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 χ was determined from the heat of fusion. Three dielectric loss peaks were observed for B2. They are termed α n, α s, and β 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 α n relaxation was almost independent of χ, but the distribution of relaxation times was broader than that for pure DL-PLA. The distribution of relaxation times for the α s relaxation also broadened with increasing χ. The relaxation strength Δε for the α s relaxation was lower than the value expected from the linearity of Δε against 1 − χ, 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 α n process indicate that the DL-PLA chains exist in the region A-1. On the other hand, the intensity of the β relaxation was not sensitive to the crystallinity, indicating that local twisting motions are not restricted, even in the region A-2.