UBC Theses and Dissertations
Physical aging in polymer and polymer nanocomposite glasses Liu, Amy
Physical aging in polymer glasses is a nonequilibrium phenomenon characterized by spontaneous processes that lead to changes in almost all physical properties. While all glasses undergo physical aging, polymer glasses are among the most widely used glasses in research and engineering due to their good glass-forming properties and wide applications. This master’s thesis uses molecular dynamics (MD) simulations to explore recently observed experimental phenomena related to physical aging in polymer glasses and polymer nanocomposites. Two manuscripts (published and to be published) have been produced in the duration of this work and are included in chapters 2 and 3. The first manuscript focuses on the effect of mechanical stresses on physical aging in polymer glasses. This is a complicated and controversial subject as experimental observations have provided evidence both for and against mechanical rejuvenation and overaging. Specifically, we use MD simulations on a coarse-grained bead spring model to investigate the stress-enhanced yielding of polymer glasses. Understanding the origin of this behavior will not only verify whether this can be considered as an example of mechanically induced overaging; it will facilitate the development of more realistic constitutive relations for the yield response of glassy polymers, a subject that has been awarded considerable attention in the last few decades. The second manuscript is concerned with a relatively new area of research, which involves altering the aging behavior of polymer glasses with the addition of nanoscopic fillers. These polymer nanocomposite systems are difficult to explore due to the complicated interactions between the fillers and the polymer matrix. Unfortunately, limited experimental work (and no simulation work known to the author) have been devoted to investigating the physical aging properties of these systems. In this work, we attempt to develop a coarse-grained polymer nanocomposite MD model to study the impact of nanoparticles on the physical aging behavior of glassy polymers. This thesis should motivate the use of simulations in conjunction with experiments when studying physical aging in glasses.
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