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Electrostrictive strains were measured in three different polymeric materials: a low modulus polyurethane elastomer, previously studied by Scheinbeim et al. (1994), and two higher modulus random copolymers of poly(vinylidene fluoride-hexafluoropropylene) [P(VDF-HFP)] with 5% and 15% HFP content. Measurements at increasing voltage (electric fields ranging from 0 to 60 MV/m) were taken using an air gap capacitance system and then converted to sample thickness. Copolymer samples with different thermal histories were compared, ice water quenched, air quenched, and slow cooled, for both compositions. The ice water-quenched 5% P(VDF-HFP) copolymer exhibited the highest strain response (>4%) with a dielectric constant of 13.9. The previously studied polyurethane elastomer exhibited the second highest strain response, >3%, with the lowest dielectric constant, 8.5. The ice water-quenched 15% HFP copolymer exhibited the lowest strain response among the three polymeric materials tested, /spl ap/3%, with a dielectric constant of 12.2. The strain energy density of the 5% HFP ice water-quenched copolymer, /sup 1///sub 2/ YS/sub m//sup 2/ (/sup 1///sub 2/ Young's modulus, Y, times the maximum electrostrictive strain, S/sub max/ squared), is the largest known for any semi-crystalline polymer: 0.88J/cm/sup 3/.