Thermoreversible gelation of syndiotactic polystyrene in benzene

The temperature-concentration phase diagram of syndiotactic polystyrene (sPS)/benzene gels establishes the existence of two compounds, C-1 and C-2, the stoichiometries of which, expressed as benzene molecules/ monomer, are 4/1 (C-1) and 1/1 (C-2). C-1 is an incongruently-melting compound as it transforms into C-2 at about the boiling point of benzene. In both compounds the chains take on a 21 helical form. Neutron diffraction investigations confirm the existence of these compounds but also disclose that compound C-1 possesses a novel crystalline form. Neutron scattering experiments reveal that sPS chains can be described by worm-like statistics in the molten state while still retaining a conformation close to the 2(1) helix. It is suggested that helix stabilization occurs by means of benzene molecules intercalated between the phenyl groups as previously proposed for isotactic polystyrene. A molecular model is put forward. The neutron scattering curves obtained in the gel state, although smeared by strong interchain scattering, are consistent with the absence of chain folding. The link between the fibrillar morphology of the gel and helix stabilization is discussed.