Nanomechanics of Glassy Polymers Under Confinement

212 Pages·2017·18.84 MB·English

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Publicly Accessible Penn Dissertations
2015
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Amit Shavit
University of Pennsylvania, [email protected]
Follow this and additional works at: https://repository.upenn.edu/edissertations
Part of the Chemical Engineering Commons
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Shavit, Amit, "Nanomechanics of Glassy Polymers Under Confinement" (2015). Publicly Accessible Penn
Dissertations. 2004.
https://repository.upenn.edu/edissertations/2004
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Many of today's nanotechnologies and processes rely on highly confined ($<30$ nm) polymer glasses
and polymer nanocomposites. These materials have aided the development of smaller and faster
computer chips, improved energy storage devices, protective coatings, and next generation materials with
enhanced mechanical, electrical, and optical properties. However, confinement of these materials
introduces interfaces, which may drastically change fundamental properties of the material. For example,
confinement of polymer glasses has shown to cause changes in the elastic modulus, which in turn may
lead to significant adverse effects on the end-use applications of these materials. Understanding how
confinement changes the properties of the material may provide us with a mechanism to control it,
enabling the production of better materials on the nano scale. Fortunately, the length scales where
confinement has an effect are reasonable to model using computer simulations. This Thesis investigates
the confinement of polymer glasses and polymer nanocomposites using molecular dynamics and Monte
Carlo simulations.
In Chapter 2, we use a coarse-grained model to investigate the effect of backbone rigidity and
confinement on polymer glasses in bulk and free-standing thin films. We extend this model throughout
the entire Thesis. In Chapters 3-4, we study the process of physical aging and its role on local mobility in
thin films, and, for the first time, we find a bulk property which may control important dynamical length
scales in the film. In Chapter 5, we investigate the mechanical properties of confined model nanopillars,
which have not been previously simulated in the context of polymer glasses, and develop close
phenomenological connections between polymer glasses and metallic glasses by looking at the local
non-affine strain. Lastly, in Chapter 6, we turn to polymer nanocomposites, and study the mechanism by
which polymers infiltrate a highly packed nanoparticle film, a method that has been recently discovered to
produce polymer nanocomposite films with very high loadings. By investigating all of these systems on
the molecular level, we explain some of the underlying physics, and our results shed light on several
controversies present in the literature.
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Dissertation
DDeeggrreeee NNaammee
Doctor of Philosophy (PhD)
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Chemical and Biomolecular Engineering
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Robert Riggleman
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glass transition, molecular dynamics, simulations, thin films
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Chemical Engineering
This dissertation is available at ScholarlyCommons: https://repository.upenn.edu/edissertations/2004
NANOMECHANICS OF GLASSY POLYMERS UNDER
CONFINEMENT
Amit Shavit
A DISSERTATION
in
Chemical and Biomolecular Engineering
Presented to the Faculties of the University of Pennsylvania in Partial
Fulfillment of the Requirements for the Degree of Doctor of Philosophy
2015
Supervisor of Dissertation
Robert Riggleman, Assistant Professor of Chemical and Biomolecular
Engineering
Graduate Group Chairperson
Raymond Gorte, Professor of Chemical and Biomolecular Engineering
Dissertation Committee:
Russell Composto, Professor of Materials Science and Engineering
Zahra Fakhraai, Assistant Professor of Chemistry
Ravi Radhakrishnan, Associate Professor of Bioengineering
Acknowledgments
I would first like to acknowledge my advisor, Dr. Robert Riggleman, for his invalu-
able guidance and support, for having an open-door policy which I fully utilized,
and for never getting annoyed of hearing “I have a bug somewhere, I just don’t
know where yet.”
I would also like to acknowledge Dr. Zahra Fakhraai for many valuable insights
and discussions over the years, as well as Dr. Russell Composto and Dr. Ravi
Radhakrishnan for more than just helpful comments.
Dr. Yeng-Long Chen is also greatly appreciated for having hosted me in Taiwan,
inviting me to be a member of his lab, and continuing to collaborate with us even
after I returned to the States. On that note, I would also like to acknowledge the
National Science Foundation for funding my Taiwan research experience through
an excellent EAPSI program from which I greatly benefitted.
I am also grateful to members of the Riggleman Lab, especially Huikuan Chao
and Jason Koski, for being incredibly helpful over the years, helping me solve bugs
in my code, and discussing challenging concepts. There are times when all you need
ii
is another pair of eyes to look at your code, and I am glad you guys were there to
provide just that.
I would also like to acknowledge close friends in the program, Richard Tourdot,
Cory Silva, Melissa Myint, and Fuquan Tu. Whether by sports like tennis, frisbee,
and volleyball, or by happy hours and commiseration, you have made this journey
much more enjoyable and sustainable.
IamalsothankfultomembersoftheHebrewTable, specificallyTomZaiderman,
Alisa Rassin, Noam Roth, Natan Katz, Guy Ozeri, and Gal Manoach. Through our
regular get togethers, you’ve reminded me of my Israeli culture.
I am extremely grateful to my family, especially my mom Orit Shavit, my dad
MosheShavit,mysisterNoamShavit,andmysavtaRinaShatil. Youwereamazing,
supportive, helpful, and loving, even from 10,000 km away. You made every trip
to Israel count, and you were the ones who suggested Chemical Engineering in the
first place.
Iamalsothankfultomygirlfriend’sfamily,especiallyPatrickandElyseNeMoyer,
fortreatingmeasfamilyandintroducingmetoseveralholidaysI’venevercelebrated
before. It’s always fun to visit, and I’m amazed at the sheer number of board games
that exist in this world.
Most importantly, I would like to thank my girlfriend Amanda NeMoyer. Your
24/7 support, love, positivity, inconceivable intelligence, and humor have made this
journey unbelievably easier, and I could not have done it without you. Thank you,
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you’re awesome.
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ABSTRACT
NANOMECHANICS OF GLASSY POLYMERS UNDER CONFINEMENT
Amit Shavit
Robert Riggleman
Many of today’s nanotechnologies and processes rely on highly confined (< 30
nm) polymer glasses and polymer nanocomposites. These materials have aided the
development of smaller and faster computer chips, improved energy storage devices,
protective coatings, and next generation materials with enhanced mechanical, elec-
trical, and optical properties. However, confinement of these materials introduces
interfaces, which may drastically change fundamental properties of the material.
For example, confinement of polymer glasses has shown to cause changes in the
elastic modulus, which in turn may lead to significant adverse effects on the end-
use applications of these materials. Understanding how confinement changes the
properties of the material may provide us with a mechanism to control it, enabling
the production of better materials on the nano scale. Fortunately, the length scales
where confinement has an effect are reasonable to model using computer simula-
tions. This Thesis investigates the confinement of polymer glasses and polymer
nanocomposites using molecular dynamics and Monte Carlo simulations.
InChapter2, weuseacoarse-grainedmodel toinvestigate theeffectofbackbone
rigidity and confinement on polymer glasses in bulk and free-standing thin films.
v
We extend this model throughout the entire Thesis. In Chapters 3–4, we study
the process of physical aging and its role on local mobility in thin films, and, for
the first time, we find a bulk property which may control important dynamical
length scales in the film. In Chapter 5, we investigate the mechanical properties
of confined model nanopillars, which have not been previously simulated in the
contextofpolymerglasses,anddevelopclosephenomenologicalconnectionsbetween
polymerglassesandmetallicglassesbylookingatthelocalnon-affinestrain. Lastly,
in Chapter 6, we turn to polymer nanocomposites, and study the mechanism by
which polymers infiltrate a highly packed nanoparticle film, a method that has
been recently discovered to produce polymer nanocomposite films with very high
loadings. By investigating all of these systems on the molecular level, we explain
some of the underlying physics, and our results shed light on several controversies
present in the literature.
vi
Contents
1 Introduction 1
1.1 Introduction to Polymers and Current Challenges . . . . . . . . . . 1
1.2 Polymer Glasses and the Glass Transition Temperature . . . . . . . 3
1.3 Aging of Polymer Glasses . . . . . . . . . . . . . . . . . . . . . . . 6
1.4 Mechanics of Polymer Glasses . . . . . . . . . . . . . . . . . . . . . 9
1.5 Towards Enhanced Materials: Polymer Nanocomposites . . . . . . . 12
1.6 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2 Influence of Backbone Rigidity on Nanoscale Confinement Effects
in Model Glass-Forming Polymers 16
2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2.2 Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
2.2.1 Simulation details . . . . . . . . . . . . . . . . . . . . . . . . 22
2.2.2 Relaxation time calculations . . . . . . . . . . . . . . . . . . 25
2.2.3 Characteristics of glass-formation . . . . . . . . . . . . . . . 26
2.2.4 Structural ordering . . . . . . . . . . . . . . . . . . . . . . . 28
2.2.5 Mechanical properties . . . . . . . . . . . . . . . . . . . . . 29
2.3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
2.3.1 Dynamics and characteristics of glass formation . . . . . . . 33
2.3.2 Local Ordering . . . . . . . . . . . . . . . . . . . . . . . . 39
2.3.3 Mechanical Properties . . . . . . . . . . . . . . . . . . . . . 42
2.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
2.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
2.6 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
3 Evolution of Collective Motion in a Model Glass-Forming Liquid
During Physical Aging 51
3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
3.2 Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
3.3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
3.3.1 Aging of thermodynamic properties . . . . . . . . . . . . . . 59
3.3.2 Aging of structural relaxation . . . . . . . . . . . . . . . . . 60
vii
3.3.3 Asymmetric approach to equilibrium . . . . . . . . . . . . . 62
3.3.4 Non-equilibrium relationship between τ and L . . . . . . 64
eff S
3.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
3.5 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
4 Physical Aging and the Local Dynamics of Glass-Forming Polymers
under Nanoscale Confinement 71
4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
4.2 Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
4.2.1 Simulation Details . . . . . . . . . . . . . . . . . . . . . . . 75
4.2.2 Aging and Age Rates . . . . . . . . . . . . . . . . . . . . . . 77
4.3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
4.3.1 Aging Behavior and Dynamics . . . . . . . . . . . . . . . . . 80
4.3.2 Local Age Rates and Dynamics . . . . . . . . . . . . . . . . 83
4.3.3 Dynamic Length Scale near T . . . . . . . . . . . . . . . . . 90
g
4.4 Discussion and Summary . . . . . . . . . . . . . . . . . . . . . . . . 93
4.5 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
5 Strain Localization in Glassy Polymers Under Cylindrical Confine-
ment 97
5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
5.2 Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
5.2.1 Simulation Details . . . . . . . . . . . . . . . . . . . . . . . 105
5.2.2 Mechanical Deformations and Analysis of Local Strain . . . 107
5.3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108
5.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118
5.5 Unpublished, Preliminary, and Ongoing Work . . . . . . . . . . . . 121
5.6 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . 133
6 PolymerInfiltrationDynamicsandLocalViscosityinGlass-Forming
Model Polymers 134
6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135
6.2 Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139
6.2.1 Simulation Details . . . . . . . . . . . . . . . . . . . . . . . 139
6.2.2 Film Thickness . . . . . . . . . . . . . . . . . . . . . . . . . 141
6.2.3 Viscosity Calculations . . . . . . . . . . . . . . . . . . . . . 143
6.2.4 Mean-Squared Displacements . . . . . . . . . . . . . . . . . 145
6.3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146
6.3.1 Polymer Infiltration Dynamics . . . . . . . . . . . . . . . . . 146
6.3.2 Surface Tension and Viscosity . . . . . . . . . . . . . . . . . 151
6.3.3 Local Viscosities and Local MSD . . . . . . . . . . . . . . . 154
6.3.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
6.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
viii