Effect of Glass Transition on Electrical Conduction Characteristics of Poly-L-lactic Acid

The temperature dependence of electrical conduction current in additive-free poly-L-lactic acid was measured from 20°C to 90°C under application of dc 2.5 kV. Although the current obeys the Arrhenius formula, a hump appears in some conditions, depending on whether the temperature is being ascended or descended, and also on whether the sample was poled or short-circuited prior to the measurements. Even under short-circuit condition, a similar hump alone appears if the sample was poled by a dc voltage beforehand. From these results, it is assumed that orientational polarization or depolarization occurs around 74°C by the glass transition, and that its resultant current and the conduction current overlap with each other. The fact that polar groups, mainly carbonyl groups, in the sample really changed their directions was confirmed by Fourier-transform infrared spectroscopy. The permittivity also increases around 74°C. These facts indicate that the number of dipoles, which become possible to rotate toward the direction of electric field, is increased by the glass transition.