The effect of 1,3:2,4-dibenzylidene-d-sorbitol (DBS) on the crystallization behaviors of poly(l-lactic acid) (PLLA) was examined in this study. A small amount (≤4 wt%) of DBS altered the crystallization rate and regime transition temperature of PLLA. First, the addition of DBS and the formation of self-assembled DBS nanofibrils both increased the nucleation rate of PLLA. Second, the curves of the spherulitic growth rate versus the crystallization temperature of PLLA were discontinuous and did not show the typical bell-shaped behavior for all samples. We found that the change in crystal structures (α′-to-α) affected the regime transition temperatures, which led to the discontinuity. The regime transition (regime II–III) temperatures of PLLA slightly decreased as the DBS amounts were increased. This indicates that the more regular structure (regime II) of PLLA formed at lower temperatures when more DBS was added. In addition, the spherulitic growth rate of PLLA was found to be mainly influenced by the fold surface free energy. When the DBS amounts were increased, the increase in the fold surface free energy decreased the growth rate of PLLA. Nonetheless, the Avrami exponent, n, was not significantly changed because the spherulitic growth geometry and nucleation mechanism of PLLA were basically the same. The Avrami plot also shows that the secondary crystallization began earlier due to the formation of DBS nanofibrils for the samples containing higher DBS amounts.
Materials Chemistry and Physics 139(1), pp.161-168