Lattice-stiffening transition in copolymer films of vinylidene fluoride (70%) with trifluoroethylene (30%)

We report the discovery of a compressibility phase transition at 160 K in c rystalline copolymer films of vinylidene fluoride (70%) with trifluoroethyl ene (30%). This phase transition is distinct from the known bulk ferroelect ric-paraelectric phase transition at 353 It and surface ferroelectric phase transition at 295 K. The new phase transition is characterized by an incre ase in the effective Debye temperature from 48 to 245 K along the [010] dir ection as the temperature falls below 160 K. This phase transition is evide nt in neutron scattering, x-ray diffraction, angle-resolved photoemission, and in the dipole active phonon modes in electron energy-loss spectroscopy.