19F NMR characterization of electron beam irradiated vinylidene fluoride–trifluoroethylene copolymers

Electron beam irradiated poly(vinylidene fluoride–trifluoroethylene) [P(VDF–TrFE)] copolymers have been recently shown to exhibit “giant” electrostrictive stains. In this work, 19F nuclear magnetic resonance (NMR) spectroscopy was used to probe structure of P(VDF–TrFE) copolymers before and after irradiation in order to elucidate the chemistry associated with the development of outstanding electroactive properties. The two-dimensional solution-state 19F NMR of as-synthesized copolymers reveals strong (∼300 Hz) geminal indirect coupling (2J) and the resulting assignments have some differences from the previous literature. The most frequently occurring sequence containing TrFE is CH2CF2CHFCF2CH2, followed by CF2CH2CHFCF2CF2. This reveals a strong preference for the TrFE units to be isolated and a slight preference for the head-to-tail addition. Both sequences contain VDF polymerized in the same direction around the central TrFE monomer unit. Resolution of bonding environments in the insoluble irradiated P(VDF–TrFE) required the use of fast (25 kHz) magic angle spinning (MAS) solid-state 19F NMR at elevated temperature. Of the resolved peaks formed by irradiation, the side group CF3CH<, was present in the highest concentration. Such CF3 pendant groups are likely to reduce crystallinity and thus impact electroactive properties. Also observed was significant formation of CF3CF2, which can be either a pendant group or a side group. Several types of CF3 end groups were observed: CF3CHF, CF3CH2, and CF3CH. The latter, indicates the formation of some unsaturated bonds upon irradiation. Small concentrations of CHF2 and CF2H were also found. Cross-linking is indirectly evidenced by broadening of the NMR spectra of the irradiated material. In the unirradiated copolymer, two conformation; of sequence CH2CF2CHFCF2CH2 are observed in the solid-state. In the solution-state in the post-irradiated solid-state, only one of these conformations exists.