Influence of Crystallinity in Molecular Motions of Poly(l-lactic acid) Investigated by Dielectric Relaxation Spectroscopy

Specimens of poly(l-lactic acid) (PLLA) with 0.43 ≤ χc ≤ 0.65 were dielectric characterized. The relaxation process associated to the dynamic glass transition shifts to higher frequencies/lower temperatures with the increase of crystallinity. From the VFT temperature dependence of relaxation times, the glass transition and Vogel temperatures were estimated, decreasing with χc increasing. The mobility enhancement in specimens with higher χc was rationalized in terms of both (i) thicker rigid amorphous phase that decreases the influence of the rigid crystalline wall on the cooperative motions of the main relaxation process and (ii) less dense mobile amorphous phase. A multicomponent character was found for the secondary relaxation process, being the sum of three components with weighted contributions that vary with crystallinity. Thus, its multimodal nature turned to be a probe of the morphology attained. The temperature dependence of each individual mode follows Arrhenius behavior. These processes tend to converge at high temperatures in a single broad process identical to the one found in amorphous PLLA. The reported behavior is unusual in the literature, emphasizing the sensitivity of secondary relaxation in PLLA to crystallinity degree, and the role of dielectric relaxation spectroscopy as an important tool to evaluate this effect.