Effects of crystallinity on dielectric properties of poly(L-lactide)

Poly(L-lactide) is attracting much attention as a biodegradable polymer. In this paper, effects of crystallinity on dielectric properties of PLLA are discussed. At 80 $C, which is above the glass transition temperature (= about 60 °C), the conductivity increases with a decrease in crystallinity. A thermally stimulated polarization current (TSPC) peak, which seems to be due to segmental-mode relaxation, appears in all the samples at temperatures around 65 to 70 °C, and it becomes smaller as the crystallinity increases. All the samples show two thermally stimulated depolarization current (TSDC) peaks at around 65 °C and around 90 to 100 °C. The lower-temperature peak seems to be due to the segmental-mode relaxation, and the other due to the normal-mode relaxation. Moreover, all the samples show a drastic increase in the real part of the complex permittivity (ε_r′) and a peak in its imaginary part (Ïμ_r.) at frequencies from 10^-1 to 10⁴ Hz, depending on the measurement temperature. The crystallinity dependencies of the two parts are similar to those of the TSPC and TSDC peaks at around 65 to 70 °C. Therefore, the increase in ε_r′ and the peak in ε_r″ are ascribable to the segmental-mode relaxation.