Dynamics of ferroelectric phase transition in vinylidene fluoride/trifluoroethylene (VF2/F3E) copolymers. I. Acoustic study

Abstract

We report the behavior of ultrasonic absorption and velocity in the ferroelectric copolymers of vinylidene fluoride (VF2) and trifluoroethylene (F3E) near the Curie point. The ferroelectric transition of VF2/F3E is strictly of the first order, however, it has the second-order character especially in the copolymers with low VF2 content. A large critical absorption due to cooperative fluctuation has been observed near the Curie point. From the temperature dependence of the critical absorption, we have found that the critical anomaly of the absorption is weak and the specific heat has logarithmic divergence at the Curie point if we ignore the effect of the distribution of the Curie point which may come from a distribution of lamellar thickness. This weak critical anomaly can be ascribed to the long-range nature of the bare interaction between dipoles due to connectivity of polymer chain or the electric dipolar interaction, the randomness of the interaction due to random copolymerization, and/or the dimensionality of the order parameter. The rotation of dipole is always accompanied by cooperative conformational change of the backbone, which is unique to a polymer chain. We have also found that a type of the dominant interaction between acoustic phonon and the order parameter depends on VF2 content.