The present paper deals with important differences in the distribution of free volume in high and low free volume polymers as indicated by a joint investigation utilizing molecular modeling and positron annihilation lifetime studies. The main focus of this paper is on the molecular modeling approach. The polymers in question are the ultrahigh free volume polymer poly(1-(trimethylsilyl)-1propyne) (PTMSP) and two polystyrene derivatives containing Si and F. Extended equilibration procedures were necessary to obtain reasonable packing models for the polymers. The transition state Gusev-Suter Monte Carlo method was utilized to prove a reasonable agreement between simulated and measured diffusivity and solubility values for the model structures. The free volume distribution was analyzed for the validated packing models and compared with respective positron annihilation data. In both cases, a bimodal distribution of free volume was observed for PTMSP while the polystyrene derivatives as other conventional glassy polymers showed a more or less monomodal behavior. Good qualitative agreement is demonstrated between size distributions of free volume elements in these polymers obtained via molecular computer modeling and experiments using positron annihilation technique.

Gas permeation, sorption, and free volume was studied in a series of glassy vinylic type Si‐containing polymers. They included p‐substituted derivatives of polystyrene, which were compared with polystyrene, and poly(vinylphenyldimethyl silane). The strongest increase in permeability and diffusion coefficients were observed for the polymer containing the Si(CH3)3 group directly attached to phenylene ring. An introduction of spacers between phenylene ring and Si(CH3)3 group results in much lower gas permeability. Positron annihilation study of free volume in these polymers showed that lifetime spectra are composed of four components. It implies that size distribution of free volume in these polymers, which are characterized by a modest level of gas permeability, is bimodal, a feature previously observed only for extra large free volume and permeability materials such as poly(trimethylsilyl propyne). © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 1612–1620, 2000

No abstract