| Two new electrostrictive polymers were discovered in the present work: the P(VDF-HFP) copolymer, and its blends with P(VDF-TrFE). “Giant” electrostrictive strains (>4%) were observed in these materials, relative to the small strains (0.01 to 1%) previously known to occur in piezoelectric materials. The ferroelectric behavior, electromechanical properties and structure were studied to gain insight into the structure/property relationship in these materials.; The effects of varying processing conditions and composition on the response of the P(VDF-HFP) copolymer were studied in Part 1. Films of 5 and 15 mol% HFP were crystallized under three different conditions: equilibrium (slow-cooled), intermediate (air-quenched) and non-equilibrium (ice-quenched). The ice-quenched samples showed the highest strain, followed by the air-quenched and then slow-cooled copolymers. The ice-quenched films, were also ferroelectric, however no D-E hysteresis loop was observed for the other conditions. X-Ray diffraction and FTIR showed that the ice-quenched samples were mostly composed of β phase crystals. The small fraction of gauche bonds (∼0.09) and the crystallinity of 60% of the 5% ice-quenched copolymer, together with a significant increase in crystallinity observed in X-Ray diffraction data after poling the film, suggest that field-induced changes in crystallinity are contributing to the enhanced strain response. The piezoelectric strain (d31) and stress (e31) coefficients decreased over time after the field was turned off, suggesting that an unstable polarization was induced by the electric field. The time of response first fast and then slow suggest that or more mechanisms occurring, one possibly involving transformation of bonds from gauche to trans, and the other involving field-induced reversible changes in crystallinity.; Blends of the 5% ice-quenched P(VDF-HFP) copolymer with P(VDF-TrFE) 68/32 mol% copolymer, were studied in Part II. The electrostrictive strains and ferroelectric behavior in the blends were strongly dependent on composition. A decrease in the remnant polarization was observed at increasing number of poling cycles for some blend compositions, also known to occur in the P(VDF-TrFE) copolymer. However, higher strain response blends, were ferroelectric. In lower response blends, the strain was dependent on the amount of gauche bonds, however in higher response blends, this seemed to be contributing only in part to the overall strain. |
