Developing Experimental Methods to Investigate Local Changes in Density and Dynamics of Thin Polymer Films Public
Han, Yixuan (Spring 2022)
Published
Abstract
In this dissertation, I address changes to local material properties in polymer thin films (density and dynamics) perturbed by interfacial effects under nanoconfinement. I measured the refractive index with ellipsometry to infer density changes in polymer thin films under confinement. I have found similar apparent increases in the thickness-dependent refractive index trend n(h) for three different polymers, polystyrene (PS), poly(methyl methacrylate) (PMMA), and poly(2-vinyl pyridine) (P2VP), despite the differences in polymer-silica substrate interactions. I tested for possible sources of non-uniform polarizability by varying the molecular weight, polydispersity, forming a film from bilayers with different interfaicial widths, and altering the surface chemistry of the substrate. However, the large unphysical apparent increase cannot be explained by any of these tests. I concluded that the use of homogeneous (uniform and isotropic) layer approximations can lead to unphysical results when film inhomogeneities and local property changes are frequently present in polymer thin films. I then developed an ellipsometric optical layer model with a depth-dependent refractive index gradient to model thin polymer films of PS, PMMA and P2VP. In the optical layer model, I proposed a linear gradient in the magnitude of refractive index, Cauchy parameter A(z), with depth dependent position z. I demonstrated the presence of a strong positive gradient in the magnitude of the refractive index (%grade) for PMMA and PS thin films, while P2VP films show a refractive index gradient that primarily fluctuates around zero for all film thicknesses. This positive gradient in refractive index indicates a higher density near the free surface, counter to common expectations of a simple free volume correlation between density and dynamics. I rationalize this denser than bulk region near the free surface based on the vapor deposited stable glasses with optimized denser molecular packings caused by the presence of the observed faster dynamics at the free surface.
In addition to probing the local density changes in polymer thin films, I developed a new experimental method using the fluorophore perylene to probe dynamics via fluorescence spectroscopy. I measured the temperature dependence of perylene doped in bulk PS, PMMA, P2VP, and polycarbonate (PC) films, defining a fluorescence intensity ``shift factor" log(a_T) based on the intensity ratio between the intensity of the first peak and the intensity of temperature invariant self-reference region (SRR). I found that the temperature dependence of log(a_T) associated with the nonradiative decay process reflects the local polymer dynamics transitioning from the liquid to glassy regimes, where the rate of nonradiative decay is influenced by cooperative alpha-relaxation in the supercooled liquid regime, and the local beta-relaxation in the glassy regime.
Table of Contents
1 Introduction 1
1.1 Synopsis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 Introduction to polymer glasses . . . . . . . . . . . . . . . . . . . . . 2
1.2.1 Dynamics of glasses . . . . . . . . . . . . . . . . . . . . . . . . 2
1.2.2 Glass transition . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2.3 Polymer basics and polymeric glasses . . . . . . . . . . . . . . 7
1.3 How are density and dynamics related? . . . . . . . . . . . . . . . . . 8
1.3.1 Density considered as a key factor to understand the glass transition. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
1.3.2 Confinement effects in polymer thin films . . . . . . . . . . . . 11
1.3.3 Summary of literature on density changes in thin polymer films 13
1.4 How are modulus and the glass transition related? . . . . . . . . . . . 17
1.4.1 Modulus behavior of bulk polymers . . . . . . . . . . . . . . . 17
1.4.2 Previous efforts to investigate modulus changes in thin polymer films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
1.5 Dissertation outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
2 Experimental Methods 40
2.1 Synopsis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
2.2 Ellipsometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
2.2.1 Basics of ellipsometry and instrumentation . . . . . . . . . . . 40
2.2.2 Light reflection at interfaces . . . . . . . . . . . . . . . . . . . 42
2.2.3 Optical modeling of the sample . . . . . . . . . . . . . . . . . 44
2.3 Fluorescence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
2.3.1 Basics of fluorescence . . . . . . . . . . . . . . . . . . . . . . . 48
2.3.2 Instrumentation of fluorescence . . . . . . . . . . . . . . . . . 50
3 Comparing Refractive Index and Density Changes with Decreasing
Film Thickness in Thin Supported Films Across Different Polymers 53
3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
3.2 Experimental methods . . . . . . . . . . . . . . . . . . . . . . . . . . 57
3.3 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 60
3.3.1 Comparing refractive index trends in thin fims of P2VP, PMMA, and PS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
3.3.2 Large apparent increase in refractive index: Testing sources of film inhomogeneities . . . . . . . . . . . . . . . . . . . . . . . 67
3.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
4 Gradient in Refractive Index Reveals Denser Near Free Surface Region in Thin Polymer Films 82
4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
4.2 Experimental methods . . . . . . . . . . . . . . . . . . . . . . . . . . 85
4.3 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 87
4.3.1 Linear gradient model in refractive index applied to PMMA thin films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
4.3.2 Fitting improvements of the linear gradient model over the homogeneous model . . . . . . . . . . . . . . . . . . . . . . . . . 95
4.3.3 Linear gradient model in refractive index applied to PS and P2VP thin films . . . . . . . . . . . . . . . . . . . . . . . . . . 97
4.3.4 Comparison of linear gradient model results with known dynamical gradients . . . . . . . . . . . . . . . . . . . . . . . . . 105
4.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112
5 Characterizing the Temperature Dependence of Perylene Doped in Various Polymer Matrices 125
5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125
5.2 Experimental methods . . . . . . . . . . . . . . . . . . . . . . . . . . 129
5.3 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 130
5.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143
6 Summary and Conclusions 148
Appendix A Implementation of transfer matrices in MATLAB 163
A.1 Synopsis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163
A.2 Reflection coeffient for p-polarization . . . . . . . . . . . . . . . . . 163
A.3 Reflection coeffcient for s-polarization . . . . . . . . . . . . . . . . . 164
A.4 Transfer matrix of the interface . . . . . . . . . . . . . . . . . . . . . 165
A.5 Transfer matrix of the film . . . . . . . . . . . . . . . . . . . . . . . . 167
A.6 Transfer matrix of a graded polymer film . . . . . . . . . . . . . . . . 168
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