In this study, structural evolution of syndiotactic polypropylene (sPP) during the early stage of cold crystallization was examined via in-situ small/wide-angle X-ray scattering (SAXS/WAXS), differential scanning calorimetry (DSC), and Fourier-transform infrared spectroscopy (FTIR). Comparison was made with the previously proposed “mesomorphic” form obtained after long-term annealing at 0 ºC, in which sPP chains are rich in trans-planar conformation.
Upon programmed heating at 3 ºC/min from quenched glassy state, formation of “precursors” was identified via WAXS to start at ca. −5 ºC before subsequent crystallization in the temperature range of 18 to 25 ºC. This precursor state was characterized by diffused scattering superimposed on the amorphous halo in WAXS profile, which showed weakly increased intensity across the scattering vector (q) range of 0.007 ~ 0.1 Å‒1 in the SAXS profile during the nucleation stage, with exothermic event in the corresponding DSC trace. The mesomorphic form obtained by quenching from the melt and long-term annealing in ice-water was confirmed to be rich in trans-planar conformation via FTIR. The corresponding SAXS profile was characterized by a mild bump across the q-range of 0.007 ~ 0.1 Å‒1 whereas the WAXS profile exhibited a broad peak centered at q ≈ 1.2 Å‒1, slightly to the right of the amorphous halo around q ≈ 1.13 Å‒1. This mesomorphic state resulted in the elimination of an incubation period in the subsequent isothermal crystallization at 15 ºC. According to combined DSC/WAXS evidences obtain during programmed heating from 0 ºC at 10 ºC/min, the pre-existence of this mesomorphic phase not only lowered the onset of crystallization from 20 to 10 ºC but also resulted in prolonged temperature range of crystallization. The corresponding SAXS-determined heterogeneity (Qinv) also revealed earlier nucleation. Hence the precursor state may be considered as a pre-ordered form that subsequently induce nucleation and growth whereas the mesomorphic phase (similar in structure but more closely packed and greater in amount) also induces nucleation but is more sluggish in the subsequent growth process; both can be categorized as embryos for crystal formation but nevertheless belong to different structural evolution routes. The more sluggish (and incomplete) transformation of the mesomorphic phase into crystalline state is attributed to entanglements locked-in by the high number of mesomorphic nanodomains formed during long-term annealing.

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