Phase-transitional behavior in vinylidene fluoride-trifluoroethylene copolymer crystals

The thermal and structural properties at normal and high pressures of vinylidene fluoride-trifluoroethylene (VDF/TrFE) copolymer crystals with VDF molar contents between 59 and 77% are examined using several types of thermal analysis systems as well as X-ray measurement. Although temperature changes in the crystal structures transform from a continuous [cooled phase (CP) to high-temperature phase (HP)] to a discontinuous one [low-temperature phase (LP) to HP] with increasing VDF molar content or external pressure for 65% VDF sample, the second-order phase transition does not appear but the transition from CP to LP occurs prior to the first-order phase transition from LP to HP. The sample with 65% VDF is composed of only CP, and LP suddenly appears in the 68% VDF sample and another LP is expected to exist in 71 and 77% VDF samples. The percolation theory may be used to explain such structural, transformation which is very sensitive to VDF content.