Piezoelectric properties of poly-β-hydroxybutyrate and copolymers of β-hydroxybutyrate and β-hydroxyvalerate

Abstract

The shear piezoelectricity was observed in oriented films of poly-β-hydroxybutyrate (PHB) and copolymers of β-hydroxybutyrate (HB) and β-hydroxyvalerate (HV). The piezoelectric stress constant 3 14 = e′ 14 − ie″ 14 (polarization/strain), the piezoelectric strain constant d 14 = d′ 14 − id″ 14 (polarization/stress), the elastic constant c = c′ + ic″ and the dielectric constant ε = ε′ − iε″ were determined at a frequency of 10 Hz over a temperature range from −150° to +150°C. Piezoelectric relaxations as well as elastic and dielectric relaxations were clearly observed at the glass transition temperature of about 15°C. In order to evaluate the piezoelectric constants ( e 2 and d 2 ) for the piezoelectric phase which consists of the crystalline region and the oriented non-crystalline region, a spherical dispersion two phase model was utilized. Assuming the appropriate fixed values for the elastic and dielectric constants in the piezoelectric phase, d 2 and d 2 were calculated as a function of temperature. For a PHB and a copolymer (17 HV/83 HB), e 2 and d 2 showed relaxations, leading to a conclusion that the instantaneous piezoelectric constant in the crystalline phase is constant independent of temperature but the piezoelectric constant in the oriented non-crystalline phase is relaxational and has the opposite sign. For a copolymer (25 HV/75 HB) and a chloroform treated copolymer (17 HV/83 HB), e 2 and d 2 were constant independent of temperature, indicating that the oriented non-crystalline phase has disappeared owing to the increased molecular flexibility due to copolymerization or annealing in chloroform vapour.