Phase-transition From a C-centered To a B-centered Orthorhombic Crystalline Form of Syndiotactic Poly(propylene)

Samples of highly syndiotactic poly(propylene) (s-PP) crystallized in a C-pseudo-centered orthorhombic form are here characterized through high resolution solid state C-13 NMR spectroscopy and wide angle X-ray diffraction. The C-13 NMR CP MAS (cross-polarization, magic angle spinning) spectra of highly disordered quench-precipitated s-PP samples (with a structure very close to a C-pseudo-centered orthrohombic form) yield additional resonances beside those already reported in the literature for s-PP samples crystallized with the chains in a fully helical (TTGG)(n) conformation (T: trans; G: gauche). From the correlation of C-13 NMR CP MAS spectra with the X-ray diffraction profiles recorded at the same temperatures, it is possible to establish that at a given temperature (approximately 100 degrees C, for our samples) a phase transition from the C- to the B-pseudo-centered orthorhombic form starts to occur. Correspondingly, the aforementioned additional resonances in the C-13 NMR CP MAS spectra progressively disappear with the onset of the phase transition. These extra resonances are indeed completely absent in the C-13 NMR CP MAS spectrum of samles crystallized in the B-pseudo-centered structure. NMR and X-ray diffraction data, according to differential scanning calorimetry, were interpreted with a recrystallization phenomenon; at temperatures below 140 degrees C for our samples, the variously sized C-pseudo-centered orthorhombic crystallites melt and readily re-crystallize in the B-pseudocentered orthorhombic form.