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 fi lms with different transition temperatures is evident in the effect of layer thickness on the ferroelectric– paraelectric phase transition in multilayer fi lms, as revealed by x-ray diffraction and dielectric measurements. The multilayer samples consisted of alternating Langmuir–Blodgett (LB) fi lms of two different copolymers of vinylidene fl uoride with trifl uoroethylene, one with 80% vinylidene fl uoride and a ferroelectric–paraelectric transition temperature on heating of 133 ± 4 °C and the other with 50% vinylidene fl uoride and a transition temperature of 70 ± 4 °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 fi lms made from an equal mixture of the two copolymers, exhibited composite behavior, 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 behavior. These results imply that the ferroelectric interaction length along the (110) direction, which is perpendicular to the fi lm plane, is approximately 11 nm.