THERMAL PHASE-TRANSITIONS OF SUBSTITUTED POLY[BIS(4-R-PHENOXY)PHOSPHAZENES]

The phase transitions observed in two substituted poly[bis(4-R-phenoxy)phosphazenes] (R = OCH3(PBMOPP) and SCH3(PBMTPP)) have been studied by DSC, X-ray diffraction, and optical microscopy. Both polymers exhibit thermotropic behavior. The influence of the size and polarity of the substituents on the thermal transitions and on the mesophase formation has been analyzed and compared with that of other substituted polyphosphazenes. The kinetics of crystallization from the mesophase and from the isotropic melt have also been analyzed for PBMOPP, PBMTPP, and poly[bis(4-ethylphenoxy)phosphazene] (PBEPP). The kinetic studies have shown that the isotherms have a behavior similar to that found in the crystallization of homopolymers, with an Avrami exponent of n = 3 for PBMOPP and PBMTPP. This exponent can be interpreted as a nucleation-controlled process with a two-dimensional growth. A strong dependence of the crystallization kinetics on the conditions of the mesophase formation has been observed, particularly the temperature of the mesophase before crystallization. In PBEPP, changes in crystallization rates and in final crystallinity for the same crystallization temperatures have been obtained depending on whether the polymer is crystallized from the mesomorphic or the isotropic state. Finally, with the assumption that the growth of the mesophase is similar to polymer crystallization, the temperature coefficient of the transformation process has been determined. Values of this coefficient are found to be related to the bulkiness and polarity of the substituents in our polyphosphazenes.