Effects of Crystallinity on Thermally Stimulated Current and Complex Permittivity of Poly(L-lactide)

Recently poly(L-lactide) synthesized from starch is attracting much attention as a biodegradable polymer. In this paper, effects of crystallinity on dielectric properties of PLLA were studied by preparing four kinds of PLLA samples with different crystallinities. A thermally stimulated polarization current (TSPC) peak was found to appear in all the samples at temperatures around 65 to 70degC, which is due to the glass transition. The magnitude of the TSPC peak was found to be in the reverse order of crystallinity. This is presumably because the segmental-mode relaxation, which brings about the TSPC, is restricted more as the crystallinity increases. All the samples show two thermally stimulated depolarization current (TSDC) peaks at around 65degC and around 90 to 100degC. The lower-temperature peak seems to be due to the segmental-mode relaxation, while the other due to the normal-mode relaxation. Moreover, all the samples show a drastic increase in the real part of complex permittivity and have a peak in its imaginary part 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 around 65 to 70degC. Therefore, the increase in the real part and the peak in the imaginary part are ascribable to the segmental-mode relaxation.