Piezoelectric relaxation in poly(γ-benzyl-glutamate)

The piezoelectric d- and e-constants, together with the elastic constant and the dielectric constant, were measured for oriented poly(γ-benzyl-glutamate) (PBG) films with various elongation ratios as a function of frequency and temperature, using an apparatus developed by us. The results are discussed in terms of a general theory of piezoelectricity for inhomogeneous systems, in particular for a disperse two-phase system. The piezoelectricity of PBG film is proved to originate from the piezoelectric and optically active symmetry of PBG crystallities and their orientation distribution by three findings: (1) the d₁₄ component of the piezoelectric matrix, which is the only component for a uniaxially or uniplanarly oriented system, is observed; (2) d₁₄ > 0 for PBDG and d₁₄ < 0 for PBLG; (3) d₁₄ is proportional to the degree of orientation F_c of PBG crystallites, as determined by x-ray diffraction. By extrapolating to perfect orientation, d₁₄ is determined to be 5 × 10^?8 cgs esu, if the side chains of PBG are rigid. The piezoelectric relaxation of PBG due to thermal motion of the side chains has a dual character: it is relaxational at lower frequencies and retardational at higher frequencies. On the assumption that the α-helical main chains surrounded by the bulky side chains are responsible for the origin of the piezoelectricity, such relaxation phenomena are interpreted in terms of the relaxation of the local elastic field in the main chains. An equivalent model having the same frequency characteristics is proposed to include the higher order structure of the PBG film.