Effect of phase transitions on adsorption properties of thin ferroelectric poly(vinylidene fluoride-co-trifluoroethylene) films

The adsorption isotherms of water, oxygen, and hexane molecules at the surface of poly(vinylidene fluoride-co-trifluoroethylene) films 30 and 5 monolayers thick obtained by the Langmuir-Schaefer technique were measured at various temperatures using quartz crystal microbalance. An effect of a ferroelectric phase transition occurring at a temperatures from 363 to 388 K on the adsorption activity of the films of the both thicknesses was disclosed. The highest adsorption was observed at a temperature T approximate to 380 K. In the case of the super-thin copolymer him 5 monolayers thick, one more maximum of the adsorbability was detected at T approximate to 300 K that corresponded to a low-temperature phase transition, which is typical of only the copolymer films thinner than 30 monolayers. The effects observed were explained by the facilitation of an adsorbate diffusion into the copolymer film upon its structural rearrangement caused by the phase transition. The results of this study allowed us to propose a new method for the determination of structural phase transitions based on studying isotherms of molecular adsorption from the gaseous phase.