| Abstract: | Molecular modeling studies have been completed on cis-PTBA(poly(tert-butylacetylene)) and Sixef44 polyimide, two glassy polymers that can be used to form gas separation membranes. The modeling studies show that polymer backbone bond rotations in PTBA are not thermally allowed. This leads to a helical structure for the cis-PTBA chains which pack as if the helices were rigid rods. Here, polymer free volume is formed by the interstitial space between adjacent helices, and gas transport occurs via continuous diffusion through the resulting channel-like free volume. On the other hand, Sixef44 exhibits a flexible polymer backbone, which leads to the formation of irregular voids. In this case, gas molecules are free to move within the voids, but transport occurs only by hopping to an adjacent void, or by void diffusion. In either case, gas transport is closely coupled to polymer backbone motion. Thus, these studies suggest two different types of free volume and gas transport mechanisms. The diffusion mechanism in glassy polymer membranes will depend on the nature of the free volume (e.g. the type of chain packing), and the polymer backbone chain flexibility. |
