Gas transport properties of side-chain crystalline polymers

The gas sorption and transport properties for a series of side-chain crystalline poly(alkyl acrylate)s are presented as a function of temperature. These polymers are shown to have unique transport properties including a significant change in permeability, or permeation switch, at the melting point.; The amorphous state permeability is observed to increase as the side-chain is lengthened, due primarily to an increase in the diffusion coefficient. The effect of side-chain length on the semi-crystalline state permeability is not strictly dependent on the level of crystallinity owing to morphology. Mixed trends are observed here that also have their basis in the diffusion coefficient. The magnitude of the permeation switch is found to vary significantly with the size of the penetrant and is also dependent on the polymer side-chain length. The penetrant dependence of the permeation switch is postulated to arise from chain immobilization of the amorphous phase by the presence of crystallites. This contrasts with the simple view of these crystals acting only as diffusion barriers, with no influence on the surrounding material. Further evidence refuting the simple two-phase model is presented that shows lower activation energies of diffusion in the molten state compared to the crystalline state. A change in composition occurs at the melting point for side-chain crystalline polymers, but this does not account for the large difference in the permeation switch magnitude between side- and main-chain crystalline polymers.; Gas solubility in amorphous poly(alkyl acrylate)s is observed to depend strongly on the alkyl composition of the polymer. An extrapolation of this solubility data is made to an alkyl volume fraction of unity, or to that of amorphous polyethylene, which shows greater values than those predicted by the Michaels and Bixler two-phase model for polyethylene. The solubility change on melting also does not conform to this two-phase model.; The thermal properties of copolymers and blends based on alkyl acrylates are shown to be tunable based on the composition and side-chain length of the materials involved. The permeation properties of one such copolymer system are reported and found to depend on the lengths of the side-chains, consistent with the trend for homopolymers.