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

摘要

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 a and 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_(3*m~06 °C o f t,he =β form. )The two melting lines intercept at a crosover temperature TQ 284 °C and crystal thickness l_9Q.6 nm, where =the relative thermal stability of two phases inverses.For crystals thicker than l_Q(practicaly hard to reach for bulk crystalization under ambient presure), the form is the stable phase; for crystals thinner than l_ Q(the comonly acesible case), a form is circumstantially more stable. With crystallinity-corrected values of the heat of fusion ΔH_( f,α)=82 MJ m~(-3)and ΔH_(o1-fb4tain6ed 3 )MfromJ =a , mcom)~bin(ation of DSC and WAXS results, we determined from the slope (= 2σ_)28a-/,)Heefe 6o..f8n 2t hme Jd mm 2Δm)αe=~ lJt(iσ-n 2gm=) ,li,n~e) t(h(at basal surfacenergy σ(which are considerably lower than those expected for tight folds, indicative of nonadjacently re-entered or loosely looped folds. The combination of lower T_v,,*SmHeffa luaes rne,nddΔer s t~h_σeΔ α phase highly competitive in the rate of nucleation at low temperatures but much less so at high temperatures as compared to the phase. The higher σ_(vealue is also cons,is)tent with the observation thathe phase is more responsive to externally added heterogeneous nucleation agents.