Molecular dynamics during crystallization of poly(L-lactic acid) as studied by broad-band dielectric relaxation spectroscopy

An investigation was carried out of the molecular dynamics of poly(L-lactic acid) before, during, and after crystallization. Experimental results were generated over a wide range of temperature and frequency by broad-band dielectric relaxation spectroscopy (DRS). An interesting finding is that the average relaxation time (defined as tau = (1)/(2)pif(max), where f(max) is the frequency at maximum loss for the CL process) does not vary with degree of crystallinity during melt and/or cold crystallization. Moreover, the temperature dependence of the average relaxation time for wholly amorphous and crystallized samples is well-described by a single Vogel-Fulcher-Tammann (V-FT) functional form. The unchanged fragility suggests that the segmental dynamics are not sensitive to the different degree of crystallinity, implying that the relaxing segments are smaller than the thickness of the amorphous layers between lamellae. Apparently, the distance between lamellae is greater than the length of the primitive segment and the characteristic size of the cooperatively rearranging domain; the length scale of the a process is thence put at less than 4 nm.