THERMOREVERSIBLE GELATION OF SYNDIOTACTIC POLYSTYRENE IN BENZENE

The temperature-concentration phase diagram of syndiotactic polystyren e (sPS)/benzene gels establishes the existence of two compounds, C-1 a nd 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 benze ne. In both compounds the chains take on a 21 helical form. Neutron di ffraction investigations confirm the existence of these compounds but also disclose that compound C-1 possesses a novel crystalline form. Ne utron scattering experiments reveal that sPS chains can be described b y worm-like statistics in the molten state while still retaining a con formation close to the 2(1) helix. It is suggested that helix stabiliz ation occurs by means of benzene molecules intercalated between the ph enyl groups as previously proposed for isotactic polystyrene. A molecu lar model is put forward. The neutron scattering curves obtained in th e gel state, although smeared by strong interchain scattering, are con sistent with the absence of chain folding. The link between the fibril lar morphology of the gel and helix stabilization is discussed.