Nano-scale structural characterization of polymers subjected to stresses and thermal variations

Macroscopic properties of polymers are determined not only by their chemical constituents, but also by their architecture or configuration. Structural details at the molecular level are extremely important for the design of high-performance polymer material. However, it is more difficult and expensive to obtain atomic level information through experiments. Computer simulation techniques can be applied to achieve this goal, in which the effect of external influence on the micro-structure of a polymer can be studied.; The evolution of micro-structure in a model polymer subjected to thermal and mechanical processes was investigate by using a domain-decomposition molecular dynamics technique. Molecular Dynamics (MD) simulations were carried out using the united atom model to understand the behavior of polymer melting and re-solidification, and structure evolution during mechanical stretching.; The MD simulations captured the events during the melting and solidification. Percentile trans and end-to-end distance (eed) versus temperature data show three distinct regions, namely the melt, melting/solidification transition stage, and the solid stage. In the melting and solidification stage, there is a sharp change in %trans and eed. Inter-molecular pair correlation distribution at different temperatures during the melting process shows that the system loses its inter-molecular order with increasing temperature. The degree of crystallinity and segmental orientation function at different draw ratios were calculated from the simulation data and compared to experimental results. The comparison showed good agreement between simulation and experiments. Stress-strain behavior of the system was studied at different temperatures and stress ramp-up rates. The results obtained show a qualitative match with the behavior of actual polymeric materials.; The simulation data of free volume distribution confirmed the experimental suggestion, i.e., chain alignment causes an increase in the number of larger, more ellipsoid free volume holes in polymers, while the density decreases with stretching. Voronoi polyhedron volume distribution for atoms at chain ends show increasing volume with increasing draw ratio, while polyhedron volume distribution for atoms not at chain ends show decreasing volume with increasing draw ratio. This phenomenon indicates the larger void during stretching is formed at chain ends.