Conformational changes of the noncrystalline chains for syndiotactic polypropylene as a function of temperature: Correlations with the crystallizations of form I and form III

The conformation and molecular mobility of the noncrystalline chains for sy ndiotactic polypropylene (sPP) samples well crystallized have been characte rized at different temperatures by high-resolution solid-state C-13 NMR spe ctroscopy. The purposes are to investigate the cause inducing the high tran s fraction of sPP chains in the noncrystalline state just after quenched at 0 degreesC from the melt and to know some correlation with the crystalliza tion of form III with the planar zigzag conformation around- 0 degreesC. Tw o samples containing form I and III crystallites were respectively crystall ized at 100 degreesC for 24 h and at 0 degreesC for 144 h from the melt, th eir degrees of crystallinity being 0.60 and 0.31. A new line shape analysis for the CHP resonance Line confirms in good accord with the previous analy sis for the CH3 line that the trans fraction is of the high level of 0.80 i n the noncrystalline state just after quenched at 0 degreesC from the melt. The trans faction is also determined for the noncrystalline components in the form I and III samples as a function of temperature by the line shape a nalysis for the CH3 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 degreesC, the trans fraction is as high as 0.73-0.80, and it seems to significantly depend on the degree o f crystallinity. In contrast, this fraction is as low as about 0.57 in regi on C above 60 degreesC, in good accord with the level at the melt. In regio n B at 15-60 degreesC, the trans fraction drastically changes as is possibl y named as trans-rich chain assembly-coiled chains transition. However, no significant conformational change is observed for the results obtained at d ifferent temperatures by the similar analysis for the CH2 resonance line, s uggesting the preferable production of some conformations mainly due to the steric hindrance between the CH3 groups including the second and third nei ghbors. C-13 spin-spin relaxation measurements for the noncrystalline compo nent in the form I sample also reveal that the molecular motion is highly l imited in regions A and B even above T-g while the rubberlike mobility is a llowable in region C. On the basis of these experimental results and the pr evious results of the preferential crystallizations of forms I and III in t hese regions, some factors affecting these crystallizations are discussed.