The effect of interlayer interactions on the ferroelectric-paraelectric phase transition in multilayered thin films of vinylidene fluoride-trifluoroethylene copolymers

The interaction between ferroelectric polymer films with different transition temperatures is evident in the effect of layer thickness on the ferroelectric paraelectric phase transition in multilayer films, as revealed by x-ray diffraction and dielectric measurements. The multilayer samples consisted of alternating Langmuir-Blodgett (LB)films of two different copolymers of vinylidene fluoride with trifluoroethylene, one with 80% vinylidene fluoride and a ferroelectric-paraelectric transition temperature on heating of 133 +/- 4 degrees C and the other with 50% vinylidene fluoride and a transition temperature of 70 +/- 4 degrees C. Samples with a repeat period of 20 LB layers (10 contiguous layers of each composition) exhibited two distinct phase transitions, indicative of minimal interaction between the two materials. Films with a repeat period of 2, or films made from an equal mixture of the two copolymers, exhibited composite behaviour, with an intermediate transition temperature and suppression of the transitions associated with the individual compositions. Films with a repeat period of 10 exhibit cross-over behaviour. These results imply that the ferroelectric interaction length along the (110) direction, which is perpendicular to the film plane, is approximately 11 nm.