Structural evolution of syndiotactic polystyrene (sPS) in a cold crystallization process was quantitatively examined with in-situ small/wide-angle X-ray scattering and differential scanning calorimetry (SAXS/WAXS/DSC). After removal of background scattering from fractal-like matrix structure, SAXS profiles obtained during programmed heating of an amorphous sPS specimen from 30 to 240 at 10°C/min can be interpreted with a similar sequence of events previously observed in cold crystallization of poly(9,9-di-noctyl-2,7-fluorene) (PFO). Specifically, the nanograin evolution of sPS involves four stages: (1) the frozen-in stage below 90°C, (2) the nucleation of oblate-like nanograins with a constant radius of gyration R g ≈2.6 nm between 90 and 130°C, (3) the growth of nanograin size to R g ≈ 3.2 nm concomitant with the emergence and development of WAXS-determined crystallinity (X c,WAXS ) from 130 to 180°C, and finally (4) the coalescence (and thickening) of the nanocrsytals into a greater size of R g ≈ 4.6 nm upon further heating up to 240°C. Developments in the DSC-determined crystallinity (X c,DSC ) coincided with nucleation and growth stages, whereas the SAXS-determined heterogeneity (Q inv ) increased steadily throughout nucleation, growth, and coalescence stages. Little changes of morphological features in the nanometer-length scale can be observed with subsequent isothermal annealing at 240°C up to 1 h; the final size of coalesced nanograins at this temperature is therefore attributed to the balance between the tendency to eliminate lateral surface via coalescence and the opposing strain field (due to locked and tightened entanglements) in the surrounding matrix. Delicate differences in nanograin size and shape during cold crystallization processes of sPS and PFO are discussed in terms of differences in chain rigidity (random coiled vs semirigid) and melt structure (isotropic vs nematic).

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 R 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,R * ≈ 294 °C of the R 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 R form is circumstantially more stable. With crystallinity-corrected values of the heat of fusion ΔH f,R ≈ 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 (= 2σ e /ΔH f ) of the melting line that basal surface energy σ e,R ≈ 8.2 mJ m -2 and σ e,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 σ e values renders the R 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 σ e,β value is also consistent with the observation that the β phase is more responsive to externally added heterogeneous nucleation agents.