Relaxation Processes in Polymers

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Subglass relaxation processes, resulting from molecular motions in otherwise glassy solids, occur commonly in amorphous polymers and in the amorphous fraction of semicrystalline ones. They have been presumed to be due to localized conformational motions decoupled from the longer range segmental motions associated with the glass transition. In appropriate circumstances, both reorientation of flexible side groups and local conformational mobility in the main chain have been proposed as mechanisms. However there is little direct evidence that these notions are correct. The present research will take advantage of dielectric relaxation data recently collected on systems deliberately chosen to be good model systems for side group relaxation and of recent progress made in the atomistic simulation of the packing and properties of bulk amorphous systems in developing a detailed molecular model for subglass relaxation due to flexible side group motion. Motions of pendant ester groups will be modelled in a bulk multi-chain environment. Similar modelling will be carried out for a relaxation of the local main chain type, the gamma relaxation in polyethylene. Previously suggested local motions will be modelled in an atomistic bulk environment and other localized motions will be sought as well. In other research, advantage will be taken of the ability to make dielectric measurements over a broader frequency or time range to experimentally investigate the shapes of a number of types of relaxation processes and to assess the ability of various phenomenological equations to represent the shapes. Since an important part of the research involves numerically intensive computing, efforts will also be made in the area of improved simulation techniques.