Molecular dynamics simulation study of norbornene–POSS polymers

Abstract

Atomistic molecular dynamics simulations have been used to delineate the effects of introducing polyhedral oligomeric silsesquioxane (POSS) moieties substituted by cyclopentyl and cyclohexyl rings as pendant groups on polynorbornene. Simulations were also performed on polynorbornene for comparison. Calculated volume–temperature behavior and X-ray scattering profiles matched well with experimental results. Most importantly, the effects of incorporating the POSS moieties into the polymer have been identified via simulations. These were judged on the basis of the increase in the glass transition temperature, retardation of the chain dynamics and improvements in the calculated elastic tensile, bulk and shear moduli of the POSS containing polymers compared to the norbornene homopolymer. The most important conclusion from the study is that aggregation of the POSS moieties is not required for the beneficial effects to be realized. Indeed, the simulations show that there is no tendency for aggregation to occur among the POSS moieties if they are well dispersed to begin with over the time scale of the simulation. Packing features are delineated with the aid of intermolecular site–site radial distribution functions. In addition, the mean squared displacement of the POSS moieties in the polymer matrix was found to be very small at all temperatures leading to a slowing of the segmental dynamics of the polymer chain, and thereby imparting the macroscopically observed stiffness. It is reasoned that the chief source of reinforcement arises from the POSS moieties behaving as strong anchor points in the polymeric matrix. This has more to do with the ponderous nature of these moieties versus any specific intermolecular interactions.