Phase transitions in polyphosphazenes

Abstract

Halogenated phenoxyphosphazenes have been prepared by solution polymerization and characterized by g.p.c., d.s.c., Fourier transform i.r. spectroscopy, optical microscopy, solution and solid state n.m.r., dilatometry and diffraction techniques (WAXS and electron diffraction). Phase transitional behaviour was monitored above the thermotropic transition [ T(1)] for each polymer and a general transformation scenario was established depending upon side group chemistry. It has been determined that side group size and substitution affects the glass transition temperature ( T g), T(1) and the melting temperature ( T m) in a way that can be scaled linearly with T(1) T m providing a useful guide to polyphosphazene behaviour. Through measuring properties as a function of temperature, a unifying perspective of the transitional characteristics of PB(4-H)PP and PB(3-H)PP polymers below as well as above T(1) has been established. A smectic mesophase where the chain backbone is extended and packed (accommodated) into pseudohexagonal arrays is primarily responsible for the properties of polyphosphazene above T(1). This arrangement also facilitates the ability of the thermotropic state to organize and solidify into an orthorhombic three-dimensional phase upon cooling from the two-dimensional thermotropic state. The polymers as crystallized from solution are of relatively low crystallinity in the monoclinic form, the level depends upon the nature and size of the side group and the conditions of precipitation.