Role of Glassy-Rubbery Interfaces on the Physical Aging of Glassy Polymer Thin Films Open Access

McGuire, Jennifer (Spring 2018)

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            Polymers in a glassy state undergo physical aging. During this process they get denser and their volume decreases. We can measure this change in volume by measuring a change in film thickness. It is known that the glass transition temperature, Tg, the temperature at which a polymer transitions from a rubbery to a glassy state, is decreased at a glassy-rubbery interface. We also know there is a link between reduction in Tg and reduction in physical aging rate in single layer films. Previous work studied the physical aging of a glassy layer atop a rubbery layer and found no change in physical aging rate despite a reduction in Tg. They believed a possible explanation for this finding was that the free surface next to the glassy layer was canceling out the effect of the glassy-rubbery interface. My goal was to eliminate this effect by studying the physical aging of a glassy layer beneath a rubbery layer. Since the silica substrate has no effect on the polymer, this reduces the system to one interface. I studied bilayer systems with rubbery poly(n-butyl methacrylate) (PnBMA) atop glassy polystyrene (PS)

            I found the physical aging rate of the glassy layer is not changed from bulk down to 142 nm of PS. Difficulty with measuring PS layers smaller than 142 nm beneath 400 nm PnBMA layers led to my decreasing the thickness of the PnBMA layer down to 150 nm. Bilayers with 150 nm PnBMA atop 150 nm PS produced aging rates in agreement with bulk data, so I was able to study PS layers of smaller thicknesses. There was no observable change in physical aging down to 57 nm of PS, but the measurements on 57 nm PS bilayers demonstrated issues with the PnBMA layer’s sensitivity to the environment. Future work will include addressing ways to increase the PnBMA layer’s resistance to changes in the environment and decreasing the PS layer thickness. Should a change in physical aging rate be observed in bilayers with smaller PS layers, future work will also study the Tg profile of the sample geometry used in this work.

Table of Contents

1 Introduction and Background    1

     1.1 Glass Transition and Physical Aging    1

     1.2 Ellipsometry    3

     1.3 How Ellipsometry Measures Physical Aging    6

     1.4 Local Tg in Thin Films    7

     1.5 Relationship Between Tg and Physical Aging    9

     1.6 Local Tg in PS/PnBMA Films    9

     1.7 Physical Aging Rate in PS/PnBMA Films    10

     1.8 Goal of Thesis    12

2 Experimental Methods    13

     2.1 Sample Preparation    13

     2.2 Ellipsometer Aging Measurements    14

     2.2.1 Verifying Instrument Stability with 1000 nm SiO2/Si Substrates    14

     2.2.2 Polystyrene (PS) Bulk Films    17

     2.2.3 Poly(n-butyl methacrylate) (PnBMA) Bulk Films    19

     2.2.4 PnBMA/PS Bilayer Films    23

3 Results and Discussion    25

     3.1 Bulk PnBMA/PS Bilayers    25

     3.2 400 nm PnBMA/PS Bilayers—Decreasing PS Layer Thickness    32

     3.3 PnBMA/150 PS Bilayers—Decreasing PnBMA Layer Thickness    33

     3.4 150 nm PnBMA/PS Bilayers—Decreasing PS Layer Thickness    34

4 Conclusions    38

References    40

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