| 摘要 |
<p>PROBLEM TO BE SOLVED: To obtain a polymeric electret material having a dielectric constant smaller than that of e.g. polyvinylidene fluoride and being excellent in piezoelectric properties, mechanical strengths, moldability, etc., by melting a polylactic acid, rapidly cooling the melt and polarizing the cooled melt. SOLUTION: A polylactic acid is produced by e.g. a method comprising directly condensing optically active L- or D-lactic acid through dehydration or comprising synthesizing a lactide being a cyclic dimer of lactic acid and subjecting this lactide to ring opening polymerization. The polylactic acid is melt-molded into a suitable shape and rapidly cooled. The rapid cooling is suitably carried out by cooling with air or water at a rate of 50-100 deg.C/min. The obtained polylactic acid molding is composed of unoriented molecular dipoles and does not show electret properties, so that it is polarized to produce a polymeric electret material. The polarization is carried out by applying an electric field of about 10-100MV/m at a temperature of about 60-180 deg.C. The obtained electret material is useful as a medical ultrasonic modifier, a piezoelectric oscillator, optical sensor or the like.</p> |
