under a saturated toluene atmosphere. The film was ~18 nm thick.
After annealed reflectivity data were shifted up a decade to allow

for easier viewing.

43

(a)

lnhornogeneous Distribution
Distribution

Rq‘ ill")

 

0.01 0.02 0.03 0.04 0.05 0.06
(b) -

d-Toluene SLD

p /10'°(A'2)

Inhomogeneous

PS SLD Distribution

 

0 200 400 600 8m
<--- air h(A) substrate --—>

Figure 3.5. a) Neutron reflectivity data and model and b) scatting
length density profile for the same film as Figure 3.4 during
exposure to a saturated toluene atmosphere. The horizontal lines
correspond to the SLD of pure d-Toluene and PS.

overall concentration of 060 in the film before and after annealing. Deuterated
toluene (SLD = 5.67x106 A'z) was used to allow for contrast with the PS during
annealing. Due to the fact that the SLDs of d-toluene and 060 are so close, and
due to the large swelling of the polymer film, it is not possible to locate the
nanoparticles during annealing for this system. However, since there is such a
low concentration of C60 in the film, and there is a high concentration of d-toluene
present, the results for the swollen film are not significantly influenced by the
presence of the 060.

During exposure to the d-toluene vapor, the film swells significantly, going
from an initial film thickness of 18 nm to a thickness of 87 nm, as shown in
Figures 3.5a and 3.5b. The reflectivity data were fitted using a model with eight
layers each with a thickness of 10 nm and varying concentration, while the ninth
layers (i.e. next to the air interface) thickness and concentration were allowed to
vary. The SLDs for pure PS and d-toluene are shown in Figure 3.5b with an
interesting observation that there is a decrease in the d-toluene concentration
closer to the substrate (curve labeled inhomogeneous distribution). Previous
studies that have considered the swelling of films upon exposure to solvents
have also reported this result.“““ 88 The lower solvent concentration at the
substrate is a result of the fact that the polymer chains are more confined at the
substrate and so they are not able to swell as much as those chains at the air
interface. Also, there appears to be a ~5 nm thick layer of pure d-toluene that has

condensed on top of the PS film, which is possible due to the fact that the

45

surface tension of d-toluene is slightly lower than that of polystyrene, allowing for
the solvent to wet the film.

From the SLD data, the equilibrium concentration of the swollen film can
be approximated as 80% by mass of solvent which agrees well with the actual
degree of swelling (79% = [87 nm — 18 nm] / 87 nm). To confirm that there is an
inhomogeneous distribution of toluene, a reflectivity model assuming a
homogeneous distribution of d-toluene of 80% by mass throughout the film (SLD
= 4.82x10'6 AZ) is also shown in Figures 3.5a and 3.5b, which does not
represent the data. Note the concentration of d-toluene determined by reflectivity
is much higher than the equilibrium mass uptake of solvent found in bulk
measurements (~27 wt%), indicating that the absorption of solvent in thin films is
different from that in bulk perhaps due to the increase in the mobility of the
chains at the air surface. In addition, the high concentration of solvent in the film
will lead to a large decrease in the viscosity of the film, which explains the ability
of the pure PS film to dewet rapidly when annealed in a saturated toluene
atmosphere.

A final note is that merely putting a layer of fullerenes on the substrate
then placing a layer of polymer on t0p does not result in a stable film. Dewetting
occurs and so it appears that the delicate gel-like layer of fullerenes and polymer

formed during spin-coating is necessary to inhibit dewetting.

46

3.3.2. Polystyrene/Polystyrene Nanoparticle Systems

Previous work has shown that nanoparticles made of collapsed and cross-
linked polystyrene chains have inhibited dewetting of thin polystyrene films under
high temperature annealing.18 These nanoparticles were used in the current
research to determine their ability to inhibit dewetting under solvent annealing.
An optical micrograph of a PS film containing 10 wt% 25 kD PSNP which has
been exposed to a saturated toluene atmosphere for 5 hours is shown in Figure
3.6. The image can be used to show that the addition of the nanoparticles inhibits
dewetting under solvent annealing just as in thermal annealing.

Neutron reflectivity experiments were also performed to determine the
location of the nanoparticles before and after annealing, and the results are
presented in Figure 3.7. In this case the nanoparticle and the polymer are
chemically similar and each is fairly soluble in the solvent, which was not the

case for the polystyrene/fullerene system. Therefore, we would not anticipate the

 

 

 

 

 

 

 

Figure 3.6. PS + 10 wt% 25 kD PSNP film after annealing with a
saturated toluene atmosphere for 5 hours. Inset shows film before
annealing. Film is ~30 nm thick.

47

El BeforeAnnealing
0 After

 

0.02 0.04 OIB 0.08 0.10
(b) q A“)

Before Annealing
dPS SLD “9’

p /10“’(A'2)

PSNP SLD

PSNP

 

0 50 100 150 200 250 300
<--- air h (A) substrate --—>

Figure 3.7. a) Neutron reflectivity data and models for two
polystyrene films with 10 wt% 25 kD PSNP. One film is immediately
after spin-coating (u) and the other has been exposed to a
saturated toluene atmosphere for 5 hours (0). b) Scattering length
density profile of the models used to fit the data. The SLD for pure
dPS and PSNP are shown on the graph. For the after annealing
sample, it is clearly seen from the SLD profile that the nanoparticles
migrate to the substrate, as shown.

48

nanoparticle preferentially segregating to the solid substrate interface during the
spin-coating process according to our hypothesis presented above. This is what
we found after fitting the data shown in Figure 3.7a, for a sample before
annealing, which indicates that the nanoparticles are initially uniformly dispersed
throughout the film as can be seen in the scattering length density profile shown
in Figure 3.7b.

However, after a sample has been exposed to a saturated toluene
atmosphere we see a change in the reflectivity profile of the film. The reflectivity
curve after exposure to toluene vapor was fitted as a 2 layer film, with the layer
near the substrate being the same thickness as the diameter of the nanoparticle,
4 nm. The scattering length density profile for the model used to fit the data
indicates that the nanoparticles have migrated to a layer near the substrate,
leaving primarily dPS in the top layer. The volume fraction of nanoparticles in the
layer near the substrate can be calculated from the SLD for that layer and is
found to be ~75. As expected, this large volume fraction of nanoparticles at the
substrate results in a densely packed layer.

Once again, the concentration of nanoparticles in the polymer film affects
their ability to inhibit dewetting since the pure polymer film would rapidly dewet
(Figure 3.1). To understand this phenomenon in more detail several films
containing varying concentrations of 25 kD PSNP have been solvent annealed
and optical micrographs are presented in Figure 3.8. It can be seen that smaller
concentrations do not result in dewetting inhibition. However, at larger

concentrations, dewetting is always inhibited, unlike the previous case with the

49

 

 

 

 

Figure 3.8. Optical micrographs of PS films with varying concentrations of 25 kD
PSNP annealed under a saturated toluene atmosphere for 1 hour. a) Pure PS
film. b) PS + 5% 25 kD PSNP film. c) PS + 15% 25 kD PSNP film. d) PS + 50%
25 kD PSNP film. All films are ~30 nm thick.
fullerenes where dewetting occurred again once the optimum concentration was
exceeded.

Knowing that the nanoparticles migrate to the substrate upon annealing it

is possible to determine the substrate surface coverage. The surface area

coverage (6) of the nanoparticles can be calculated from the equation:

3 h
=- _ ¢PSNPX77

2 2“ (3.2)

50

where h is the film thickness, a is the nanoparticle radius, ¢ps~p is the total
volume fraction of nanoparticles in the film and his the packing density of
spheres in two-dimensions, assumed to be 0.9 for maximum 2-dimensional
packing. From Figure 3.8, we find that dewetting is inhibited once the
nanoparticle concentration in the film reaches ~10%. With a volume fraction of
0.1, we find that the surface coverage of the nanoparticles is ~1, indicating there
is a complete monolayer of nanoparticles at the substrate. Therefore, at
concentrations large enough so that at least a monolayer of nanoparticles can
form at the substrate, dewetting will be inhibited.

A previous study has explained the migration of nanoparticles to the
substrate in a polymer film through a gain in the system’s total entropy.80 For

segregation to occur, the following relation should be met:

3 2
akBTl:£:l > kBT + 61:1]
0 o

where or represents the degrees of freedom gained by releasing a polymer

(3.3)

molecule’s monomer unit from substrate constraints (i.e. being next to the hard
substrate rather than the softer nanoparticle layer), which is of order 1, k3 is
Boltzmann’s constant, T is the temperature, a is the components’ monomeric
interaction energy,89 and a is a Kuhn or monomer length (~0.1-1 nm). This
relation is the result of a balance between conformational entropy gained by the
polymer chain moving away from the wall and sitting on the rougher nanoparticle
layer (Mafia/013) with the loss of translational entropy available to the

nanoparticle (~kBT) and the mixing enthalpy loss between the nanoparticle and

51

polymer molecule (£[a/oj2).89 For the system studied, the interaction energy term
for the PSNP in PS is of order 0.1-1 kBT for dispersion forces. Therefore,
segregation will occur for a nanoparticle radius of order 1-10 rim and dewetting
can be inhibited assuming the concentration of nanoparticles is high enough for a
monolayer to form.

Therefore, polystyrene nanoparticles can be added to inhibit dewetting in
solvent annealed PS films. The nanoparticles are entropically driven to the
substrate during the annealing process to form a jammed, closely packed
monolayer, just as seen in thermally annealed films.18 This layer shields the
adverse van der Waals forces which lead to dewetting, thereby stabilizing the film

by acting as an effective solid coating to promote wetting.

52

3.4. Conclusion

We have shown that addition of nanoparticles to thin polymer films inhibits
and even eliminates dewetting upon exposure to a saturated solvent atmosphere
at room temperature, as has been seen before in high temperature annealing.
Exposure of a pure polymer film shows solvent is rapidly absorbed lowering the
viscosity allowing for dewetting under equilibrium solvent concentration
conditions. However, the presence of nanoparticles in polymer films severely
retards dewetting and appears to be a general phenomenon that is not limited to
the type of nanoparticle used, as we have shown through the use of two different
nanoparticles, one that is chemically dissimilar to the polymer matrix (fullerenes)
and one that is the chemically the same (PSNP). We hypothesize that the
fullerenes form a ~2 nm thick gel-like layer next to the solid substrate that shields
the unfavorable substrate-polymer interactions thereby eliminating dewetting.
The gel-like layer apparently forms during the spin-coating process due to phase
separation of the fullerenes to the substrate. The concentration of fullerenes used
is important to inhibit dewetting, as there is a optimum concentration where both
above and below dewetting will occur. Additionally, we have shown that using
PSNPs in PS films can also inhibit dewetting upon solvent annealing, just as has
been seen upon high-temperature annealing. In this case, the nanoparticles are
initially homogeneously distributed in the film than entropically driven to the
substrate during the annealing process to form a jammed monolayer which

shields the‘adverse van der Waals forces which lead to dewetting.

53

Chapter 4
Influence of Substrate Surface Energy on the Dewetting of

Polymer/Nanoparticle Films

4.1 . Introduction

Manufacturing uniform, thin polymer films which remain stable on a variety
of solid substrates has become of technological importance. Numerous
applications employ these films, including dielectric coatings, nanolithography
resist layers and chemical and biological sensors." 2 Difficulties arise, however,
due to the fact that in many cases molecularly thin films can easily become
unstable, resulting in dewetting of the film.

Whether a liquid will wet and remain stable on a solid substrate is
dependent upon the spreading coefficient which is a thermodynamic parameter

defined as:90

5:73—7L-75L (4.1)
where ys and n are the solid and liquid surface free energies, respectively, and
731. is the interfacial free energy between the two. If the spreading coefficient is
positive for a given system, the liquid will spread over the substrate, and under
equilibrium conditions, remain stable. A negative value for the spreading
coefficient results in a nonwetting film. From this, one can see that a substrate

with a lower surface energy is more likely to have a negative spreading

coefficient, and thus more likely for a film to dewet from it.

54

A number of recent studies have shown that the addition of nanoparticles
to polymer films can retard and even eliminate dewetting. Nanoparticles such as
fullerenes,14 dendrimers,72 polystyrene nanoparticles18 and even quantum dots"0
have been shown to inhibit dewetting upon thermal annealing. Fullerenes and
polystyrene nanoparticles have also been shown to inhibit dewetting of polymer
films upon solvent annealing.” 9‘ The ability of the nanoparticles to inhibit
dewetting is dependent upon their location in the film as well as their interaction
with the polymer, which has been demonstrated through molecular dynamics
simulations.19

In the current work, we investigate the ability of nanoparticles to inhibit
dewetting of polystyrene films on hydrophilic and hydrophobic substrates. In
addition, the films were annealed under a saturated solvent atmosphere. Two
different nanoparticles were used, with the first being polystyrene nanoparticles,
so there were minimum enthalpic interactions between the polymer and the
nanoparticles at the monomer level, leading to enhanced entropic interactions.
Also, by altering the molecular weight of the nanoparticle, it was possible to alter
its size, giving nanoparticles which range in size from ~5-8 nm. The second type
of nanoparticle was fullerenes. These nanoparticles are chemically dissimilar to
the polystyrene film, allowing for possible phase separation, as well as being

much smaller in size (~ 0.7 nm) than the polystyrene nanoparticles.

55

4.2. Experimental

The polystyrene standard obtained from Scientific Polymer Products, Inc
(weight average molecular weight (M) = 75.7 kD, polydispersity index (PDI) =
1.17) was used as received. Fullerenes (060 or buckyballs) were purchased from
Term-USA and were used as received. Polystyrene nanoparticles (PSNP) were
synthesized from linear precursors (M... = 25, 60 and 140 kD).77 The procedure to
make the nanoparticle is to drip a solution of the linear precursor, which contains
20 mol% crosslinking groups, into hot solvent to activate the crosslinking process
and produce a nanoparticle from a single molecule. By varying the molecular
weight, and thus the size of the chain of the linear precursor, we are able to
obtain different size nanoparticles. The nanoparticles were rid of any silicon
containing impurities using a hydrofluoric acid wash. This was done by first
digesting any silicon compound by stirring the nanoparticles in a hydrofluoric acid
(HF) bath for several hours. Toluene was then added to the bath to dissolve the
nanoparticles. The mixture was then allowed to settle and separate, then the HF
layer was decanted off, leaving only the organic solution. The solution was then
dripped into methanol to precipitate the silicon-free nanoparticles which were
dried under vacuum at 40°C until all the solvent had been removed. As
discussed above the procedure was done to ensure that silicon contaminants did
not alter the surface energy of the PSNP which may have led to false wettability
of the nanoparticle filled films. A previous study in our lab demonstrated some

presence of silicon containing compounds in PSNP samples when x-ray photon

56

spectroscopy was performed necessitating the above procedure to ensure
accurate results.

Polystyrene/nanoparticle solutions were made using ACS reagent grade
toluene. Stock solutions of both polystyrene and the nanoparticles were first
made, and then mixed at the correct amount to get the desired overall mass
concentration as well as the desired mass concentration of nanoparticles to
polystyrene (0.01-20%). The resulting solutions were filtered using a 0.2 pm
PTFE filter before use. In addition, all solutions were sonicated immediately
before use to ensure the nanoparticles were dispersed.

Hydrophilic and hydrophobic silicon wafers were used to support the
polymer films. The hydrophilic silicon substrates were cleaned with an acid
solution of 70:30 by volume 96% H280. and 30% H202 for 1 hour to remove any
organic contaminants. The wafers were then rinsed with copious amounts of
Millipore water and dried under nitrogen. For these substrates, the solution was
spin-coated directly onto the wafer to produce the polymer film for subsequent
study. A silanizing agent Sigmacote® (Sigma-Aldrich), which is a clear, colorless
solution of a chlorinated organopolysiloxane ((SiCI2C4H9)20, 2.5%) in heptane
(97.5%) was employed to prepare the hydrophobic surfaces. The silicon wafer
was flooded with the solution for 1 minute and then the excess was rinsed off and
the wafer was dried under nitrogen. The surface energy of silicon wafers treated
with Sigmacote® was found to be 28 s: 1 mN/m based on contact angle

measurements with polar (water) and non-polar (methylene iodide) liquids while

57

the surface energy of a bare wafer was 71 :l: 1 mN/m. All wafers were used within
one day of treatment.

Spin-coating directly onto the hydrophobic substrates was not possible
due to their poor wettability. Instead, solutions were spin-coated onto freshly
cleaved mica sheets (Ted Pella, Inc.). The films were then transferred to the
hydrophobic substrates by one of two methods. The first was by floating the film
onto the surface of clean, deionized water and picking the film up with the
silanized wafer. The second method involved “flipping” the film onto the silanized
wafer. The mica sheet was first placed with the film side down onto the top of the
silanized wafer. Water was then dispensed into the gap, displacing the film and
causing it to fall to the hydrophobic substrate. All films were dried under nitrogen
to remove any water left behind by the transfer process. Film thicknesses were
determined to be 20-50 nm thick by ellipsometry.

Films were determined to be uniform and continuous upon inspection with
optical microscopy. Annealing of the films was performed by exposing them to a
saturated toluene environment to observe the dewetting process. Reflective
mode optical images (bright and dark field) were obtained in situ using an
Edmund Industrial Optics optical microscope with a JAI camera attachment.

Samples for neutron reflectivity measurements were prepared on 2 inch
diameter silicon wafers that were 1 mm thick to ensure the substrate would not
warp and give erroneous results. Samples were prepared in the same way as
described above for samples used in the dewetting experiments. Neutron

reflectivity experiments were carried out at two different facilities. The first was

58

the POSY2 reflectometer at the Intense Pulsed Neutron Source at Argonne
National Laboratory. The intensity of the instrument is 100 neutrons per pulse
with wavelengths (An) in the range of 2.5 - 16 A. The second was the SPEAR
reflectometer at the Los Alamos Neutron Scattering Center at Los Alamos
National Laboratory. Measurements of the reflectance (R) were taken at different
angles of incidence (0° < 0 < 3°) and then spliced together by fitting the data in
the partially overlapping wave vector (q) ranges. The overall q range that could
be obtained from the wavelengths and angles possible was 0 - 0.25 A'1 where q
= 41tsin0/itn. The reduced reflectivity data was analyzed using the program
Motofit.79 To do this, models were approximated which consisted of a series of
layers of constant concentration with discrete parameters. In addition, layer
roughness was included in the modeling using values on the order of 1-10 A. The
reflectivity of the model was then calculated and then the model adjusted until the
best fit was determined by minimizing 78 using the Levenberg-Marquardt

algorithm.

59

4.3. Results and Discussion

4.3.1 . Polystyrene/Polystyrene Nanoparticle Systems

We have reported in Chapter 3 that 25 kD PSNP (radius (a) = 2.1 nm) can
inhibit dewetting of thin PS films on a high energy substrate (piranha cleaned
silicon wafer) under solvent annealing.91 This inhibition occurs due to the fact that
the nanoparticles, which are initially uniformly dispersed throughout the film,
migrate to the polymer-substrate interface creating a jammed layer.18 If the
concentration of the nanoparticles is large enough, to ensure that at least a
tightly packed monolayer will form at the substrate, dewetting is inhibited. The
nanoparticle migration is caused by an overall system entropy gain.80 A polymer
chain next to the substrate loses many conformations and so suffers a large
entropy loss. By moving a nanoparticle to the substrate a translational entropy
loss of only ~kBT occurs, where k3 is Boltzmann’s constant and T, temperature,
which is much less than the entropy loss endured by each monomer unit in the
polymer chain near the hard substrate. This is actually quite a strong driving
force that scales with the nanoparticle size which is tempered a little by the loss
of mixing enthalpy between the polymer and nanoparticle at the monomer level.89
Regardless we have found that polystyrene nanoparticles strongly segregate to

‘8' 3° which we will also demonstrate

the solid substrate during thermal annealing
occUrs during solvent annealing below.
The surface area coverage (0 of the segregated nanoparticles can be

calculated through a simple mass balance from the equation:

60

3 h
6 = E£ZJ¢PSNP X 77

(4.2)
where h is the film thickness, ¢ps~p is the total volume fraction of nanoparticles in
the film and 77 is the packing denSity of spheres in two-dimensions, assumed to
be 0.9 for maximum 2-dimensional packing. Using equation 4.2, we can calculate
that the volume fractions needed to form a monolayer for larger nanoparticles as
discussed below.

Optical micrographs of PS + PSNP films on silicon wafers, which have
been annealed under a saturated toluene atmosphere, are shown in Figure 4.1.
The concentrations used in these cases are 17% for the 60 kD PSNP (a = 2.8
nm, 8: 1.2) and 23% for the 140 kD PSNP (a = 3.8 nm, 8: 1.2). It is clear from
the figure that the larger nanoparticles inhibit dewetting on a high energy
substrate just as the 25 kD PSNP has been shown to do in our previous work.91
The nanoparticles migrate to the substrate upon annealing, where they form a
jammed layer which shields the adverse van der Waals forces (Figure 4.1c).‘8' 3°

For a low energy substrate, the adverse van der Waals forces emanating
from the substrate are much stronger, and therefore dewetting is anticipated to
occur much faster than with an unstabilized film. In other words, the spreading
coefficient is highly negative (see equation 4.1) to result in a much faster
dewetting rate.27 An image of a pure PS film which was spin-coated onto freshly
cleaved mica and then flipped onto a wafer treated with Sigmacote® is presented

in Figure 4.2. Complete dewetting of PS on a high energy, silicon wafer substrate

occurs on the order of an hour of exposure to a saturated toluene atmosphere.

61

 

 

 

 

 

 

 

Figure 4.1. Optical micrographs of PS + PSNP films (~30 nm thick)
on piranha cleaned wafers annealed for 5 hours under a saturated
toluene atmosphere. a) P3 + 17% 60 kD PSNP (0:1 .2). b) PS +
23% 140 kD PSNP (9:1 .2). c) A schematic of how the monolayer of
PSNP stabilizes the film against the adverse van der Waals forces.
The arrows represent the extent of the van der Waals force
interaction.

62

 

Figure 4.2. Optical micrographs of a pure PS film (~30 nm thick) on

a silanized wafer after annealing under a saturated toluene

atmosphere for 15 minutes. Inset shows film prior to annealing.

From Figure 4.2, one can see that complete dewetting is achieved much more
rapidly for the lower energy substrate. After only 15 minutes, the once uniform
film has dewetted into an array of polymer droplets.

The ability of the PSNP to create a stable film on the lower energy
substrate was investigated. Images of annealed PS + 25 kD PSNP films are
shown in Figure 4.3 where we find that the concentration of nanoparticles is
crucial to the ability of the films to remain stable. From Figure 4.3, one can see
that although only a monolayer of nanoparticles is needed to inhibit dewetting on
the high energy substrates, more than a monolayer is needed to inhibit dewetting
for the low energy substrate. In Figure 4.3b, the concentration of nanoparticles

would result in a monolayer of nanoparticles at the substrate once they migrate.

63

 

Figure 4.3. Optical micrographs of PS + 25 kD PSNP films on a silanized wafer
after annealing under a saturated toluene atmosphere. a) PS + 5% 25 kD PSNP
after annealing for 15 minutes (0:05). b) PS + 10% 25 kD PSNP after annealing
for 15 minutes (6:1 .0.) 0) PS + 15% 25 kD PSNP after annealing for 5 hours
(6:1 4.) d) PS + 26% 25 kD PSNP after annealing for 5 hours (0:2. 5). All films
are ~30 nm thick.

However, dewetting occurs on the same time frame as that of pure PS.
Increasing the concentration slightly retards dewetting, however holes still form,
as seen in Figure 4.3c. If the concentration is raised high enough so that 2 layers
are present at the substrate, as in Figure 4.3d, inhibition of dewetting is achieved.

The stronger forces present from the lower energy substrate require a larger

concentration and hence a thicker layer of nanoparticles to inhibit dewetting.

Larger nanoparticles were then investigated to determine if they would
inhibit dewetting on low energy substrates. Images of PS films with larger PSNP
are presented in Figures 4.4 and 4.5. In Figure 4.4, one can see that for the 60
kD PSNP filled film, below a monolayer concentration (8: 0.9) dewetting is seen
(Figure 4.4a), as expected. At a concentration slightly above that needed to form
a monolayer (8: 1.4), dewetting is inhibited as shown in Figure 4.4b. The same
is true for the largest nanoparticles used, 140 kD. Below a monolayer
concentration (8 = 0.9) dewetting is observed (Figure 4.5a). Once the
concentration reaches that needed to form a monolayer (8 = 1.2), dewetting is

inhibited on the silanized wafer, as seen in Figure 4.5b.

 

 

 

Figure 4.4. Optical micrographs of PS + 60 kD PSNP films on silanized
wafers annealed under a saturated toluene atmosphere. a) PS + 12% 60
kD PSNP after annealing 30 minutes (0:0.9). 0) PS + 20% 140 kD PSNP
after annealing 1 day (0:1 .4). Films are ~30 nm thick.

65

 

 

 

 

Figure 4.5. Optical micrographs of PS + 140 kD PSNP films on silanized
wafers annealed under a saturated toluene atmosphere. a) PS + 16% 60
kD PSNP after annealing 30 minutes (0:09). b) PS + 23% 140 kD PSNP
after annealing 20 hours (0:1 .2). Films are ~30 nm thick.

The stability of polymer films on silicon substrates is dependent upon the
interplay of short- and long- range van der Waals forces acting across the film.
Seemann, et al. first investigated these interactions through the effective
interfacial potential of the system.” 83 They found that increasing the size of the
oxide layer of the silicon wafer can alter the stability of the film. Krishnan, et al.
furthered this work by including terms to incorporate an alkane chain brush as
well as a nanoparticle layer in the film, leading to a five layer van der Waals
theory for the interfacial potential within the non-retarded approximation.92 They
found that increasing the size of the alkane chain at the substrate enhances
dewetting, which is similar to what we observe through the addition of a silanized
layer (Sigmacote®) on our silicon wafers. In addition, they found that increasing
the size of the nanoparticle layer leads to stabilization of the film, consistent with
our results. We have found that the layer thickness needed to shield the van der

Waals forces and stabilize the film against dewetting on a Sigmacote® treated

66

wafer is on the order of 5 nm, the size of the 60 kD PSNP or a double layer of 25
kD PSNP.

Therefore, PSNPs are able to inhibit dewetting on both low and high
energy substrates under solvent annealing. This inhibition is dependent upon
creating a layer thick enough to shield the adverse van der Waals forces when
the nanoparticles migrate to the polymer-substrate interface. In the case of the
high energy substrate, these forces are not as strong, so even the smallest
PSNP can inhibit dewetting assuming a full monolayer is present to create a
jammed state. This jammed layer acts as a buffer at the substrate, shielding the
adverse forces and immobilizing the film, eliminating dewetting from occurring.
However, for the lower surface energy, the forces are much stronger, therefore,
either a second layer of nanoparticles is needed (25 kD PSNP) or a single layer
of larger nanoparticles (60 or 140 kD PSNP) are required to shield or retard the

forces.

67

4.3.2. Polystyrene/Fullerene Systems

We have previously reported that the addition of 060 nanoparticles to thin
polystyrene films can inhibit dewetting upon solvent annealing.” 9‘ The ability of
the fullerenes to inhibit dewetting is dependent upon the formation of a fullerene-
enriched layer at the substrate which shields the adverse surface forces
emanating from the substrate, as well as increasing the local viscosity prohibiting
the film from moving. Simply putting a layer of fullerenes on the substrate and
then placing a pure PS film on top does not inhibit dewetting, therefore this is not
merely a surface energy effect.

Fullerenes were added to PS films in order to determine their ability to
promote wetting on high energy versus low energy substrates. Optical
micrographs of PS with 3 wt% 030 films are presented in Figure 4.6. It was not
possible to produce a uniform film on the low energy substrate by directly spin-

coating the polymer solution. Therefore, these films were spin-coated onto

 

 

 

 

Figure 4.6. Optical micrographs of PS + 3 wt% 060 films (~30 nm thick)
spin-coated onto mica and floated onto a) a piranha cleaned wafer and
annealed for 5 hours and b) a silanized wafer and annealed for 15
minutes. Annealing was performed by exposing the films to a saturated
toluene atmosphere. Insets show films prior to annealing.

68

freshly cleaved mica, floated onto filtered water and then picked up with the
substrate. This was done for both the low and high energy substrates to ensure
that there would be no difference in the films. We have previously shown through
neutron reflectivity experiments that the fullerenes separate to the polymer-
substrate interface upon spin-coating. Therefore, there is a fullerene-enriched
layer ~ 2 nm thick at the substrate containing ~30 vol% 060 fullerenes.38 For the
high energy substrate (piranha cleaned silicon wafer) shown in Figure 4.6a, it is
clear that dewetting is inhibited as seen before for films spin-coated directly onto
a wafer. Therefore, the manufacturing process of spin-coating onto mica and
transferring the film onto a wafer does not alter the ability of the nanoparticles to
inhibit dewetting.

However, a different result is seen in Figure 4.6b. Here, the fullerenes do
not inhibit dewetting on the low energy (Sigmacote®) treated silicon wafer. Due
to the fact that the surface energy is much lower in this case, the adverse forces
initiating dewetting are much stronger, as discussed earlier and quantified by
Krishnan, et al.92 Because these forces are much stronger in the case of the
Sigmacote® substrate, the 2 nm fullerene enriched layer does not shield them
promoting dewetting. This is expected due to the fact that the 25 kD PSNP,
which are larger than 2 nm, were not able to shield these forces at a monolayer
coverage.

An alternative method to manufacture the films was tried to see if
dewetting could be eliminated. Spin-coating of the polymer solution was done on

mica and then the film was transferred to the wafer by flipping it over. Our

69

 

Figure 4.7. Optical micrographs of PS + 3 wt% 060 films (~30 nm thick)
spin-coated onto mica and flipped onto a) a piranha cleaned wafer and
annealed for 1 hour and b) a silanized wafer and annealed for 15 minutes.
Annealing was performed by exposing the films to a saturated toluene
atmosphere. Insets show films prior to annealing.

previous work has shown that by flipping the film onto the silanized wafer, the
fullerene-enriched layer is located at the air interface prior to annealing.91 Optical
micrographs of PS with 3 wt% 060 films flipped onto the substrate are presented
in Figure 4.7. In this case, the films on both the high energy and low energy
substrates dewet upon exposure to a saturated toluene atmosphere. It was
expected that the nanoparticles would migrate to the substrate to inhibit
dewetting as has been seen with the polystyrene nanoparticles,91 however
dewetting still occurred, even in the case of the high energy substrate.

In order to determine the reasoning behind dewetting for the nanoparticle
polymer films where the fullerenes were initially located at the air interface,
neutron reflectivity experiments were performed on a variety of substrates. Films

composed of linear polystyrene, 060 and PSNP were spin-coated onto mica and

70

flipped onto silanized wafers. The low energy substrates were used to produce a
smoother film to decrease error and noise in the reflectivity data. A low energy
substrate results in a smoother film due to the fact that when the film is
transferred to the substrate from mica with water, the water will be preferentially
attracted to the high energy or hydrophilic surface, mica, displacing the film
evenly. If a high energy substrate had been used, the water would not be
preferentially attracted to one surface, resulting in an uneven transfer of the film.
The PSNP were added to ensure that the film would remain stable on the
silanized water as discussed above. Two different PSNP were used to eliminate
dewetting, 60 kD and 140 kD, chosen because we have previously shown that
they both are large enough to shield the adverse van der Waals forces and
eliminate dewetting on the low energy substrate for a monolayer coverage, the
condition we use here. Due to the fact that the background film is composed of
protonated PS, it is not possible to distinguish between the bulk film and the
PSNP due to the fact that they have the same scattering length density (SLD) of
1.41x106 A'2 to the neutrons. However, it is possible to determine the location of
the fullerenes in the film because there is sufficient contrast in the SLD of 060
(5.73x10'6 A'2) compared to protonated PS. In both cases, similar results were
found and are presented in Figures 4.8 and 4.9, with the data and model fits in
Figures 4.8a and 4.9a and the scattering length density profiles in Figures 4.8b
and 4.90. Before annealing, the fullerenes are located in an enriched layer at the

air interface, as seen before, since the film was flipped.91 This can be seen by

71

(a)

10'8 1:1 PS+60 kD PSNP+3% 060 before annealing
o PS+60 kD PSNP+3% C60 after annealing

Rq4 (if)
orb

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

002 004 _1 0 0.08 0.10
(b) q (A I
3.0 :
NA 1 ' 60 1
ii , L—r
so 1.5 _-_., 1" t .
E 1’ i
1.0 -- -- ~ ~-
0-5 " — PS+60 kD PSNP+3% 06,, before annealing ‘
---- PS+60 kD PSNP+3% C60 after annealing
0.0 ' " : . . . .

 

 

 

 

 

0 50 1CD 150 200 250
<--- air h (A) substrate -—->

Figure 4.8. a) Neutron reflectivity data and models for a PS + 60 kD PSNP + 3
wt% Ceo flipped film before and after annealing under at saturated toluene
atmosphere for 2.5 hours. b) The scattering length density profiles for the models
used in a). The film was formulated to ensure that the PSNP are at sufficient
concentration to form a monolayer.

U PS+14OK PSNP+3% 060 before annealing
o PS+140K PSNP+3% C60 after annealing

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

0.02 0.04 A4 0.06 0.08 0.10
(b) GI I .
3.0 _
NA I 60] .
=5, I ,
«92 1.5 ‘, .
E I
1.0
0-5 ""' — PS+140K PSNP+3% 06,, before annealing '
---- PS+140K PSNP+3% 060 before annealing
0'0 _ .. . . .

 

 

 

 

0 100 2(1) 300
<--- air h(A) substrate -—->

Figure 4.9. a) Neutron reflectivity data and models for a P8 + 140 kD PSNP + 3
wt% 060 film before and after annealing under a saturated toluene atmosphere
for 5 hours; b) The scattering length density profiles for the models used in a).

The film was formulated to ensure that the PSNP are at sufficient concentration
to form a monolayer.

73

the large SLD at the air interface followed by a homogeneous SLD profile within
the film.

However, after annealing an interesting observation is made. There is a
layer of pure polystyrene located at the substrate interface for both cases, with a
thickness approximately the same as the diameter of the PSNP used (5.5 nm for
60 kD and 7.5 nm for 140 kD). This is suggested from the SLD profile by the dip
in the SLD next to the substrate to a value equal to that for pure protonated
polystyrene. Using other SLD profiles did not represent the data as well which we
confirmed through extensive modeling. It is assumed that these layers are the
polystyrene nanoparticles which have migrated to the substrate to inhibit
dewetting and have excluded the Can fullerenes. The fullerenes, on the other
hand, move away from the air interface, but they do not diffuse down to the
substrate during the annealing time as anticipated. Instead, a uniform dispersion
of the nanoparticles in the film is seen located above the PSNP layers. Thus, the
fullerenes are soluble in the polystyrene matrix at this concentration and do not
preferentially migrate to the substrate most likely by a combination of enthalpic
and entropic forces. Entropically, there is not much gain for the polymer to push
the fullerenes to the substrate since they are so small and almost equivalent to
the monomer size. Enthalpically, there may be enhanced mixing energy between
the polystyrene monomer units with the fullerene surface since we have found it
is possible to disperse ~2 vol% fullerenes (~3.5 wt%) in polystyrene.89

The surface energy of a pure fullerene layer is quite high and so during

the PSNP stabilized annealing period they are driven from the air interface and

74

mix with the PS presumably by the arguments given above. We find it interesting
though that the spin-coated layer of fullerenes ~2 nm thick on the silicon wafer
interface remains stable during annealing and in fact eliminates dewetting. The
spin-coating process must create a rather delicate layer containing ~30 vol%
fullerenes38 that is stable, or metastable and trapped in this configuration, to

produce a macroscopically stable system.

75

4.4. Conclusion

In this work we have shown that dewetting of thin polymer films upon
solvent annealing can be inhibited using nanoparticles. PSNP can inhibit
dewetting in PS films on both hydrophilic and hydrophobic substrates. In the case
of hydrophilic substrates, a monolayer of nanoparticles must be present at the
substrate to form a jammed state, shielding the adverse van der Waals forces.
On a lower energy substrate, these forces are stronger. Therefore, either a
higher concentration of smaller nanoparticles are required (25 kD PSNP) or a
monolayer of larger nanoparticles are needed (60 or 140 kD PSNP). Fullerenes,
which have been shown to inhibit dewetting on a hydrophilic substrate in a
previous study,91 are unable to inhibit dewetting on the lower energy substrates.
The ~2 nm fullerene enriched layer that forms at the substrate upon spin-coating
is not able to retard the stronger van der Waals forces present from the
hydrophobic substrate. Additionally, if the film is initially flipped so that the
fullerene layer is at the air interface, dewetting occurs for both the hydrophilic and
hydrophobic substrates under a saturated toluene atmosphere. This is due to the
fact that the fullerenes do not segregate to the polymer-substrate interface upon
annealing. A unique system containing fullerenes, polystyrene nanoparticles and
linear polymer was used to argue this where we found the fullerenes will
homogeneously distribute through the film during annealing. Thus, during spin-
coating a delicate but robust thin layer of fullerenes forms which can stabilize a

polymer film under not too adverse conditions.

76

Chapter 5

Application of Technology to Chemical Sensors

5.1. Introduction

In the present work, we look at the issues associated with polymer film
dewetting in sensor applications. Upon exposure to saturated solvent
atmospheres, polymer films are plasticized, reducing their T9 and allowing for
dewetting to occur. This type of dewetting has been seen in the development of
chemical sensors,9 since the sensors are tested, or “challenged” with streams of
saturated solvents to test their response before use.

One common type of sensor used is a surface acoustic wave (SAW)
device. These devices have input and output transducers patterned on a
piezoelectric substrate. When the input transducer is excited, it generates a
surface acoustic wave that propagates across the substrate creating an output
signal at the receiving transducer. To make the device a sensor, a polymer
coating designed to selectively absorb a specific analyte is deposited onto the
substrate along the propagation path. When the analyte is present, it is absorbed
by the polymer coating causing a minute increase in the mass of the film that can
then be measured by a shift in the SAW operating frequency relative to an
uncoated reference device. When dewetting occurs in a sensor, the output
frequency can become erratic or stop altogether due to the reduced analyte
detection area, leading to a faulty sensor.9

The. usual approaches for stabilizing thin polymer films often involve

modification of the polymer, the substrate or the interaction between the two,

77

such as crosslinking,1o sulfonation and metal complexation,11 surface
treatments,12 or grafting."3 These methods, however, may impact the absorption
sensitivity of the film or may not be appropriate for rapid sensor manufacture and
thus cannot be used in sensor applications. Consequently, an alternate method
for eliminating dewetting is needed.

In this work it has been shown that the addition of nanoparticles to
polymer films can inhibit dewetting under solvent annealing. The location of the
nanoparticles at the polymer-substrate interface upon annealing leads to
stabilization of the film. in the work presented here, this novel strategy of using
nanoparticles to inhibit dewetting is adopted for use in vapor annealed polymer
films used in chemical sensors. Fullerenes (060) will be added to various
polymers used in sensor applications to determine their ability to eliminate
dewetting when the polymer films are exposed to various solvent vapors. In
addition, sensor performance will be tested with and without nanoparticles to

determine their influence.

78

5.2. Experimental

Polyepichlorohydrin (PECH) was purchased from Scientific Polymer
Products, Inc. with an approximate value of weight average molecular weight
(M) = 700 kD. Modulated differential scanning calorimetry (MDSC) was used to
determine the glass transition temperature (T g) of PECH which was
approximately -22.5QC (TA Instruments Q1000). DKAP polymer, a
poly(dimethylsiloxane) derivative utilized in sensor applications was synthesized
at and provided by Sandia National Laboratories with a reported T9 of ~ 20°C.
BSP3, another polymer with a T9 below room temperature which is used in
sensing applications, was also provided by Sandia National Laboratories.
Fullerenes (Cso or buckyballs) were purchased from Aldrich Chemical Company
and were used as received.

Polymer/nanoparticle solutions were made using ACS reagent grade
chloroform or toluene. Stock solutions of both polymers and the nanoparticles
were first made, and then mixed at the correct amounts to get the desired overall
mass concentration as well as the desired mass concentration of nanoparticles to
polymer (0.01-20%). The resulting solutions were filtered using a 0.2 pm PTFE
filter before use. In addition, all solutions were sonicated immediately before use
to ensure the nanoparticles were well dispersed.

Initial dewetting samples were manufactured by spin-coating the solutions
onto silicon wafers cleaned with an acid solution of 70:30 volume ratio of 96%
H2804 and 30% H202 for one hour, rinsed with Millipore water and dried with

nitrogen. Spin-coating was done using a speed of 5000 rpm for 40 seconds,

79

producing films 20-50 nm thick per ellipsometry. In addition, spray-coating was
utilized to prepare samples to mimic the actual manufacture of chemical sensors.
This was performed using a syringe pump which fed polymer solution to a
nebulizer (Burgener Research Inc.) at a flow rate of 5 thr along with a
regulated stream of nitrogen at 50 psi. The resulting mist was sprayed through a
300 by 800 um shadow mask for various times to produce differing film
thicknesses. In addition to coating acid cleaned silicon wafers, the polymer
solutions were also sprayed onto as-received functional sensors supplied by
Sandia National Laboratories to determine the influence of the coating procedure

on their performance.

80

5.3. Results and Discussion

5.3.1. Dewetting Inhibition

Polymers commonly used in sensor applications were studied to
determine if the dewetting inhibition effect demonstrated by fullerenes upon
solvent exposure was a phenomenon specific to polystyrene, or a general
phenomenon independent of the polymer system. Dark field optical micrographs
of the dewetting process for one of the polymers of interest in sensor
applications, polyepichlorohydrin (PECH), are shown in Figure 5.1. The films
were spin-coated onto silicon wafers and then exposed to a saturated methanol
atmosphere which is a solvent for PECH. As seen with the pure polystyrene, the
pure PECH film dewets upon exposure to the solvent atmosphere as shown in
Figure 5.1a. After 45 minutes, hole formation is observed and after 20 hours of
exposure to methanol vapor a distinct breakup of the film is seen. Absorption
experiments using PECH and methanol were performed as described previously

in Section 2.3. It is found that the amount of methanol absorbed by the PECH is

     

- . _ .‘_-, ._ . ' L 351 om [,- . y . ‘ .

Figure 5.1. Optical micrographs of spin-cast (a) pure PECH and (b)
PECH + 1 wt% 060 films after exposure to a saturated methanol
atmosphere for 20 hours. Insets show films prior to annealing.

81

much less than that seen in the case of toluene and PS, leading to a much higher
polymer concentration (~1035 mg/ml). Because of this, the solution To is
calculated to be ~ -65°C (assuming a similar T,I for methanol to that for toluene),
and the viscosity is several orders of magnitude larger, explaining why the hole
growth is much slower for this system.“

A PECH film with 1% by mass addition of fullerenes after exposure to
methanol for 17 hours is shown in Figure 5.1b. Again, addition of the fullerenes
inhibits, and at this time scale, eliminates dewetting of the polymer film. The
white dots represent dust particles deposited during the annealing process
exemplifying the robustness of this technology since particles frequently act as
nucleation sites for dewetting." 3‘

Evaluating the influence of fullerene nanoparticles on sensor performance
requires spray-coating of the polymer solutions onto the SAW substrate. Unlike
spin-coating, spray-coating does not result in a uniform, smooth film, at least for
the conditions used in the current procedure. In addition, we were not able to
observe the coating as it is being deposited; therefore it is unclear whether a
poor coating is inherent to the spraying process or results from rapid dewetting
once the polymer is on the substrate. Despite this film non-uniformity the above
spray-coating procedure was followed to mimic an actual procedure used to
manufacture SAW sensors. Bright field optical micrographs of spray-coated
PECH onto piranha-cleaned silicon wafers are shown in Figure 5.2. As

demonstrated in the insets, the coatings are inhomogeneous and of varying

82

    

Figure 5.2. Optical micrographs of spray-coated films after

exposure to a saturated methanol atmosphere. (a) Pure PECH after

42 hours and (b) PECH + 10 wt% 060 after 47 hours of exposure.

Insets show films prior to annealing.
thicknesses with gaps between deposited polymer on the silicon wafer. Once the
pure PECH coating is exposed to a saturated methanol vapor the same
dewetting effect is present with the polymer dewetting from the surface forming
droplets, just as seen polystyrene films as shown in the previous chapters. In the
case of PECH with 10% fullerenes by mass (Figure 5.2b), the dewetting is
inhibited completely. Here, it is possible to use 10 wt% fullerenes due to the fact
that the initial fullerene concentration in the solution was only 0.05 mg/ml so
aggregates do not form. It is clear that gross dewetting, seen in Figure 5.2a, does
not occur when nanoparticles are present. In fact, in the presence of fullerenes, it
appears that exposure to the saturated solvent atmosphere has increased the
overall wetted area of the polymer on the substrate.

To further generalize this phenomenon another system was studied with a
bright field optical micrograph of the wetting/dewetting process for the polymer
DKAP shown in Figure 5.3. A solution of pure DKAP in chloroform was spray-

coated onto a silicon wafer and exposed to methanol vapor for 17 hours (Figure

83

 

ated (a) pure DKAP
and (b) DKAP + 10 wt% C50 films after exposure to a saturated
methanol atmosphere for 17 hours. Insets show films prior to
annealing.

5.3a), with the inset showing the coating immediately after spray-coating. It is
clear that the coating has dewetted and the polymer is agglomerated for less
surface coverage. However, as shown in Figure 5.3b, the coating consisting of
DKAP with 10% fullerenes by mass demonstrates a different behavior. Again,
this coating was spray-coated onto a silicon wafer prior to exposure to a
saturated methanol atmosphere for 17 hours. No appreciable signs of dewetting
are seen and the coating surface coverage appears to have remained constant,
thus the presence of the nanoparticles has again inhibited dewetting. So,
dewetting is prevented regardless of whether the coatings are initially smooth
and uniform (spin-coating) or non-uniform (spray-coating) and it is suspected that

the solid-like layer also forms during the spray-coating procedure.

84

5.3.2. Sensor Performance

Live sensors that exhibit dewetting under similar conditions to those above
were tested to discern the influence of nanoparticles on wetting behavior as well
as sensor performance. SAW sensors used in Sandia National Laboratories’
uChemLabTM chemical sensor were used for the tests. A bright field optical
micrograph of a live SAW sensor with polymer films produced by spray coating is
shown in Figure 5.4. The left side of the sensor has a coating of PECH with 10%
fullerenes by mass while the right side has been spray-coated with pure PECH. It
is clear through observation of the figure that the coating of polymer with
fullerenes has wetted the substrate to a greater degree than the coating
consisting of pure polymer, which produces a very poor coating. Once again, it is
unknown whether the poor film results during the coating process or from
dewetting aftenNards. Nonetheless, it is clear that the presence of the
nanoparticles results in a superior film.

A goal of this work was to determine if the addition of nanoparticles to

 

 

Figure 5.4. Optical micrograph of a live SAW sensor.
The left side (Line 1) is coated with PECH + 10 wt%
060. The right side (Line 2) is coated with pure PECH.

85

polymer coatings can result in more stable, reproducible sensors. To this end,
the response of live sensors having coatings with and without the addition of
fullerenes was observed. The sensors were exposed to various solvents to test
their response, including: dimethyl methyl phosphonate (DMMP), methyl
salicylate (MS), water and methanol (MeOH), in that order. The compounds
DMMP and MS are surrogates for aggressive biochemical agents. Methanol
vapor represents a chemical that produces gross dewetting in most sensors and
water was tested due to its ubiquitous presence. A saturated vapor stream of
each solvent was sprayed across the live sensor for a period of five seconds,
three times each.

The response curve for a live SAW sensor is shown in Figure 5.5 with one
line containing pure PECH and another line with PECH containing 10%
fullerenes by mass (Figure 5.4). The curves have been shifted to allow for easier
viewing with the peaks in the response curve corresponding to each challenge
with solvent vapor. The response curves for the pure PECH film are shown in
Figure 5.5a with curve A showing the response immediately after film deposition.
The response is very erratic with an unstable baseline, leading to the belief that
the polymer is dewetting from the sensor surface during the exposure. After one
day, the same sensor was challenged, and as can be seen in curve B, the sensor
shows no response at all and the sensor is effectively inoperable. Comparing this
result to the optical micrographs shown in Figure 5.2 as well as Figure 5.4, it is
clear that a non-uniform film that has dewetted from the substrate is not desirable

for satisfactory sensor performance.

86

 

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9 o
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o
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Reference Time (seconds)

 

 

 

Figure 5.5. Response curves for a live SAW sensor.
(a) Response to challenges for line 2 coated with pure
PECH (A) immediately after spray-coating and (B)
after sitting for 1 day. (b) Response to challenges for
Line 1 coated with PECH with 10 wt% 060 (C)
immediately after spray-coating, (D) after sitting for 3
days and (E) after spraying more solution 4 days after
' the initial deposition.

87

The line with the fullerene/PECH film was challenged and the response
curves are shown in Figure 5.5b. Curve C was the response immediately after
the film was sprayed. Three days later the same sensor was challenged again
and the response (curve D) demonstrates that the sensitivity to DMMP has
actually gotten stronger while the others have decreased in magnitude. In curve
E, more fullerene/PECH solution was sprayed onto the line four days after the
initial deposition, to determine if a stronger sensor response occurred, and the
device was challenged again clearly revealing a stable response with increased

sensitivity to DMMP.

88

5.4. Conclusion

The ability to inhibit dewetting of polymer films with the addition of
nanoparticles has been found to be possible with the use of a variety of different
polymers. Therefore, this appears to be a general phenomenon which would
allow for the possibility of use on many types of sensor polymers. Thus,
nanoparticles, in particular fullerenes, can greatly increase the robustness of
SAW and other sensors that require thin polymer films due to their greater
stability. By using this technique, it is possible to manufacture more robust,
reliable sensors. Note that the films are extremely thin, requiring only a minute
amount of nanoparticles, and so the overall cost of the particles is not prohibitive

to their utilization.

89

Chapter 6

Summary and Conclusions

In this work we have shown that when a polymer is exposed to a saturated
solvent atmosphere, the film is plasticized, lowering both its viscosity and glass
transition temperature, potentially leading to dewetting. When dewetting occurs,
holes first rupture and then grow in size until they coalesce, eventually forming a
pattern of droplets through Rayleigh instabilities. The time scale of dewetting for
polystyrene films annealed under a saturated toluene atmosphere has been
shown to be dependent upon both the molecular weight of the films and the initial
film thickness, different from what has been seen in thermal annealing. Here, the
films exhibit shear-thinning behavior due to the large strain rate they experience
as they move across the substrate. Dewetting of the polymer films can be very
detrimental due to the fact that many applications require the films to remain
stable over time.

It has been shown in this work that the addition of nanoparticles to
polymer films can eliminate dewetting. Fullerene nanoparticles eliminate
dewetting of polystyrene films on silicon wafers upon exposure to a saturated
toluene atmosphere due to presence of a fullerene enriched layer approximately
2 nm in thickness at the polymer-substrate interface, as determined by neutron
reflectivity. The enriched layer forms during the spin-coating process due to
phase separation of the fullerenes because of their poorer solubility. The
fullerenes remain in this layer even after the film is plasticized by the solvent

atmosphere due to the favorable interfacial interactions between the fullerenes

9O

and the substrate. It is speculated that the polymer chains are not wholly
confined in this layer, as its thickness is less than the radius of gyration of a
polystyrene chain (~7 nm). Therefore, the fullerenes are dispersed in segments
of polymer chains that also extend into the pure polystyrene layer above. There
is an optimum concentration of fullerenes which must be used in order to
eliminate dewetting. Below this concentration, holes will develop in the film and
grow, although complete dewetting may not occur. Above the optimal
concentration, the fullerenes will aggregate forming micron sized particles which
can act as nucleating sites and induce dewetting.

It has also been shown in the current work that polystyrene nanoparticles
can also inhibit dewetting of thin polystyrene films. In this case, there are
minimum monomeric enthalpic interactions between the nanoparticles and the
polymer to represent a more ideal system. In addition these nanoparticles are
more soluble in the solvent than fullerenes, therefore phase separation does not
occur during the spin-coating process. Reflectivity data has verified that the
polystyrene nanoparticles are initially homogeneously dispersed throughout the
film. Upon annealing, however, the reflectivity data shows that the nanoparticles
migrate to the polymer-substrate interface and inhibit dewetting of the film
provided there is a large enough concentration to form a jammed layer at the
substrate. The segregation of nanoparticles can be attributed to a gain in entropy
due to the nanoparticles located at the substrate as opposed to the linear

polymer chain. Also, through the calculation of the Hamaker constant of the film,

91

it is verified that with the nanoparticles at the substrate, the film will remain
stable, as opposed to being unstable if they were to move to the air interface.

The fact that the fullerenes and polystyrene nanoparticles inhibit dewetting
of polystyrene films through different mechanisms can be explained due to their
chemical and physical differences. Fullerenes are chemically dissimilar to
polystyrene and are less soluble in toluene. Therefore, the fact that we see a
phase separated layer upon spin-coating is understandable. In addition, they are
found to aggregate, which at high enough concentrations can result in the
formation of clusters of aggregates that are micrometers in size. Polystyrene
nanoparticles, however, have essentially the same chemical makeup as the
polymer chains in which they are dispersed and are more soluble in solution.
Because of this, phase separation does not occur during spin-coating and the
polystyrene nanoparticles do not aggregate as the fullerenes do to form colloidal
scale particles, which can initiate dewetting. Therefore, there is no maximum
concentration of nanoparticles that can be used to eliminate dewetting in the
films.

The surface energy of a substrate is important to the wetting behavior
induced by the nanoparticles. Due to the much lower surface energy of a
silanized water, there are stronger adverse van der Waals forces to initiate
dewetting. Unfortunately, it was found in this work that the fullerene nanoparticles
were not able to eliminate dewetting on the lower energy substrates. The 2 nm
layer that is formed during spin-coating is not thick enough to shield the polymer

film from the substrate, and so it is able to rupture and dewet. Polystyrene

92

nanoparticles are much larger however, and therefore they have been shown to
inhibit dewetting on a silanized wafer. Again, the mechanism for dewetting
inhibition is the nanoparticles segregating to the substrate forming a jammed
layer.

Finally, we have been able to apply the result found herein to the
application of chemical sensors. Fullerenes were shown to be able to inhibit the
dewetting of several specific polymers used for detection of harmful agents in
sensor applications. When the polymers are exposed to a variety of saturated
vapors, they dewet in a manner similar to that of polystyrene. However, the
addition of the fullerenes to these polymers eliminated dewetting regardless of
whether the films were initially smooth and uniform (spin-coated) or not (spray-
coated). Additionally, tests on a live SAW sensor showed that the film with
fullerenes gave a more stable response and proved more reliable than the film

without nanoparticles.

93

Appendices

94

Appendix A

Principles of Neutron Reflectivity

With technology moving to the nano-scale, many different techniques are
required to characterize materials with sub-nanometer accuracy. One such
technique which is very popular is that of neutron reflectivity. Neutron reflectivity
is a technique which uses neutrons to determine the depth-composition profile for
thin planar samples, giving detailed information about the structure and interface
of a sample.93 It is similar to the techniques of x-ray reflectivity and light
ellipsometry, however it can be more useful in the characterization of polymers.
The reason for this is due to the significant difference in which a neutron interacts
with different isotopes of elements. The neutron analog to the difference in
electron density when working with x-rays and refractive index with light is the
coherent neutron scattering length (be). This value is significantly different for a
deuterium atom versus a hydrogen atom, allowing for the possibility to label
specific materials and produce a large contrast compared to the protonated
counterpart. In addition, neutrons are able to penetrate through many
engineering materials without being absorbed, which is a problem in the other
techniques. Finally, neutron reflectivity is a non-destructive technique that does
not require special operating conditions, making it applicable to many
experiments.

Previously it was stated that neutron reflectivity operates similar to
reflectivity using x-rays and light. Light reflects and refracts depending upon the

refractive index of the material. This is the same case for neutron reflectivity,

95

where here the neutrons reflect and refract depending upon the neutron

refractive index (n,) of the material as given by the equation:

1_ lip.
2n

"-=

J (A-1)

where xln is the neutron wavelength and ,02 is a quantity known as the scattering
length density (SLD). The SLD of a material is given by the sum of the products
of the atomic number density (N) and the coherent scattering length (be) for each
element. When a beam of neutrons at an incident angle of 6, hits an interface
between two media with neutron refractive indices n,- and n,-+1, it will either reflect

at an angle equal to 6), or will refract with an angle 3+1 according to Snell’s law:
n J. cos 6?]. = nj+1 cos 6’].+1 (A2)
At the air interface of the sample (no=1) this equation becomes:
cos «9, = r21"1 cos 60 (A.3)

There is a critical angle (60) below which the angle of refraction is zero, thus
cosflc=n1. The position of this critical angle provides information about the
average SLD of the sample. Above this angle, reflection and refraction of the
incident beam occurs and the reflectivity measured contains information about
the change in SLD over the depth of the film.

One can use the Fresnel reflection coefficient given by:

nj Sln 6’]. - n].+1 sm l9].+1

 

r. =

n+1 _ . - (A.4)
nj sm (9]. + nj+1 sm (9}.+1

to determine the reflectivity, R, given by:

96

R = r 13141 (AS)
(the actual Fresnel reflectivity is given by the product of the Fresnel reflection
coefficient and its complex conjugate, but because the materials used in this
study do not have complex components, it is simply the reflection coefficient
squared). By imposing the boundary condition that the wave vector transfer in the
direction perpendicular to the surface (qz) must be equal on both sides of the

interface, we can write equation A5 in terms of Q2 in the form:

- ~2
%

 

R(q) = )6

(A5)

 

 

Where qc is the critical value below which point total reflection occurs and q2 is

given by:

47: .
61z = 73111 5’ (A.7)

n

where 0 is the angle of incidence. An interesting note is that above qc the
reflectivity drops off proportional to qz" at large values of qz for sharp interfaces.
Therefore, reflectivity is usually plotted as qu‘ versus qz as the data
asymptotically reach a limiting value.

Two different instruments were used to collect neutron reflectivity data in
the work presented here. The first was the POSY2 reflectometer94 (see Figure

A.1) of the Intense Pulsed Neutron Source at Argonne National Laboratory and

97

Shutters

    
 

Collimator .
Goniometer

3He Linear PS

Figure A.1. Schematic of the POSY2 reflectometer at
Argonne National Laboratory.

Target
Moderator Slits Detector

    
 
 
 

Incoming
protons

f
/

Beam

Sample Changer/ Stop
Goniometer

 

    

l
Collimator Choppers Mirror

Figure A.2. Schematic of SPEAR at Los Alamos National Laboratory.

98

the other was the Surface Profile Analysis Reflectometer (SPEAR, see Figure
A2) of the Lujan Center at Los Alamos National Laboratory. Both instruments
are pulsed source time-of-flight reflectometers. The neutrons are produced by the
method of spallation, where protons hit a metal target, releasing neutrons from
the target’s nucleus. The neutrons then pass through moderators, choppers and
collimators which select only neutrons with the appropriate energy and
wavelength as well as focus the beam to the desired size. These instruments use
a neutron beam which has a range of wavelengths of roughly 2-14 A. To obtain a
greater q range, measurements are also taken at various angles of incidence
(0.4-2.5°) and the data are combined. These values allow for a possible q range
of about 0.007-0.25 A“. After the neutrons hit the sample, the sum of the
reflected waves (see Figure A3) is measured using a 3He linear position
sensitive detector. Based upon the time to reach the detector and the position,
the intensity of the sum of the reflected neutrons can be determined.

Samples for neutron reflectivity were prepared on 2 inch diameter silicon
wafers. The samples were either spin-coated directly onto a clean silicon wafer,
or spin-coated onto a freshly cleaved mica surface and transferred to a

hydrophobic silicon wafer through the use of water. The wafers used for

 

 

Figure A.3. Schematic of the reflection and refraction of a beam of
neutrons through a thin film (1) on a substrate (2) in air (0).

99

experiments were at least 1 mm in thickness to avoid the potential for the wafer
to warp, thus ensuring that only one reflection would be seen. The films used
ranged in thickness from about 15-40 nm. The POSY2 instrument required the
samples to be positioned vertically, while the samples were placed horizontally in
the SPEAR instrument. Due to the low flux available to the POSY2 instrument,
measurement times ranged from 24-40 hours depending on the sample
composition. For SPEAR, measurement times were much shorter at 2-3 hours
per sample. In addition, the SPEAR instrument allowed for the possibility to
measure samples in situ through the use of an aluminum chamber. The sample
was placed on a platform while a reservoir below the sample was filled with
solvent. The chamber was then closed allowing it to become saturated with
solvent vapor. The chamber was invisible to the incident neutrons and therefore
only the sample reflectivity was measured.

Experiments were designed so that there is sufficient contrast between the
different materials used in order to “see” nanostructures using neutrons. By using
materials with different SLDs, and thus different neutron refractive indices, it is
possible to determine the composition throughout the film. The different materials
used in this work and their scattering length densities are listed in Table A.1.
From the table one can see that there is a large difference between the PS and
dPS SLDs. By simple isotopic substitution, it is possible to create a material that
is chemically very similar, but can still be differentiated through the use of
neutrons. Another observation is that there is also a significant difference in the

SLD of 060 compared to PS. Therefore, the samples used for the experiments

100

Table A.1. Scattering length densities (SLDs) for the
materials used in the reflectivity studies.

 

 

 

 

 

Material SLD (x106) A-z
PS X 1.41
dPS 6.42
060 5.73
d-Toluene 5.67

 

 

 

 

presented in this work consisted of PS with 060 nanoparticles and dPS with PS
nanoparticles. The films which used a background of PS required longer
measurement times due to the fact that the hydrogen atoms created a large
amount of incoherent scattering.

Once data are collected by the detector, they must be properly reduced.
First, the reflected beam intensity is normalized to the intensity of the incident
beam. This is done by measuring the direct beam in what is called a transmission
run, where no sample is present and the beam hits the same detector.
Transmission runs are taken every time the beam is started up or any time the
detector is changed to ensure accurate results. Next, the data sets for the same
sample which used different angles of incidence are combined. The data sets are
merged together by scaling the reflectivities of the higher angle runs so that they
match those of the lower angle runs in the overlapping regions. Only the lowest
angle run is not shifted because this is where the total reflection region, Fl=1,

exists. Once the data are properly reduced it can be analyzed.

101

Appendix B

Neutron Reflectivity Data Interpretation

Due to the highly complex math involved for even a single interface,
neutron reflectivity data are interpreted through the use of models and fitting
routines. The models may consist of various layers, each having its own
thickness (d) and SLD (see Figure 8.1). It is also possible to give each layer a
specific roughness (a) to allow for a diffuse interface. There are several
programs available to the interpretation of reflectivity data, and the one used in
the present work is called Motofit.79' 95

Motofit is a macro specifically designed to work within the analysis
package IGOR Pro.96 It works by having the user input the known parameters in
the system as well as estimated values for the unknown parameters. The known
parameters are held constant while the unknowns are unconstrained. Due to the
fact that there can be several solutions for the data depending on the number of

parameters left unconstrained, it is assumed that one knows as much as possible

 

 

 

 

0 air no
1 I d1 n1
2 I 02 n2
”14 t dm-t nm-1

 

  
    

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kfibcrafi‘érst‘trm931's::l‘.'\;::t1 3:.:v'.\'«3“.\9~:122:1:-‘—::..~.;_;‘ :‘,~:.“;3,;::_:.: ‘ , : ,3 2:
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.\. .. .' .. 1:1: 1. 3 .3. 4 , . .ag..~.-; . .- .. . x)... ~ ' x ;.;,.:..
5. . . . 1.. t..: 4. ,1 .7 "1+
V ‘ ‘ ' ‘ -‘ 1' ‘ ‘ , ‘ V ‘ "' ~ ‘ 'v ‘ '- ‘ ‘ r _.
.\'.....‘-.,. . ... ‘.... >-.I‘,““<.-:,171IL~-‘1\u|":-’v.h :.....: “‘1 ... .1‘ .. ... ......_..
C 1 '11‘3 _. .‘ .‘_<;.. _ .‘H’.’ \‘ .,'_ . . . . 1,...

   

.. '. ’ . ' "x.'. , ‘ . . .‘ ‘ , ' - x . . ' :.."\
4»- ~. I. <<<<< .... ._...,.... «a ,"— .134. ._._g ..A,
Q : a:.‘~.\‘}:~:-.lx~,:~:~“\.-';~:\:$._ ::.-.\2; :. v\~.::.—‘;::--- ‘ 1 —.--- .- -. . ~ 3..

Figure 8.1. Illustration of a reflectivity model for a film
having m layers, each with a distinct thickness (d) and
neutron refractive index (n).

102

about the sample through the use of other techniques to compliment the
reflectivity data. Using a least squares method (Levenberg-Marquardt), the
program is used to determine the best fit to the data. Once the data have been
appropriately modeled, the SLD ‘ profile (p(z)) can be deduced and then be
converted into a volume fraction profile (¢(z)) through the use of the equations:
pz-p
¢1 (z) = _§_)__z
,0] "pz
¢1(2)+¢2 (z)=1 (8.2)

where the subscripts 1 and 2 denote the two different materials that the film is

(3.1)

composed of. This enables one to know the composition of the film throughout
the entire depth.

The reduced data obtained from the experiments were loaded into Motofit
in the form of 3 columns, q, F? and error in Fl. Once the data were loaded it was
automatically displayed in a plot. The reflectivity fit panel (see Figure 8.2) was
then used to enter in the model parameters desired. All data were fitted against Fl
vs. q and weighted with the error in R. In this case there is some bias towards the
low q region so it is important to make sure that the critical edge is fit well or it
could lead to a large increase in the chi squared value.

For the model, there are several constant parameters which the values
are already known (see Table 8.1). These values are input into the model and
locked. Next, initial values for the other parameters are entered into the model.
Some of these values, such as the SLD for pure materials, are known and can

therefore be locked as well. The unknown values are left unconstrained and an

103

 

Figure 3.2. Image of the reflectivity panel window in Motofit.

initial fit to the data is made. At first, the roughness of each layer is locked at 4 A
while the other unknown parameters are fit. This is due to the fact that the data
isn’t as sensitive to the roughness, especially when the data does not extend to
large q values. Until a good initial fit to the data has been obtained, the
roughness can oscillate considerably to give an unrealistic resurt. Once there is a
good initial fit to the data, the roughness of the layers can then be fit to yield a
realistic value.

Table 8.1. Known parameters used in all reflectivity models.

 

 

 

 

 

 

 

 

 

Parameter Value
Scale 1
SLD top 0
SLD base 2.07
Sigma_base 4
Solvent 0
penetration

 

104

Appendix C
Additional Data

The following figures and tables are examples of reflectivity data collected
and the models fitted using the Motofit program. The data was collected to
supplement the data presented in Chapters 3 and 4 of this work. Also included,

are the parameters used to fit the reflectivity data presented in Chapters 3 and 4.

105

(a) 10'8

El 75 kD PS on piranha wafer

 

(b) 0.02 0.04 1) 0.06 0.08 0.10

 

 

 

 

 

p /10'6(A'2)

 

 

 

 

 

 

 

 

 

I— 75 kD PS on piranha wafer
50 100 150 200 250 300
<—— air 2 (A) substrate -—->

 

Figure 0.1. a) Neutron reflectivity data and model for a polystyrene
film on a piranha cleaned silicon wafer. b) Scattering length density
profile of the model used to fit the data.

106

Table C.1. Fitted values for the model parameters used in Figure C.1.

 

 

 

 

Layer “$23988 p (21:6 (X) bkg x2
0 - 0 -
1 297 1.41 5.9 2.09E-06 57.15
base - 2.07 4

 

 

 

 

 

 

 

 

107

El 63 kD dPS on piranha wafer

 

0.02 0.0 0.08 0.10
(b)

4 _1 0.06
NA )

p/1of< '2)

 

— 63 kD dPS on

0 50 100 150 200
<—— air 2 (A) substrate ——->

Figure C.2. a) Neutron reflectivity data and model for a deuterated

polystyrene film on a piranha cleaned silicon wafer. b) Scattering
length density profile of the model used to fit the data.

108

Table C.2. Fitted values for the model parameters used in FigureC.2.

 

 

 

 

Layer Thltz’lzqess pa 1:6 (X) bkg x2
0 - 0 -
1 215 6.41 8. 2 2.07E-06 48.00
base - 2.07 4

 

 

 

 

 

 

 

 

109

El 82 kD dPS on piranha wafer
o 82 kD dPS on

wafer

 

 

 

 

 

 

 

 

  

 

‘\
6 .- 7‘ _
I
I
5 ._ 1 . ..
1
A I'
N
'.< 4-- . I. ..
V
1° '.
o \
E 3 .-. ,.. ‘. -
Q I
I
2 “

 

 

 

 

 

 

— 82 kD dPS on piranha wafer
---- 82 kD dPS on Sigmacote® wafer

 

 

 

 

 

50 100 150 200 250
<—— air 2 (A) substrate —-->

Figure C.3. a) Neutron reflectivity data and models for deuterated
polystyrene film. One was spin-coated onto a piranha cleaned
silicon wafer while the other was floated onto a Sigmacote® treated

wafer. b) Scattering length density profiles of the models used to fit
the data.

110

Table C.3. Fitted vaers for the model parameters used for the
piranha cleaned sample in Figure C.3.

 

 

 

 

 

 

- 6
Layer “$231988 pay; (2) bkg x2
0 - 0 -
1 257 6.41 5.7 1.70E-05 22.92
base - 2.07 4

 

 

 

 

 

 

Table C.4. Fitted values for the model parameters used for the
Sigmacote® treated sample in Figure C.3.

 

 

 

Thickness p x106 0 2
La er . bk
y (A) (A2) (A) 9 x
0 - 0 -
1 224 6.41 8.7 1.40E-05 1 10. 7

 

base

 

 

 

2.07 4

 

 

 

 

 

111

El 75 kD PS + 3% 060 after annealing

Hq‘ (K4)

_L
l

 

0.02 o. 0.08 0.10
(b)

04 _1 0.06
(NA )

p /10"’(A‘2)

— 75 kD PS + 3% 060 after annealing

 

0 100 200 300 400
<—- air 2 (A) substrate -—->

Figure C.4. a) Neutron reflectivity data and models for polystyrene
with 3 wt% C60 films on piranha cleaned silicon wafers. One was
tested without annealing and the other was first annealed with a
saturated toluene vapor. b) Scattering length density profiles of the
models used to fit the data.

112

Table C.5. Fitted values for the model parameters used in Figure C.4.

 

 

 

Thickness p x106 0 2
La er _ bk
y (A) (A2) (A) 9 x
o - of -
1 387 1.41 3

 

4.62E-08 188.9
2 27 2.5814 3.6

base - 2.07 4

 

 

 

 

 

 

 

 

 

113

(a) 10'8

El 75 kD PS + 3% C60 before annealing
o 75 kD PS + 3% 060 after annealing

Rq“ (Aj)
orb

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

0.02 0 04 0 0.08 0.10
-1
(b) q (A )
3.0 g f f
I |
5 \1_
A 2.0 __...,...._ ' 1
N I ;
S i I 9
‘90 1.5 ,. : 1 ...,....-..
a f '
1.0 E
0'5 — 75 kD PS + 3% C60 before annealing '
---- 75 kD PS + 3% C60 after annealing
0.0 , - - -

 

 

 

 

 

0 50 100 150 200 250 300
<—— air z(A) substrate -—->

Figure C.5. a) Neutron reflectivity data and models for polystyrene
with 3 wt% C60 films on piranha cleaned silicon wafers. One was
tested without annealing and the other was first annealed with a
saturated toluene vapor. b) Scattering length density profiles of the
models used to fit the data.

114

Table C.6. Fitted values for the model parameters used for the
before annealed sample in Figure C.5.

 

 

 

 

 

 

 

 

 

 

 

Thickness p x106 0 2
La er . bk
y (A) (A 2) (A) 9 X
0 - 0 -
1 293 1.41 7.4
8.33E-O7 53.14
2 20 2.3 3.3
base - 2.07 4

 

Table 0.7. Fitted values for the model parameters used for the after

annealed sample in Figure 0.5.

 

 

 

 

 

 

 

 

 

 

 

Thickness p x106 6 2
La er - bk
y (A) (A2) (A) 9 x
O - 0 -
1 260 1.41 7.1
5.93E-07 71.28
2 23 2.4732 2.9
base - 2.07 4

 

115

 

 

Table 0.8. Fitted values for the model parameters used for the

normal film before annealing in Figure 3.2.

 

 

 

 

 

 

 

 

 

 

Thickness 0 x106 0 2
La er - bk
y (A) (A 2) (A) 9 x
O - O -
1 300 1.41 4
1.14E-05 393.2
2 14 2.4 4
base - 2.07 4

 

 

Table 0.9. Fitted values for the model parameters used for the
normal film after annealing in Figure 3.2.

 

 

 

 

 

 

 

 

 

 

 

Thickness p x106 0 2
La er - bk
y (A) (A2) (A) 9 x
O - 0 -
1 294 1.41 4
8.27E-06 97.9
2 15 2.8 4
base - 2.07 4

 

Table C.10. Fitted values for the model parameters used for the
flipped film before annealing in Figure 3.2.

 

 

 

 

 

 

 

 

 

 

 

Thickness p x109 0 2
La er . bk
y (A) (A2) (A) 9 x
0 - O -
1 16 1.41 4
1.38E-05 35.3
2 293 2.3 4
base - 2.07 4

 

116

 

 

 

Table C.11. Fitted values for the model parameters used for the

film before annealing in Figure 3.4.

 

 

 

 

 

 

 

 

 

 

 

Thickness p x109 0 2

La er . bk

y (A) (A2) (A) 9 x

O - O -

1 1 58 1 .41 4

1 .54E-O6 14.7

2 1 7 3.33 4

base - 2.07 4

 

Table 0.12. Fitted values for the model parameters used for the

film after annealing in Figure 3.4.

 

 

 

 

 

 

 

 

 

 

 

Thickness p x106 0 2
La er - bk
y (A) (A2) (A) 9 x
0 - 0 -
1 1 58 1 .41 4
1 .54E-06 6.7
2 20' 3.1 4
base - 2.07 4

 

117

 

 

Table C.13. Fitted values for the model parameters used for the
film during annealing in Figure 3.5.

 

 

 

 

 

 

 

 

 

 

 

 

' 6
Layer ”"2239” “(212? (X) bkg x2
0 - 0 -
1 72 5.67 4
2 1 00 5.1 1
3 1 00 4.84 1
4 1 00 4.76 1
5 100 4.7 1 2.80E-06 9.5
6 1 00 4.63 1
7 1 00 4.49 1
8 1 00 4.06 1
9 1 00 3.17 1
base - 2.07 4

 

 

 

 

 

 

 

 

118

Table C.14. Fitted values for the model parameters used for the
before annealed sample in Figure 3.7.

 

p><106

 

 

 

 

 

 

 

 

 

Layer “322953 (A,2) (X) bkg X2
0 - O -
1 268 5.92 4 1.66E-05 73.2
base - 2.07 4

 

Table C.15. Fitted values for the model parameters used for the
before annealed sample in Figure 3.7.

 

 

 

 

 

 

 

 

 

 

 

Thickness p x106 0 2
La er . bk
y (A) (A 2) (A) 9 x
O - O -
1 248 6.41 4
1.28E-O6 54.4
2 40 2.68 4
base - 2.07 4

 

119

 

 

Table 0.16. Fitted values for the model parameters used for the

film before annealing in Figure 4.8.

 

 

 

 

 

 

 

 

 

 

 

Thickness p ><106 o 2
La er - bk
y (A) (A 2) (A) 9 x
0 - 0 -
1 20 2.34 4
3.37E-06 14.00
2 241 1.41 4
base - 2.07 4

 

Table 0.17. Fitted values for the model parameters used for the

film after annealing in Figure 4.8.

 

 

 

 

 

 

 

 

 

 

 

Thickness p x106 0 2

La er . bk

y (A) (A 2) (A) 9 x

0 - 0 -

1 215 1 .51 4

6.22E-06 11.15

2 55 1.41 4

base - 2.07 4

 

120

 

 

Table 0.18. Fitted values for the model parameters used for the
flipped film before annealing in Figure 4.9.

 

 

 

 

 

 

 

 

 

 

 

Thickness p x106 0 2
La er - bk
y (A) (A2) (A) 9 x
O - 0 -
1 20 2.7 4
1.25E-06 28.40
2 312 1.41 4
base - 2.07 4

 

Table 0.29. Fitted values for the model parameters used for the
flipped film after annealing in Figure 4.9.

 

 

 

 

 

 

 

 

 

 

 

Thickness p x109 0 2
La er _ bk
y (A) (A 2) (A) 9 7‘
0 - 0 -
1 282 1.51 4
1.06E-05 50.99
2 75 1.41 4
base - 2.07 4

 

121

 

 

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