Ferroelectric properties of polymeric bilayer systems

Bilayers of VDF-TrFE ferroelectric copolymer thin films have been characterised in terms of polarisation behaviour and dielectric nonlinearities. The effective properties of the ferroelectric bilayers have been investigated and compared with those of the individual layers in the stack. It has been demonstrated that the effective permittivity of the bilayer is not fully described by a simple series model. The remanent polarisation Pr in the bilayer shows essentially a linear decrease with temperature and reaches zero at around 120 ° C, i.e. above the Curie temperatures of the two copolymers involved. The two observations are attributed to electric bias fields in the bilayer which cause a coupling between the two layers and lead to a temperature dependence of the polarisation which is phenomenologically similar to observations in superlattices.