Mechanical Properties and Reliability of Semi-Crystalline Polymers

The major effort of this project has been the study of the physical and mechanical properties of crystalline polymers and the changes that take place over long-time periods. The study of the long-time crystallization, at ambient temperature, of a series of linear and branched polyethylenes showed a slow but steady density increase with time that could be demonstrated to be due to a decrease in the content of the liquid-like region and an increase in the partially ordered interfacial region. Significantly, the core-crystallinity level remains unaffected by the long time density increase. A detailed study of the overall crystallization kinetics of a set of model random copolymers, the hydrogenated polybutadienes, that covered a wide range in molecular weight and counit content was competed. A basic understanding of the crystallization process in random copolymers, and of the structural factors which control long- time crystallization from the melt was developed. For the first time it was possible to separate the influence of molecular weight and co-unit content as independent variables that govern crystallization.