Formation of Polymer Glasses: Impact of Mechanical Deformation and Cooling Conditions on Physical Aging Open Access

Gray, Laura (2015)

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Polymer glasses are ubiquitous materials used in many applications from containers to gas separation membranes to nanolithography templates. Out of equilibrium glasses exhibit a time-dependent decrease in density during structural relaxation towards equilibrium. How stress and mechanical deformation impart mobility to polymer glasses have been studied primarily for materials where the glassy state was formed stress free. In this dissertation, I have explored the effect of cooling rate and stress applied during the glass formation process on the physical aging rate of the subsequent glass.

First, we conducted systematic experiments to determine the possible causes of accelerated aging of polymer films with decreasing film thickness previously observed. We used ellipsometry to measure the physical aging rate of polystyrene films by quantifying the time-dependent decrease in film thickness that results from an increase in average film density during aging. We found that polystyrene (PS) films held in a free-standing state while cooling through the glass transition temperature, Tg, exhibited film-thickness dependent aging rate whereas films held in a supported state exhibited aging independent of film thickness. We found that PS films cooled slower through Tg aged slower. However, this did not explain the accelerated, faster physical aging observed for free-standing films, as free-standing films cool faster than supported films, yet aged slower.

We constructed an experimental apparatus to apply a known stress to free-standing films during the thermal quench. As the magnitude of stress during vitrification increases, the physical aging rate quickly transitions over a small range of stresses to a faster aging rate, indicating the resulting glass is less stable. To explore this unique finding, we constructed a computer-controlled apparatus to measure and apply stress and strain to polymer films during vitrification in order to characterize the temperature-dependent stress build up and yield stress near Tg.

The results presented in this dissertation are important to consider when preparing glasses as stress is common, often due to thermal expansion coefficient mismatch between the material and the mold. These results also indicate that stress upon cooling can change the state of the glass, altering the physical aging of the polymer.

Table of Contents

Chapter 1: Introduction and Scope of Dissertation ...1
1.1 Introduction to Glasses and Open Questions ...1
1.2 Outline of Dissertation ... 7 Chapter 2: Background ...10
2.1 Synopsis ... 10
2.2 Potential Energy Landscape Description of Glasses ...11
2.3 Molecular Mobility Enhanced by Deformation ...15
2.4 Stress-Induced Landscape Tilting ...19
2.5 Stresses in Glass Formation ...23
2.6 Anomalous Physical Aging and Mechanical Properties in Thin Polymer Films ...26
2.6.1 Accelerated Physical Aging with Decreasing Film Thickness .... 26
2.6.2 Free Volume Diffusion Model ... 30
2.6.3 Rubbery Stiffening in Polymer Thin Films ... 31
2.7 Summary ... 35 Chapter 3: Physical Aging Behavior of Glassy Polymer Films: Effect of Quench Conditions ...36
3.1 Synopsis ...36
3.2 Introduction ... 37
3.3 Experimental Methods ... 41
3.4 Results and Discussion ...44
3.4.1 Effect of Polymer Chemical Structure: Stiff Backbone vs. Flexible C-C Backbone with Bulky Side-Groups ... 44
3.4.2 Free-Standing vs. Supported Quench ... 48
3.4.3 Effect of Annealing Conditions on Physical Aging Rate .... 51
3.4.4 Effect of Quench Rate on Physical Aging Rate ... 53
3.4.5 Applied Stress during Quench due to Thermal Expansion Mismatch ... 59
3.5 Summary and Conclusions ... 66 Chapter 4: Formation of Polymer Glasses under Stress: Effect on Physical Aging ... 67
4.1 Synopsis ... 67
4.2 Introduction ... 68
4.3 Experimental Methods ... 71
4.4 Results and Discussion ... 73
4.5 Summary and Conclusions ... 84 Chapter 5: Mechanical Tensile Testing Apparatus for Controlled Deformation of Polymer Films during Vitrification ... 86
5.1 Synopsis ... 86
5.2 Introduction ... 86
5.3 Experimental Techniques ... 91
5.3.1 Mechanical Tensile Testing Apparatus: Components and Calibration ... 91
5.3.2 Sample Preparation ...103
5.4 Results and Discussion ... 104
5.4.1 Mechanical Behavior of Polystyrene above Tg under Constant Strain Rate Deformation ... 104
5.4.2 Stress Relaxation of Polystyrene above Tg ...114
5.4.3 Polystyrene Creep above Tg ...118
5.5 Summary and Conclusions ...120 Chapter 6: Stress Build Up and Yield in Polymer Films near the Glass Transition ... 123
6.1 Synopsis ... 123
6.2 Introduction ... 124
6.3 Experimental Techniques ...128
6.4 Results and Discussion ...129
6.4.1 Stress Build up on Cooling at Constant Length ...129
6.4.2 Yield above the Glass Transition Temperature ... 132
6.5 Summary and Conclusions ...138 Chapter 7: Summary... 139 Bibliography ... 145

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