SOLVENT QUALITY AND PHASE-STABILITY IN SYNDIOTACTIC POLYSTYRENE-SOLVENT SYSTEMS

The phase behavior of syndiotactic polystyrene in solvents with differ ent qualities has been investigated. The concentration dependence of t he melting points of the beta- and delta-phases has been measured, and the experimental data have been used to construct the corresponding l iquidi. The different solvents can therefore be classified in order of increasing solvent quality for polystyrene: decalin < 1,2,4-trichloro benzene < o-xylene < 1,2-dichlorobenzene < chlorobenzene < chloroform. The stability of the beta-phase increases with decreasing solvent qua lity. The opposite influence of this solvent quality is observed for t he delta-phase. This solvent quality induced inversion of the phase st ability explains the peculiar behavior in good solvents like chlorofor m and the formation of thermoreversible gels in these solvents. The st udy of the phase behavior and the analogy with the melting behavior of mixtures of low molecular mass substances reveal the formation of an incongruent melting polymer-solvent compound. A two-step mechanism com posed of a coil to helix transition followed by an intermolecular asso ciation is proposed.