Phase transition from a C‐centered to a B‐centered orthorhombic crystalline form of syndiotactic poly(propylene)

Abstract

SUMMARY:Samples of highly syndiotactic poly(propy1ene) (s-PP) crystallized in a C-pseudo-centered orthorhombic form are here characterized through high resolution solid state 13C NMR spectroscopy and wide angle X-ray diffraction. The 13C 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 j n conformation trans; G: gauche). From the correlation of 13C 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°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 13C NMR CP MAS spectra progressively disappear with the onset of the phase transition. These extra resonances are indeed completely absent in the 13C 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 "C for our samples, the variously sized Cpseudo-centered orthorhombic crystallites melt and readily re-crystallize in the B-pseudocentered orthorhombic form.

Morphology development during isothermal crystallization. II. Isotactic and syndiotactic polypropylene blends

“…This process of secondary infilling crystallization, subsequent to primary growth, is prevalent in s PP and contrasts with the more compact structures (less variation of L B during crystallization) during i PP morphology development. This observation is also consistent with the strong tendency of s PP to form multiple endotherms in DSC experiments 10, 23–33. In addition to the differences of the i PP and s PP homopolymers during growth, the shapes of the correlation functions are different at the late stages of crystallization.…”

“…The s PP sample melts at temperatures approximately 15–20 °C below those of the i PP homopolymer. Multiple melting is a common feature in s PP homopolymer10, 23–33 and is observed for the s PP component in the i PP/ s PP blends with i PP concentrations up to 50 wt %. The multiple endotherms of the s PP component are labeled endotherms I and II in Figure 1.…”

“…This observation suggests a limited application range for this blend pair. The melting behavior of s PP homopolymer has also been extensively studied 10, 20–33. Although a comparison of the equilibrium melting point of 100% syndiotactic s PP relative to that of 100% isotactic i PP is ambiguous,10, 21, 24, 29, 33 the observed s PP melting point, for comparable tacticities (pentad content % rrrr = % mmmm), is generally less than that of i PP under practical crystallization conditions 10.…”

Nonisothermal crystallization and melting behavior of syndiotactic polypropylenes of different microstructure

“…The higher the heating rate used, the shorter is the time available for the diffusion of the molecular segments onto the growing recrystallizing lamellae, so the remelt process is decreased 17, 34. The recrystallization depends on the stability of the primary and secondary crystallites,21 and this is confirmed by the results of Figure 2, since when the number of defects in the polymer decreases the relative weight of the higher endotherm is lower 35. On the other hand, Figure 5 shows that with the first endothermic peak at 140 °C in the total heat flow there is one endothermic peak in the in‐phase contribution and one exothermic peak in the out‐phase contribution, the latter associated with cold‐crystallization, so it is possible to find new evidence to confirm that the double melting peak is associated with recrystallization processes.…”

Evolution of a Metallocenic sPP with Time: Changes in Crystalline Content and Enthalpic Relaxation

“…[12][13][14][15][16][17][18] Accordingly, great efforts have been focused on elucidating its structure. [19][20][21][22] Four crystalline forms have been described so far and have been named as follows: [9,20] two different orthorhombic crystalline forms denoted as Form I (ordered and disordered, both with helical conformations) and the metastable Form II, the trans-planar Form III, and the monoclinic Form IV. In addition, a mesomorphic trans-planar modification has also been reported.…”