Gas transport behavior of semicrystalline syndiotactic polystyrene

Solid-state structure, crystalline morphology, crystallization kinetic, and gas transport properties of thermally crystallized semicrystalline syndiotactic polystyrene (s-PS) have been investigated in this study. Solid-state structure of syndiotactic polystyrene (s-PS) after crystallization from the melt and the glassy state was examined by differential scanning calorimetry (DSC), density and wide angle x-ray diffraction analysis (WAXS). The measurements confirmed the low density of both crystalline forms, which in the case of a crystalline form was smaller and in the case of β crystalline form was slightly larger than the density of the glassy amorphous s-PS. Positron annihilation lifetime spectroscopy (PALS) experiments have been also carried out to study the free volume properties of these materials.; The crystalline morphologies of s-PS containing pure α and β phase were examined via atomic force microscopy (AFM) and optical microscopy (OM). The β crystalline phase exhibited typical spherulitic morphology. The morphology of the a crystalline phase of limiting ordered modification (α″-form) consists of individual randomly oriented lamellae imbedded in the matrix of amorphous s-PS. The a crystalline phase of limiting disordered modification (α′-form) exhibited small spherulite-like morphology with lack of regular birefringence pattern. The crystalline morphology observations were in good agreement with the crystallization kinetic data.; Oxygen and carbon dioxide gas permeability, diffusion and solubility of semicrystalline s-PS containing different crystalline forms were studied as a function of crystallinity. Syndiotactic polystyrene containing the β crystalline form exhibited decrease of permeability with crystallinity. On the contrary, s-PS containing less dense a crystalline form showed increase of permeability with crystallinity, meaning highly gas permeable to small gas molecules. Unusual gas transport behavior of the α crystalline form was attributed to porous crystalline structure containing the nanochannels. Despite the porous structure, α crystalline form showed very low oxygen and carbon dioxide solubility compared to gas solubility in the amorphous phase. The proposed diffusion model explained the characteristic features of the gas permeation behavior for chemically “inert” small gas molecules in the permeation medium consisting of glassy amorphous polymer with dispersed porous crystalline phase containing the nanochannels. The diffusions of oxygen gas in amorphous and in crystalline phases possess the same activation mechanism of diffusion, which is controlled by the amorphous phase. The diffusions of various gas molecules were also investigated. The diffusion coefficient linearly decreased with increasing size of gas molecules. The big-size gas molecules, such as Ar, O₂, N₂ and CH₄, were expected to go through only in the parallel direction along the channel in the α″ crystal while the small-size gas molecules, such as He and Ne, could diffuse in the parallel and also the perpendicular directions in the channel.