Comparing Measures of Global and Local Material Properties of Polystyrene under Different Boundary Conditions: Smooth, Chain-Tethered, and Rough Walls 公开
Huang, Xinru (Spring 2018)
Abstract
In this dissertation, I measure both global and local material properties of polystyrene (PS) under different substrate conditions using ellipsometry and fluorescence. The first part of my dissertation focuses on investigating the correlation between the temperature-dependent specific volume vsp and the variations in the glass transition temperature Tg for supported PS films. I find that both liquid and glassy vsp behave nonmonotonically as a function of film thickness h. They start to deviate from bulk at ~120 nm, and at 65 nm have a maximum increase of (0.4 ± 0.2) % before dropping back to the bulk value at ~40 nm. This small amount of increase in vsp is the same order of magnitude as Lipson’s theoretical prediction and all within the experimental error of previous studies ± 1 %. The dramatic density increase for films thinner than 30 nm is consistent with some previous studies, but I believe this is not real due to the breakdown of Lorentz-Lorenz formula.
For rest parts of this dissertation, I use fluorescence spectroscopy to locally measure the glass transition temperature Tg near end-tethered and rough substrates without the free surface effect, which is ill-defined and uncontrollable. To achieve such local measurements, a 12 ± 1 nm chemically labeled PS layer with trace amounts of a fluorescent Tg sensitive pyrene dye is inserted at a position z away from different interfaces, thereby mapping out local Tg(z) profiles. I observe an optimum grafting density of σ = 0.011 chains/nm2 that results in a maximum Tg increase of 49 ± 2 K, which corresponds to the “mushroom-to-brush” crossover regime. The length scale over which this Tg(z) perturbation persists from the interface is ~100-125 nm. Then, I make rough interfaces by exposing flat silica substrates to hydrogen fluoride vapor for different lengths of time to achieve various roughnesses. The local Tg(z = 0) right at the rough substrates increases monotonically up to 111 ± 2 ⁰C with increasing root-mean-square roughness Rrms from 0.5 nm to 10.9 nm. Comparing these two studies suggests chain tethering plays a dominant role in deciding the long length scale of Tg(z) near dissimilar polymer-polymer interfaces.
Table of Contents
Chapter 1: Introduction........................................................................ 1
1.1 Polymer Basics..................................................................................................... 1
1.2 Glass Transition.................................................................................................... 8
1.2.1 Glass Transition in Thin Polymer Films....................................................... 11
1.2.1.1 Polymer-Air Interface (free surface)............................................... 13
1.2.1.2 Polymer-Substrate Interface (wall)................................................. 14
1.2.1.2 Polymer-Polymer Interface............................................................. 21
1.3 Outline of Dissertation ................................................................................... 23
1.4 References........................................................................................................ 26
Chapter 2: Experimental Methods..................................................... 32
2.1 Measurements with Ellipsometry....................................................................... 32
2.2 Measurements with Fluorescence....................................................................... 37
2.3 Comparison of Tg Values Reported by Ellipsometry and Fluorescence............. 42
2.4 References.......................................................................................................... 46
Chapter 3: Changes in the Temperature-Dependent Specific Volume of Supported Polystyrene Films with Film Thickness.......................................... 48
3.1 Synopsis.............................................................................................................. 48
3.2 Introduction........................................................................................................ 49
3.3 Experimental Methods....................................................................................... 55
3.4 Results and Discussion....................................................................................... 56
3.4.1 Temperature-Dependent of Film Thickness and Index of Refraction......... 56
3.4.2 Ellipsometry Data Fitting............................................................................ 61
3.4.3 Lorentz-Lorenz Equation as a Measure of Density and Specific Volume.. 73
3.4.4 Changes in Specific Volume and Density with Decreasing Film Thickness 78
3.5 Conclusions........................................................................................................ 85
3.6 References.......................................................................................................... 86
Chapter 4: Optimizing the Grafting Density of Tethered Chains to Alter the Local Glass Transition Temperature of Polystyrene near Silica Substrates: the Advantage of Mushrooms over Brushes.............................................................. 93
4.1 Synopsis.............................................................................................................. 93
4.2 Introduction........................................................................................................ 94
4.3 Experimental Methods....................................................................................... 95
4.4 Results and Discussion....................................................................................... 97
4.5 References........................................................................................................ 105
Appendix.................................................................................................................. 108
Chapter 5: Experimental Study of Substrate Roughness on the Local Glass Transition of Polystyrene............................................................................... 119
5.1 Synopsis............................................................................................................ 119
5.2 Introduction...................................................................................................... 119
5.3 Experimental Methods..................................................................................... 122
5.3.1 Roughening Silica Substrate without Changing Surface Chemistry......... 122
5.3.2 Sample Preparation and Fluorescence Measure of Local Tg(z=0) next to Rough Substrate 125
5.4 Results and Discussion..................................................................................... 127
5.4.1 Local Increases in Tg next to Roughened Substrates................................ 127
5.4.2 Characterizing Substrate Roughness......................................................... 131
5.4.3 Other Possible Influences of Increasing Substrate Roughness: Increased Chain Adsoption? 137
5.5 Conclusions...................................................................................................... 141
5.4 References........................................................................................................ 142
Chapter 6: Summary and Conclusions............................................ 146
References................................................................................................................ 152
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