Conformational Changes of the Noncrystalline Chains for Syndiotactic Polypropylene as a Function of Temperature: Correlations with the Crystallizations of Form I and Form III

Abstract

The conformation and molecular mobility of the noncrystalline chains for syndiotactic polypropylene (sPP) samples well crystallized have been characterized at different temperatures by high-resolution solid-state 13 C NMR spectroscopy. The purposes are to investigate the cause inducing the high trans fraction of sPP chains in the noncrystalline state just after quenched at 0 °C from the melt and to know some correlation with the crystallization of form III with the planar zigzag conformation around 0 °C. Two samples containing form I and III crystallites were respectively crystallized at 100 °C for 24 h and at 0 °C for 144 h from the melt, their degrees of crystallinity being 0.60 and 0.31. A new line shape analysis for the CH 2 resonance line confirms in good accord with the previous analysis for the CH 3 line that the trans fraction is of the high level of 0.80 in the noncrystalline state just after quenched at 0 °C from the melt. The trans fraction is also determined for the noncrystalline components in the form I and III samples as a function of temperature by the line shape analysis for the CH 3 resonance line. As a result, it is found that there are three temperature regions, regions A, B, and C, where the trans fractions are greatly different. In region A below 15 °C, the trans fraction is as high as 0.73-0.80, and it seems to significantly depend on the degree of crystallinity. In contrast, this fraction is as low as about 0.57 in region C above 60 °C, in good accord with the level at the melt. In region B at 15-60 °C, the trans fraction drastically changes as is possibly named as trans-rich chain assembly-coiled chains transition. However, no significant conformational change is observed for the results obtained at different temperatures by the similar analysis for the CH 2 resonance line, suggesting the preferable production of some conformations mainly due to the steric hindrance between the CH 3 groups including the second and third neighbors. 13 C spin-spin relaxation measurements for the noncrystalline component in the form I sample also reveal that the molecular motion is highly limited in regions A and B even above Tg while the rubberlike mobility is allowable in region C. On the basis of these experimental results and the previous results of the preferential crystallizations of forms I and III in these regions, some factors affecting these crystallizations are discussed.