Polymorphism, thermal behavior, and crystal stability in syndiotactic polystyrene vs. its miscible blends

Four crystal types (alpha, beta, gamma, delta) and some mesopbases/sub-modifications have been identified and discussed in syndiotactic polystyrene (sPS). The alpha- and beta -forms are the main crystal packing forms in thermally-processed sPS, while the gamma- and delta-crystals are identified only in solvent-treated sPS. In addition, the delta- and gamma-forms are of a monoclinic crystal cell (with helical chain conformation) and the cell dimensions depend on the types and amount of residual solvent trapped in the crystal. The delta- and gamma-crystal in solvent-treated sPS are more like mesophases that transform readily to the alpha-, beta- or beta'-crystal upon heating the solvent-treated sPS to high temperatures near melting. This review thus focuses on studies of the alpha, beta-crystals in sPS, and provides comprehensive discussions on the thermal behavior, crystal structures, thermodynamics, kinetics, and stability of these two major crystal packings (alpha vs. beta) in sPS upon melt crystallization in comparison with cold crystallization. Analyses of melting behavior, diffractograms, or IR spectra, etc. of sPS can be complicated by the presence of co-existing polymorphic crystals. In general, a total of four melting peaks (labeled as P-I, P-II, P-III, P-IV from low to high temperatures) have been identified in a melt-crystallized sPS that typically contains mixed fractions of both crystals. By refining the techniques of obtaining sPS with individually isolated alpha- or beta-crystal, recent studies have been able to correct suspected inaccuracy of some thermodynamic and kinetic measurements in earlier studies and to interpret the relative stability of the various crystals in sPS. sPS samples could be prepared such that they contained purely isolated alpha- or beta-crystal, and the individual crystal types are used for more precise characterization of analysis. The P-I and P-III melting peaks are attributed to the beta'-crystal while the P-II and P-IV peaks are attributed to the alpha-type. In addition, kinetic and thermodynamic characterization has been thoroughly performed on individually isolated crystal types. The alpha-crystal of sPS has a lower melting temperature than the beta-crystal, with T-m,alpha(0) = 281.7 degreesC and T-m,beta(0) = 288.7 degreesC. The crystallization kinetics of the alpha-crystal is a heterogeneous nucleation with higher rates while the beta-crystal is a homogeneous nucleation with lower rates. The beta'-type is more thermodynamically stable than the alpha-type; but the alpha-type is kinetically more favorable. In addition, although there is literature report concerning a transformation of delta or gamma mesophase crystals to alpha- or beta-crystal; there is no evidence showing a solid-solid transition from the alpha- to beta -crystal or beta- to alpha-crystal during normal thermal processes. It suggests that both alpha- to beta-crystal are stable solid and transformation between them can only be achieved by melting and re-packing. This could be fully explained using a stability/metastability chart of free energy vs. temperature. Nevertheless, the individual melting/reorganization of these two crystals might undergo crystal transformation via solid-liquid-solid transition. The crystallization kinetics of beta'-crystal is a homogeneous nucleation with lower rates. By comparison, crystallization kinetics of the alpha-crystal is a heterogeneous nucleation with higher rates. Microscopy characterization also revealed a highly nucleated crystallization of the alpha-crystal. The effect of blend miscibility on the polymorphism behavior in sPS is also discussed. The effects of miscibility on polymorphism was investigated by studying miscible blends of sPS with atactic polystyrene (aPS) or sPS with poly(1,4-dimethyl phenylene oxide) (PPO). Miscible blends containing sPS have been found to favor growth of beta-crystal than neat sPS when subjected to the same melt crystallization conditions. (C) 2001 Elsevier Science Ltd. All rights reserved.