Dielectric behaviour of copolymers based on 2,2,2-trifluoroethyl methacrylate and cyano co-monomers

In the frame of a research aiming at developing new dielectric polymers containing CCN and CF substituents with strong dipole moment, statistical copolymers based on cyano monomers such as acrylonitrile (AN), methacrylonitrile (MAN), methylvinylidene cyanide (MVCN) and 2,2,2-trifluoroethylmethacrylate (MATRIF), were synthesized and characterized. Elemental analysis has shown that the molar percentages of AN and MAN in the copolymers were 45 mol.%, while only 5 mol.% of MVCN was incorporated in the poly(MVCN-co-MATRIF) copolymer. These copolymers exhibit glass transition temperatures, T_g, in the range of 70-90 °C. The dynamic dielectric analyses and their complex permittivities of these copolymers were studied versus the temperature and the frequency. Evidence of an α-relaxation phenomenon in the glass transition region, which is confirmed by the Vogel-Fulcher-Tammann (VFT) temperature dependence of the relaxation times, was assigned to the cooperative reorientation motions of the cyano groups. The values of dielectric strength (Δε) for the copolymers based on MATRIF were determined by Havriliak-Negami (HN) fitting from the dispersion curves, and can be related to the polarity of the monomer unit and to the packing of the macromolecular chains. These relaxations are sometimes overlapped by conduction phenomena due to ionic impurities at low frequencies and high temperatures dipolar losses. In the glassy state, the permittivity values of AN and MAN copolymers show an increase of polarity which makes them candidates for some applications amongst advanced electrical materials such as dielectric layer for capacitors.