Development of the molecular design rules of ultra-permeable poly[1-(trimethylsilyl)-1-propyne] membranes

Poly[1-(trimethylsilyl)-1-propyne] (PTMSP) is known to have the highest per meability coefficients for glassy polymers. However, unlike other glassy me mbranes such as polycarbonates or polysulfones, the high permeability of PT MSP decays as a function of time. A series of measurements of the gas perme ability coefficients were reported in this work to highlight the effect of the physical ageing on the permeability of N-2 gas molecules through PTMSP membranes. In order to develop the molecular design rules for this high per meability, and the molecular structural parameters mostly responsible fur t he physical ageing, molecular dynamics techniques were used to investigate the effect of the presence of the double bond. the Si-atom, the bulky trime thylsilyl side group, the co-operative effect of the singular methyl and th e trimethylsilyl side groups on the diffusion coefficients of nitrogen and argon through the polymer. It was shown that a combination of a high volume fraction, high torsion barriers around the single bonds in the main backbo ne and high specific electrostatic interactions due to the presence of the Si-atom are all necessary for high diffusion coefficients of gases through PTMSP membranes. This combination was also shown to provide the necessary v oids fur the diffusion process as well as for characterising the polymer wi th low cohesive energy densities and extra rigidity resulting in an excepti onally high permeability. Loss in free-volume due to the physical ageing ma y thus lead to lower permeability coefficients. (C) 2000 Elsevier Science L td. All rights reserved.