EFFECT OF SUBSTITUENTS ON THE THERMAL TRANSITIONS AND DEGRADATION BEHAVIOR OF POLY[BIS(R-PHENOXY)PHOSPHAZENES]

The glass-to-rubber (T(g)), melting (T(m)), and crystal-liquid crystal (T(1)) phase transitions of poly[bis(4-R-phenoxy)phosphazenes] with R = (CH3)2N- and (CH3)3C- were studied by calorimetry, X-ray diffraction, and optical microscopy. The crystallinity of these polymers depended strongly on the annealing conditions and heating/cooling rates as well as the size and polarity of the substituent (R) groups. These data were combined with previous results, and the phase transitions, crystallinity, and thermal stability were compared for a series of five poly[bis(R-phenoxy)phosphazenes] where R = -C2H5, -C(CH3)3, -OCH3, -SCH3, or -N(CH3)2. Large bulky side groups or polar substituents increased T(g) values and inhibited crystalline formation. Although no simple relationship was found between T(1) and the substituent size or polarity, the transitions for our five polymers were consistent with Magill's empirical correlation between (T(m) - T(g))/(T(m) - T(1)) and T(1)/T(m) for semicrystalline polyphosphazenes. This correlation suggested that side-chain mobility and the temperature range of conformational disorder, T(m) - T(1), were governed by steric interactions between side groups. The thermal degradation mechanism of poly[bis(R-phenoxy)phosphazenes] was found to depend on the polarity of the "R" substituent.