The thickness dependence of the phase transition temperature in PVDF

It was found recently that in the Langmuir–Blodgett ultrathin vinylidene fluoride (PVDF) films there is ferroelectric phase transition of the first order. Earlier in the bulk PVDF this phase transition was not observed because the melting temperature of this ferroelectric polymer (∼170 °C) is lower than the point of the possible phase transition. Therefore this polymer was treated for a long time as pyroelectric. In the present work we investigate PVDF Langmuir–Blodgett films at the nanoscale and the film thickness interval, where ferroelectric phase transition disappears and transition from ferroelectric to pyroelectric state takes place. This phenomenon is explained by the finite-size effect at the nanoscale using Landau–Ginzburg–Devonshire (LGD) theory and by the Weiss mean field model.