Glass Transition and Physical Aging Behavior of Thin Free-Standing and Supported Polystyrene Films Pubblico

Abstract

In this dissertation, I have explored the glass transition and physical aging of thin and ultrathin polystyrene (PS) films held in both the supported and free-standing state with ellipsometry. We measured the physical aging rate of supported PS films as a function of aging temperature and film thickness, find a reduced aging rate for the ultrathin films at all aging temperatures. These results have shown that the enhanced mobility at the free surface known to be responsible for glass transition temperature (Tg) reductions in supported and free-standing films also causes reductions in the physical aging rate for supported films. The distance from the surface at which dynamics are perturbed from bulk values grows as the temperature is reduced.

We also measured the Tg of ultrathin high molecular weight (MW) free-standing PS films and report the first observation of two Tgs in ultrathin high MW free-standing films. We believe these two Tgs are caused by two different but simultaneous mechanisms of Tg reduction. The lower transition is MW dependent and has previously been observed, while the upper transition is MW independent, has not been seen before in these films, and matches the Tg reduction previously measured in low MW free-standing films. Physical aging measurements on films held between the two transitions demonstrate that the upper transition is an actual glass transition. Additionally, by measuring the thermal expansion of these films above, below, and between the two transitions, we find that the majority of the film (~90%) solidifies at the upper transition while only ~10% of the film remains mobile until freezing out at the second transition.

In addition to these free surface based confinement effects, the physical aging rate of thin (500 nm thick) free-standing PS films can be reduced by nearly a factor of two when the stress applied during the quench is reduced. This stress is due to the thermal expansion mismatch between the film and the frame. Finally, the background, theory, and data needed to perform a complete thermoviscoelastic calculation of this stress is described and shown to be a significantly non-trivial task.

Table of Contents

Chapter 1: Introduction ........................................................................................... 1 1.1 Polymer Basics ..................................................................................................... 1 1.2 Glass Transition ................................................................................................... 6

1.2.1 Fundamentals of the Glass Transition ................................................................... 6

1.2.2 Measurement of Tg with Ellipsometry ................................................................... 8

1.2.3 Physical Aging ................................................................................................. 10 1.3 Confinement ...................................................................................................... 15

1.3.1 Tg Reductions in Ultrathin Supported PS films ...................................................... 15

1.3.2 Tg Reductions in Ultrathin Free-Standing Polymer Films ........................................ 17

1.4 Outline of Dissertation ........................................................................................ 20 1.4.1 Chapter Descriptions ........................................................................................ 20

1.4.2 Additional Work not Included in this Dissertation .................................................. 23

Chapter 2: Dual Tgs in Ultrathin Free-Standing Polystyrene Films ......................... 25

2.1 Introduction ...................................................................................................... 25 2.2 Experimental ..................................................................................................... 27 2.3 Results and Discussion ........................................................................................ 30 2.4 Summary .......................................................................................................... 40

Chapter 3: Physical Aging in Ultrathin Supported Polystyrene Films ...................... 41

3.1 Introduction ....................................................................................................... 41 3.2 Experimental ..................................................................................................... 46 3.3 Results and Discussion ........................................................................................ 48 3.4 Conclusions ....................................................................................................... 62 3.5 Addendum ......................................................................................................... 63

Chapter 4: Physical Aging of Thin Free-Standing Polystyrene Films: Stress Effects 65

4.1 Introduction ...................................................................................................... 65 4.2 Experimental ..................................................................................................... 69 4.3 Results and Discussion ........................................................................................ 70

4.3.1 Physical Aging of Films Quenched and Measured Free-standing via Ellipsometry ....... 70

4.3.2 Thermal Stresses Imparted to Free-standing Films by Rigid Frames on Cooling ........ 75

4.3.3 Correlating Physical Aging Rate with Thermal Expansion Mismatch and Applied

Stress .................................................................................................................... 83

4.4 Conclusions ....................................................................................................... 91

Chapter 5: Above, Below, and In-Between the Two Glass Transitions of

Ultrathin Free-Standing Polystyrene Films: Thermal Expansion Coefficient and

Physical Aging ....................................................................................................... 92

5.1 Introduction ....................................................................................................... 92 5.2 Experimental ...................................................................................................... 96 5.3 Results and Discussion ........................................................................................ 98

5.3.1 Thermal expansion above, below, and between the two transitions ......................... 98

5.3.2 Physical aging below and between the two transitions ......................................... 109

5.4 Conclusions ...................................................................................................... 119 Chapter 6: Summary ............................................................................................ 121 6.1 Introduction ..................................................................................................... 121 6.2 Free Surface Length Scale .................................................................................. 123

6.3 Two Tgs in Ultrathin High MW Polystyrene Films .................................................... 127

6.4 Quench Stress Effects on Physical Aging .............................................................. 130

Chapter 7: Appendix - Thermoviscoelastic Stress Calculation .............................. 132

7.1 Introduction ..................................................................................................... 132 7.2 Elastic Stress Calculation ................................................................................... 136 7.3 Viscoelastic Effects ............................................................................................ 137 7.3.1 Basics of Viscoelasticity ................................................................................... 137

7.3.2 Time-Temperature Superposition ..................................................................... 139

7.3.3 Working with Viscoelastic Functions .................................................................. 140

7.3.4 Thermoviscoelasticity ..................................................................................... 143

7.4 Thermoviscoelastic Solution for Free-Standing and Supported Films ......................... 145

7.4.1 Analytical Solution .......................................................................................... 145

7.4.2 Sources of Data And Complete Representation in Mathematica ............................. 146

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School	Laney Graduate School
Department	Physics
Degree	PhD
Submission	Dissertation
Language	English
Research Field	Physics, General Physics, Condensed Matter Chemistry, Polymer
Parola chiave	Polymer Physical Aging Stress Confinement Thermal Expansion Ellipsometry Free-Standing Film Polystyrene Thin Film Glass Transition
Committee Chair / Thesis Advisor	Roth, Connie, Emory University
Committee Members	Weeks, Eric, Emory University Family, Fereydoon, Emory University Bucknall, David, Georgia Institute of Technology Urazhdin, Sergei , Emory University

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