Dielectric relaxation in blends of amorphous poly(DL-lactic acid) and semicrystalline poly(L-lactic acid)

We investigated the thermal and dielectric behavior of blends consisting of semicrystalline poly(L-lactic acid) (L-87) and amorphous poly(DL-lactic acid) (DL-74 and DL-7). Two series of blends L-87/ DL-74 coded as B1 and L-87/DL-7 coded as B2 were used where the code number indicates molecular weight M-w in kg/mol. The DSC thermograms of quenched blends exhibited a single glass transition at about 326 K followed by the exothermic peak due to crystallization of L-87. The relatively sharp glass transition indicates that both components are miscible in the molten state. The degree of crystallinity Z was determined from the heat of fusion. Three dielectric loss peaks were observed for B2. They are termed alpha(n), alpha(s), and beta as reported previously [Macromolecules 2003, 36, 210] and are assigned to the normal mode relaxation of DL-PLA dissolved in the amorphous region, the segmental mode process, and local twisting motions of the main chains, respectively. The dielectric relaxation time for the alpha(n), relaxation was almost independent of chi, but the distribution of relaxation times was broader than that for pure DL-PLA. The distribution of relaxation times for the a, relaxation also broadened with increasing chi. The relaxation strength Deltaepsilon for the alpha(s) relaxation was lower than the value expected from the linearity of Deltaepsilon against 1 - chi, indicating the existence of two amorphous regions A-1 and A-2. In the region A-1 the segmental motions are allowed, but in the region A-2 local segmental motions are heavily damped. The data for the a. process indicate that the DL-PLA chains exist in the region A-1. On the other hand, the intensity of the beta relaxation was not sensitive to the crystallinity, indicating that local twisting motions are not restricted, even in the region A-2.