Thermodynamic Characterization of Polymorphs in Bulk-Crystallized Syndiotactic Polystyrene via Small/Wide-Angle X-ray Scattering and Differential Scanning Calorimetry

• Published in: Macromolecules 2009-06, Vol.42 (12), p.4200-4207
• Main Authors: Su, C. H, Jeng, U, Chen, S. H, Cheng, C.-Y, Lee, J.-J, Lai, Y.-H, Su, W. C, Tsai, J. C, Su, A. C
• Format: Article
• Language: English
• Subjects: Applied sciences; Exact sciences and technology; Organic polymers; Physicochemistry of polymers; Properties and characterization; Structure, morphology and analysis
• ISSN: 0024-9297; 1520-5835
• DOI: 10.1021/ma900384b

By means of in situ small/wide-angle X-ray scattering (SAXS/WAXS) and differential scanning calorimetry (DSC), we examined evolutions of lamellar crystal thickness for α and β crystals, respectively, in bulk-crystallized syndiotactic polystyrene (sPS) during the partial melting−reorganization process upon progressive heating up to 290 °C. For the SAXS data analysis, the Kratky−Porod approximation proves to be particularly helpful in extracting the crystal thickness when approaching final melting where crystalline lamellae (near equilibration with the melt) exist in low concentrations as dispersed entities instead of in arrays. On the basis of the crystal thicknesses at elevated temperatures under solid−melt equilibration, we constructed melting lines of the two separate forms in the Gibbs−Thomson phase plane. The extrapolated (to infinite lamellar thickness) equilibrium melting temperature T m,α* ≈ 294 °C of the α form is moderately lower than T m,β* ≈ 306 °C of the β form. The two melting lines intercept at a crossover temperature T Q ≈ 284 °C and crystal thickness l Q ≈ 9.6 nm, where the relative thermal stability of the two phases inverses. For crystals thicker than l Q (practically hard to reach for bulk crystallization under ambient pressure), the β form is the stable phase; for crystals thinner than l Q (the commonly accessible case), the α form is circumstantially more stable. With crystallinity-corrected values of the heat of fusion ΔH f,α ≈ 82 MJ m−3 and ΔH f,β ≈ 146 MJ m−3 obtained from a combination of DSC and WAXS results, we determined from the slope (= 2σe/ΔH f) of the melting line that basal surface energy σe,α ≈ 8.2 mJ m−2 and σe,β ≈ 26.8 mJ m−2, which are considerably lower than those expected for tight folds, indicative of nonadjacently re-entered or loosely looped folds. The combination of lower T m*, ΔH f, ΔS f, and σe values renders the α phase highly competitive in the rate of nucleation at low temperatures but much less so at high temperatures as compared to the β phase. The higher σe,β value is also consistent with the observation that the β phase is more responsive to externally added heterogeneous nucleation agents.