F. Auriemma et al., PHASE-TRANSITION FROM A C-CENTERED TO A B-CENTERED ORTHORHOMBIC CRYSTALLINE FORM OF SYNDIOTACTIC POLY(PROPYLENE), Macromolecular chemistry and physics, 196(12), 1995, pp. 4011-4024
Samples of highly syndiotactic poly(propylene) (s-PP) crystallized in
a C-pseudo-centered orthorhombic form are here characterized through h
igh resolution solid state C-13 NMR spectroscopy and wide angle X-ray
diffraction. The C-13 NMR CP MAS (cross-polarization, magic angle spin
ning) spectra of highly disordered quench-precipitated s-PP samples (w
ith a structure very close to a C-pseudo-centered orthrohombic form) y
ield additional resonances beside those already reported in the litera
ture 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 a
t the same temperatures, it is possible to establish that at a given t
emperature (approximately 100 degrees C, for our samples) a phase tran
sition from the C- to the B-pseudo-centered orthorhombic form starts t
o 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 abs
ent in the C-13 NMR CP MAS spectrum of samles crystallized in the B-ps
eudo-centered structure. NMR and X-ray diffraction data, according to
differential scanning calorimetry, were interpreted with a recrystalli
zation 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.