Topology driven and thermally driven dynamic properties of glass forming materials

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2021-12

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Abstract

The temperature dependence of relaxation time is one of the most important topics in the study of glass forming process. This thermally driven aspect of the glass forming event has been extensively studied in both experimental and theoretical domain. To study the dynamics at equilibrium state below the nominal glass transition temperature is especially challenging due to the long-time aging problem, but progress has been made. Recently, the problem is finessed by McKenna and co-workers who measured the dynamics in the upper bound region of 20-million-years-old amber (20 Ma) and amorphous Teflon film with extremely low fictive temperatures. The results strongly challenged the idea of ideal glass transition and supported that the dynamics of glass forming system do not diverge at temperatures above 0 K. These data also provide an opportunity to examine the glass transition models built for predicting the dynamics of the material in the glass forming process. In this dissertation, five glass transition models which can be categorized as configurational entropy glass transition models (generalized entropy theory (GET), the random first order transition theory (RFOT), and the DiMarzio and Yang model) and elastic glass transition models (elastically collective nonlinear Langevin equation theory (ECNLE) and the shoving model) are evaluated by the dynamic data of 20 Ma amber and amorphous Teflon. The results suggest that all the elastic glass transition models can predict non-diverging behavior below glass transition temperature. The configurational entropy models also can predict non-diverging behavior when the appropriate temperature dependence of configurational entropy is applied. However, all the glass transition models evaluated in this dissertation predict longer relaxation times than that of the experimental data suggest. In addition to thermally driven glass forming event, recently some simulation studies suggest that the ring-like molecular topology can lead to topological glass state which means the system can be kinetically frozen due to the circular topology. Such frozen state can occur at the temperature far above glass transition temperature, and the driving force of the formation of such topological glass is the interpenetration of ring molecules. The extremely slow dynamics at temperature far above glass transition temperature (which is a feature of topological glass) is first seen in this study of highly entangled circular polymer poly(3,6-dioxa-1,8-octanedithiol) (PolyDODT). The most entangled cyclic PolyDODT melt we studied has about 300 entanglements calculated based on weight average molecular weight. Multiple characterizations have done on the cyclic PolyDODT to quantify the purity, the results suggest the high purity, but we cannot guarantee the cyclic PolyDODT studied in this work is free of linear chain due to the limitation of current characterization technique. The linear rheology properties of cyclic PolyDODT is studied in this work. In addition to the slow dynamics of highly entangled cyclic molecule, a well-defined rubbery plateau is observed which is never seen before due to the size of cyclic molecule studied before is relatively small. Since the molecular weight of cyclic PolyDODT melts we have are big, we studied the behavior of solutions of cyclic PolyDODT to expand the range of number of entanglements. The concentration dependence of plateau modulus is found similar with that of linear polymer solution plateau modulus~C^2.3. After proper scaling we found the less entangled cyclic PolyDODT has a Rouse-like behavior η∝Zw^1.0 until the entangled level is up to that of most entangled ring (polystyrene ring with about 13 entanglements) studied recently. When the cyclic PolyDODOT get further entangled the viscosity varies as η∝Zw^6(solution data is consistent with melt data after scaling) which is very different from that of linear polymer( η∝Mw^3.4). We also found the concentration dependence of steady state recoverable compliance of cyclic PolyDODT solution is weak compared to the common linear polymer (for example: Polystyrene) and the molecular weight dependence of steady state recoverable compliance follows Js~Mw^2 which is similar with many studied cyclic polymer.

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Keywords

Glass Transition, Configurational Entropy Glass Transition Model, Elastic Glass Transition Model, Cyclic Molecular Topology, Rheology

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