Polyphosphazenes: Thermomechanical transitions

Abstract

Four linear phosphazene homopolymers with structure [NPX2]n, where X = Cl, OCH2CF3, OCH2C3F7, and OCH2C3F6CHF2, two copolymers with mole ratios of the alkoxy substituents OCH2CF3/OCH2C3F7 and OCH2CF3/OCH2C3F6CHF2 approximately 1/1, and two physical blends with overall compositions approximating those of the copolymers were examined between −180° and 250°C in an inert atmosphere using a fully automated torsional braid apparatus. Crystal/melt, crystal/crystal, glass, and glassy-state transitions are documented on the basis of the thermomechanical experiments. In addition to the assignment of transition temperatures, the noteworthy findings include the narrow range of values of the glass transition temperatures of the four homopolymers, the increasing negative slope of the modulus versus temperature in the glassy state with increasing size of the substitutent, the presence of a transition immediately above Tg for the copolymers, and the incompatibility of the blends. A glassy-state transition in each of the fluoroalkoxypolyphosphazene materials is tentatively attributed to motion of the substituents. An additional glassy-state transition occurs in the heptafluorobutoxy- and octafluoropentoxy-substituted materials. The transition temperatures of specimens preheated to 250°C were essentially the same as those preheated to 150°C.