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

Four crystal types (alpha, beta, gamma, delta) and some mesopbases/sub-modi fications have been identified and discussed in syndiotactic polystyrene (s PS). The alpha- and beta -forms are the main crystal packing forms in therm ally-processed sPS, while the gamma- and delta -crystals are identified onl y in solvent-treated sPS. In addition, the delta- and gamma -forms are of a monoclinic crystal cell (with helical chain conformation) and the cell dim ensions depend on the types and amount of residual solvent trapped in the c rystal. 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. Th is review thus focuses on studies of the alpha, beta -crystals in sPS, and provides comprehensive discussions on the thermal behavior, crystal structu res, thermodynamics, kinetics, and stability of these two major crystal pac kings (alpha vs. beta) in sPS upon melt crystallization in comparison with cold crystallization. Analyses of melting behavior, diffractograms, or IR s pectra, etc. of sPS can be complicated by the presence of co-existing polym orphic 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 crys tals. By refining the techniques of obtaining sPS with individually isolate d alpha- or beta -crystal, recent studies have been able to correct suspect ed inaccuracy of some thermodynamic and kinetic measurements in earlier stu dies and to interpret the relative stability of the various crystals in sPS . sPS samples could be prepared such that they contained purely isolated al pha- or beta -crystal, and the individual crystal types are used for more p recise characterization of analysis. The P-I and P-III melting peaks are at tributed to the beta ' -crystal while the P-II and P-IV peaks are attribute d to the alpha " -type. In addition, kinetic and thermodynamic characteriza tion has been thoroughly performed on individually isolated crystal types. The alpha -crystal of sPS has a lower melting temperature than the beta -cr ystal, 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 nucl eation with higher rates while the beta -crystal is a homogeneous nucleatio n 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 o f 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 th at 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 e xplained using a stability/metastability chart of free energy vs. temperatu re. Nevertheless, the individual melting/reorganization of these two crysta ls 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 -cr ystal is a heterogeneous nucleation with higher rates. Microscopy character ization also revealed a highly nucleated crystallization of the a-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 mis cible blends of sPS with atactic polystyrene (aPS) or sPS with poly(1,4-dim ethyl phenylene oxide) (PPO). Miscible blends containing sPS have been foun d 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.