Electric Field-Induced Polarization Responses of Noncentrosymmetric Crystalline Biopolymers in Different Frequency Regimes - A Case Study on Unidirectionally Aligned β-Chitin Crystals.

A dielectric medium containing noncentrosymmetric domains can exhibit piezoelectric and second-harmonic generation (SHG) responses when an electric field is applied. Since many crystalline biopolymers have noncentrosymmetric structures, there has been a great deal of interest in exploiting their piezoelectric and SHG responses for electromechanical and electro-optic devices, especially owing to their advantages such as biocompatibility and low density. However, exact mechanisms or origins of such polarization responses of crystalline biopolymers remain elusive due to the convolution of responses from multiple domains with varying degrees of structural disorder or difficulty of ensuring the unidirectional alignment of noncentrosymmetric domains. In this study, we investigate the polarization responses of a noncentrosymmetric crystalline biopolymer, namely, unidirectionally aligned β-chitin crystals interspersed in the amorphous protein matrix, which can be obtained naturally from tubeworm (LS) tube. The mechanisms governing polarization responses in different dynamic regimes covering optical (>10 Hz), acoustic/ultrasonic (10-10 Hz), and low (10-10 Hz) frequencies are explained. Relationships between the polarization responses dominant in different frequencies are addressed. Also, electromechanical coupling responses, including piezoelectricity of the LS tube, are quantitatively discussed. The findings of this study can be applicable to other noncentrosymmetric crystalline biopolymers, elucidating their polarization responses.