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Degree

Doctor of Philosophy, University of Akron, Polymer Science, 2005.

Abstract

Molecular weight (MW) and architecture are two important parameters of a synthetic polymer. Their roles on polymer properties including dynamics have not been well understood yet. In this thesis, we have used various techniques including light, neutron scattering and dielectric spectroscopy to elucidate their influences on polymer dynamics within a broad time (frequency) range, covering chain, segmental relaxation and fast dynamics. Comparisons between different polymers were made to understand the role of chemical structure in determining MW dependence of the dynamic behavior. Experimental results showed that different physical properties studied appear to have similar molecular weight dependence in the sense that they all saturate when chains approach Gaussian coil behavior. We demonstrate that the difference in the molecular weight dependence for various polymers does not correlate with either the difference in the Kuhn segment length or molecular weight between entanglements. Instead, we propose to introduce an additional parameter, mR (molecular weight associated with each step of the Random walk in the approximation of Gaussian chain) that might be important for characterizing the molecular weight dependence of chain statistics and many physical properties. The most intriguing result is that the molecular weight dependence of the fast dynamics, sound velocity and fragility observed in polystyrene is opposite to the one observed in polyisobutylene, although Tg increases with molecular weight in both cases. We speculate that difference in symmetry of the monomer structures is responsible for the opposite behavior. Studies of the influence of architectures on fast and segmental relaxation were also carried out. We found that in the case of polybutadiene both of them scale better with total molecular weight instead of the molecular weight of each arm, as suggested by the chain end free volume model. Analysis of the branching effect on the segmental relaxation illustrates similarity to the blending of the same polymer with different molecular weights.

Keywords

Polymer Dynamics; Glass Transition; Fast dynamics; Fragility; Molecular Weight Effect; Depolarized Light Scattering

Advisor

Alexei P Sokolov

Document number: akron1115933800
Permalink: http://rave.ohiolink.edu/etdc/view?acc_num=akron1115933800

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