Effects of Confinement and Interfaces on Stress Relaxation and Stiffness in Polymer Films and Nanocomposites Characterized by Novel Fluorescence Techniques

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Advancements in nanotechnology have led to the production of devices with components with sub-100 nm size scales. Studies have investigated how properties of polymers change when confined to nanoscale dimensions. However, stiffness-confinement studies have reported increases, decreases, and invariance with confinement for different polymer/substrate pairs, making it difficult to obtain general trends in such behavior. The major focus of this dissertation is to develop a fundamental understanding of stiffness-confinement effects. A novel fluorescence technique is developed to study stress relaxation and stiffness in PS films and nanocomposites. It is shown that stress relaxation occurs over hours despite the films being tens of degrees above Tg. Stiffness-confinement studies demonstrate that single-layer films stiffen with confinement with stronger effects in the rubbery state. Bilayer film studies show that stiffness is enhanced near a substrate and reduced near a free surface. The fluorescence/trilayer technique is used to directly characterize stiffness gradients enabling the first comparison of stiffness gradient length scales obtained using two different techniques – fluorescence and atomic force microscopy (AFM). Both show agreement that stiffness gradients depend on whether the model nanocomposites are confined or bulk. Fluorescence demonstrates that thermal history impacts magnitudes and length scales associated with stiffness perturbations in model nanocomposites at room temperature. It is demonstrated that PS is more susceptible to substrate perturbations in the rubbery state and more susceptible to surface perturbations in the glassy state. The tunability of stiffness-confinement behavior is shown in PS films containing a plasticizer. The fluorescence approach is extended by using a broader class of vibronic coupling dyes to characterize stress relaxation and Tg. These studies help identify trends in stiffness-confinement behavior and help to provide consensus among various reports. This dissertation provides an understanding of Tg-confinement behavior of PS in complex geometries such as polymer brushes and supported nanorods. In brushes grown from nanoparticles, Tg and fragility decrease with sufficiently low molecular weight while Tg breadth increases. In PS nanorods, intermediate molecular weight PS exhibited no change in Tg with reduced rod diameter. However, high molecular weight PS exhibited reductions in Tg with decreasing rod diameter due to intrinsic size effects.

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  • 03/13/2018
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