Abstract
Cold crystallization process of isotactic polypropylene (iPP) was examined via a combination of small/wide-angle X-ray scattering (SAXS/WAXS), differential scanning calorimetry (DSC), and Fourier transform infrared spectroscopy (FTIR). The approach adopts model-fitting with the ellipsoid form factor and a distorted-FCC structure factor supplemented with the Kratky-Porod method in the analysis of SAXS data. Our WAXS results indicate that the as-quenched (from melt to −10 ºC) “mesomorphic” iPP shows two peaks at scattering vector q ≈ 1.3 and 1.7 Ǻ−1. The mesomorphic fraction remained fairly constant around 0.35 up to 70 ºC, followed by decrease at ca. 70 ºC with concomitant increase in crystallinity due to partial transformation of mesophase into α crystals. Interestingly, a minor fraction of this mesophase persists up to 180 ºC, after apparent melting of all α crystals. As for the SAXS results, we have used model-fitting with ellipsoid form factor P(q) and distorted-FCC lattice structure factor S(q) to elucidate further details in the structural evolution process. After removal of background scattering, SAXS profiles obtained during programmed heating of glassy iPP from −10 to 180 ºC at 3 ºC/min can be interpreted with a sequence of events consistent with previously observations in cold crystallization of poly(9,9-di-n-octyl-2,7-fluorene) (PFO) and syndiotactic polystyrene (sPS). Specifically, the structural evolution of iPP nanograins involves four stages: (1) emergence of precursors (or oblate-like nanograins) of constant radius of gyration Rg ≈ 2.2 nm between −10 and 10 ºC in the absence of WAXS-detectable crystalline peaks, (2) the growth of nanograins to Rg ≈ 2.6 nm with concomitant development of WAXS-determined crystallinity (Xc,WAXS) from 10 to 50 ºC, (3) the coalescence of the oblate nanocrystals upon further heating above 50 ºC, and (4) melting above 130 ºC. Parallel evolution of chain conformation upon heating of glassy iPP was also monitored via FTIR spectroscopy. Results show that chain conformation in the mesomorphic phase is 31 helical, the same as in α crystals of iPP. In the temperature range of 25 to 40 ºC, there were significant increases in the peak area of the long-sequence (10 to 14 monomer units) 31 helix bands of different sequential lengths, signifying further development of mesomorphic or crystalline phases; this was then followed by a rapid decrease in intensities of the helix bands above 160 ºC, consistent with melting of α crystals indicated by parallel WAXS results although the presence of residual 31 helices extends deep into the melt state up to 210 ºC. We conclude that nanograins in the early stage of cold crystallization (which exhibit no crystalline peaks in the WAXS profile before subsequent transformation to crystals) of iPP coincides with this mesomorphic phase previously observed in quenched iPP glasses.