ABSTRACT
Four typical classes of polymeric piezoelectrics are shown in Table 1. The first group is optically active polymers. Most of the polymers in this group are biological materials, e.g., cellulose derivatives such as cellulose triacetate, cellulose diacetate, and cyanoethyl cellulose, proteins such as collagen and keratin, as well as synthetic polypeptides such as polY-'Y-methyl-glutamate (pMG) and polY-'Y-benzyl-glutamate (pBG). The origin of piezoelectricity in these polymers is ascribed to the internal rotation of dipoles related to asymmetric carbon atoms which give the polymer optical activity. The piezoelectric effect in these polymers can be observed when the polymer film is oriented uniaxially. It is found that the piezoelectric constant d14 in these polymers is not zero, which means that the shear stress in the plane of orientation causes electric polarization perpendicular to the plane of stress. Piezoelectric effect has also been observed in synthetic optically active polymers such as polypropylene oxide (PPO) and poly-Ji-hydroxybutyrate (PHB). The second group of piezoelectric polymers is poled polar polymers (electret polymers). Polyvinyl chloride (PVC), polyvinyl fluoride (PVF), polyacrylonitrile' (PAN), odd nylons such as nylon-H, and copolymers of vinylidene cyanide and vinyl acetate [P(VDCNNAc)] are examples belonging to this group. Piezoelectricity in these polymers is caused by orientation of dipoles in the polymer chains which are frozen in during the poling process.
