Influence of physical aging of ferroelectric vinylidene fluoride copolymer films on their structural and electrophysical characteristics

General problems of structural changes which occur at storage of two-phase crystalline polymers with a metastable structure at room conditions are considered on the example of a vinylidene fluoride - hexafluoropropylene copolymer. The physical aging occurs in flexible-chain crystalline polymers (polyethylene, fluoropolymers under consideration, etc.), where, due to low glass transition temperatures, the liquid-like dynamics of amorphous phase chains is realized at room temperature through cooperative micro-Brownian motions with short relaxation times. Taking into account that covalent-bound sections of the chains of the amorphous phase can enter the crystallites, the noted mobility may initiate changes in the size of the latter. Such a possibility is proved by the example of the noted copolymer. At low-temperature crystallization of a vinylidene fluoride - hexafluoropropylene copolymer from a solution the formed alpha-phase crystals have little perfection. The size of the crystals increases when the films are stored in room conditions. Because of the crystal polymorphism, at the same time a certain fraction of gamma-phase crystals which are present in initial films undergo a polymorphic transformation gamma -> alpha. These processes lead to an increase in crystallinity. Moreover, during such processes additional structuring is observed, which is reduced to the displacement of various kinds of intra-chain defects into the amorphous phase (and especially into the surface). Since the copolymers under study are ferroelectrics, they were studied by piezo force microscopy. It was found that despite the crystallization predominantly in nonpolar alpha-phase, piezo force microscopy revealed a domain structure, which formation mechanism is discussed. The structural changes at physical aging of the films affect the character of the noted domain structure. Thus, it is suggested that the mechanism of the described structural changes is realized through the developed cooperative mobility of the chains in the amorphous phase, which characterizes the processes of rotational diffusion.