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, F-19 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 F-19 NMR of as-synthesized copolymers reveals strong (similar to300 Hz) geminal indirect coupling ((2)J) 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 F-19 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 -CRF2 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. (C) 2002 Published by Elsevier Science B.V.