Recent developments of piezoelectric polymers - polyurea, odd-nylon, vinylidenecyanide copolymer, and polylactic acid

Polyvinylidene fluoride and its copolymers are well known piezoelectric polymers. Besides these a number of piezoelectric polymers have been investigated. Thin films of polyurea are synthesized by vapor deposition polymerization in vacuum and the piezoelectric constant of aromatic polyurea is d/sub 31/ = 10 pC/N, which is stable until 200/spl deg/C. Odd-numbered nylons exhibit the field-induced rotation of the hydrogen-bonded molecular sheets. The piezoelectric constant increases above the glass transition temperature (d/sub 31/ = 17 pC/N). Vinylidene cyanide/vinylacetate copolymers are non-crystalline polymers and show the piezoelectric constant d/sub 31/ = 10 pC/N after poling above the glass transition temperature. The large dielectric relaxation strength results in the large remanent polarization. Elongated films of helical polylactic add show the shear piezoelectric constant d/sub 14/ = -10 pC/N. Strong optical activity and biodegradation are noticeable. All these polymers show the ferroelectric hysteresis and polarization reversal.