Atomic-level description of stress in dense polymeric systems

摘要

A new framework is being developed to describe stress production and relaxation in dense polymeric systems. Stress is traditionally defined on the molecular scale, with the chains being regarded as entropic springs. In this view, the polymeric chains are considered to be always in tension and stress is due exclusively to bonded interactions along the chains. Stretching the chains leads to a retractive force of an entropic nature. Non-bonded interactions between atoms belonging to neighboring chains are considered to lead to hydrostatic stress only. The new description is defined on the atomic rather than on the molecular scale. It takes into account all (bonded and non-bonded) interactions, and makes no apriori assumption about the behavior of any component of the system. Since it is defined on the atomic scale, the description captures all fast and slow relaxation modes that correspond to short and long wavelength perturbations, respectively. It is therefore equally applicable to small and large volumes of material and to short and long chains. This report presents a brief overview of this new framework. It is discussed which assumptions made in the molecular theory are justified by atomistic simulation results and which are not. It is shown that both theories are in qualitative agreement with the experimentally observed physical picture, while the atomic scale description offers the advantages mentioned above.