Interplay of the polymer stiffness and the permeability behavior of silaneand siloxane polymers

The effect of the modification of the molecular structure on the stiffness of the polymeric backbones in relation to the diffusion coefficients of typ ical rubbery and glassy silane and siloxane polymers at different temperatu res was investigated. The inflexibility in the polymeric chains as deduced by higher values for the persistence length was shown to correspond consist ently to lower values for the self-diffusion coefficients. Increasing the t emperature resulted in decreasing the persistence length of the silane poly mers and increasing it in the case of the siloxane polymers. This was found to be due to the fact that the bond angle about the oxygen atoms is approx imately 144 degrees and rotations about the oxygen atoms for the trans isom eric states will bring the side groups on the neighboring Si-atoms in close proximity to each other thus increasing the torsional energies of the tran s isomeric states considerably. The obtained simulation results showed an e xcellent agreement to those determined experimentally. (C) 2001 Elsevier Sc ience Ltd. All rights reserved.