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

Abstract

Four crystal types (α, β, γ, δ) and some mesophases/sub-modifications have been identified and discussed in syndiotactic polystyrene (sPS). The α- and β-forms are the main crystal packing forms in thermally-processed sPS, while the γ- and δ-crystals are identified only in solvent-treated sPS. In addition, the δ- and γ-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 δ- and γ-crystal in solvent-treated sPS are more like mesophases that transform readily to the α-, β″- or β′-crystal upon heating the solvent-treated sPS to high temperatures near melting. This review thus focuses on studies of the α, β-crystals in sPS, and provides comprehensive discussions on the thermal behavior, crystal structures, thermodynamics, kinetics, and stability of these two major crystal packings (α vs. β) 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 α- or β-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 α- or β-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 β′-crystal while the P-II and P-IV peaks are attributed to the α″-type. In addition, kinetic and thermodynamic characterization has been thoroughly performed on individually isolated crystal types. The α-crystal of sPS has a lower melting temperature than the β-crystal, with T m, α 0=281.7°C and T m, β 0=288.7°C. The crystallization kinetics of the α-crystal is a heterogeneous nucleation with higher rates while the β-crystal is a homogeneous nucleation with lower rates. The β′-type is more thermodynamically stable than the α″-type; but the α″-type is kinetically more favorable. In addition, although there is literature report concerning a transformation of δ or γ mesophase crystals to α- or β-crystal; there is no evidence showing a solid–solid transition from the α- to β-crystal or β- to α-crystal during normal thermal processes. It suggests that both α- to β-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 β′-crystal is a homogeneous nucleation with lower rates. By comparison, crystallization kinetics of the α-crystal is a heterogeneous nucleation with higher rates. Microscopy characterization also revealed a highly nucleated crystallization of the α-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 β-crystal than neat sPS when subjected to the same melt crystallization conditions.