Abstract
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 approxima- tionprovestobeparticularlyhelpfulinextractingthecrystalthicknesswhenapproachingfinalmeltingwhere 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, weconstructedmeltinglinesofthetwoseparateformsintheGibbs-Thomsonphaseplane.Theextrapolated (to infinite lamellar thickness) equilibrium melting temperature Tm,R* ≈ 294 � C of the R form is moderately lower than Tm,β* ≈ 306 � C of the β form. The two melting lines intercept at a crossover temperature TQ ≈ 284 � C and crystal thickness lQ ≈ 9.6 nm, where the relative thermal stability of the two phases inverses. For crystals thicker than lQ (practically hard to reach for bulk crystallization under ambient pressure), the β form is the stable phase; for crystals thinner than lQ (the commonly accessible case), the R form is circumstantially more stable. With crystallinity-corrected values of the heat of fusion ΔHf,R ≈ 82 MJ m -3 and ΔHf,β ≈ 146 MJ m -3 obtained from a combination of DSC and WAXS results, we determined from the slope (= 2σe/ΔHf) of the melting line that basal surface energy σe,R ≈ 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 Tm*, ΔHf, ΔSf, and σe values renders the R phase highly competitive in the rate of nucleation at low temperatures but much less so at high temperatures as compared tothe βphase.Thehigher σe,βvalueisalsoconsistentwiththeobservationthatthe βphaseismoreresponsive to externally added heterogeneous nucleation agents.
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