Molecular dynamics simulation study of norbornene-POSS polymers

Atomistic molecular dynamics simulations have been used to delineate the ef fects of introducing polyhedral oligomeric silsesquioxane (POSS) moieties s ubstituted by cyclopentyl and cyclohexyl rings as pendant groups on polynor bornene. Simulations were also performed on polynorbornene for comparison. Calculated volume-temperature behavior and X-ray scattering profiles matche d well with experimental results. Most importantly, the effects of incorpor ating the POSS moieties into the polymer have been identified via simulatio ns. These were judged on the basis of the increase in the glass transition temperature, retardation of the chain dynamics and improvements in the calc ulated elastic tensile, bulk and shear moduli of the POSS containing polyme rs compared to the norbornene homopolymer. The most important conclusion fr om the study is that aggregation of the POSS moieties is not required for t he beneficial effects to be realized. Indeed, the simulations show that the re 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. Pac king features are delineated with the aid of intermolecular site-site radia l distribution functions. In addition, the mean squared displacement of the POSS moietes in the polymer matrix was found to be very small at all tempe ratures leading to a slowing of the segmental dynamics of the polymer chain , and thereby imparting the macroscopically observed stiffness. It is reaso ned that the chief source of reinforcement arises from the POSS moieties be having as strong anchor points in the polymeric matrix. This has more to do with the ponderous nature of these moieties versus any specific intermolec ular interactions. (C) 2000 Elsevier Science Ltd. All rights reserved.