Thermomechanical trnasitions in polyphosphazenes

Abstract Unoriented and oriented samples of two phosphazene homopolymers with chemical structure [NPX 2 ] n , where X = CF 3 CH 2 O and p ‐CH 3 C 6 H 4 O, and three unfilled and filled alkoxy‐type polyphosphazene copolymers, plus one filled aryloxy‐type copolymer were examined with a Rheovibron viscoelastometer. Measurements were made from −120°C to temperatures above the T (1) transition of the respective polymers. The effects of orientation and repeated temperature cycling through the T (1) transition temperatures were studied. The overall physical behavior of these polyphosphazenes was dominated by changes that occurred in the primary softening dispersion and the mesomorphic or T (1) transition region. The enthalpy of the T (1) transition and the T (1) temperature were investigated by differential scanning calorimetry (DSC) as a function of temperature cycling in the T (1) region; for instance, the change in enthalpy in the T (1) region is about an order of magnitude larger than it is at T m for the unfilled polyphosphazene homopolymers. The T g 's of the polyphosophazenes were also determined. Density measurements were made on oriented and unoriented specimens by the flotation method. Although an increase in density (and crystallinity) was observed when specimens were cycled through the T (1) transition, no significant difference in density was found between oriented and unoriented materials. Some discussion of the molecular origin of the T (1) transition is included.