Effect of isotacticity on formation of mesomorphic phase of isotactic polypropylene

The effect of isotacticity on the mesomorphic phase-forming properties of isotactic polypropylene (iPP) is reported. Only iPP having high isotacticity (iPP-HT) can form the mesomorphic phase by a rapid quench, but iPP having low isotacticity (iPP-LT) cannot form the mesomorphic phase though it crystallizes by the same quenching rate. These results suggest that the mesomorphic phase of iPP is closely related to the stereoregularity of the polymer chain. The mesomorphic phase in the iPP-HT shows longer regular 3/1 helices than in the crystalline iPP-LT. The structure and formation mechanism of the mesomorphic iPP are considered analogous to those of liquid crystals since the average length of the long regular 3/1 helices in the mesomorphic iPP acting as rodlike segments is sufficiently large to fulfill a criterion for the formation of liquid crystalline phase. From this criterion, the minimum requirement of isotacticity to form the mesomorphic phase is also estimated.