Effect of physical aging of poly(1-trimethylsilyl-1-propyne) films synthesized with TaCl₅ and NbCl₅ on gas permeability, fractional free volume, and positron annihilation lifetime spectroscopy parameters

The effect of physical aging on the gas permeability, fractional free volume (FFV), and positron annihilation lifetime spectroscopy (PALS) parameters of dense, isotropic poly(1-trimethylsilyl-1-propyne) (PTMSP) films synthesized with TaCl₅ and NbCl₅ was characterized. As-cast films were soaked in methanol until an equilibrium amount of methanol was absorbed by the polymer. When the films were removed from methanol, film thickness initially decreased rapidly and was almost constant after 70 h in air for both catalysts. This timescale was much longer than the timescale for complete methanol desorption (ca. 5 h). From the film-thickness data, the reduction in FFV with time was estimated. For samples prepared with either catalyst, the kinetics of FFV reduction were well-described by a simple model based on the notion either that free-volume elements diffuse to the surface of the polymer film and are subsequently eliminated from the sample or that lattice contraction controls polymer densification. Methane permeability decreased rapidly during the first 70 h, which was the same timescale for the thickness change. The decrease in methane permeability was smaller in films prepared with NbCl₅ than with TaCl₅. The logarithm of methane permeability decreased linearly as reciprocal FFV increased, in accordance with free-volume theory. The PALS results indicate that the concentration of larger free-volume elements (as indicated by the intensity I₄) decreased with aging time and that the other PALS parameters were not strongly influenced by aging.