Balancing crystalline and amorphous domains in PLA through star-structured polylactides with dual plasticizer/nucleating agent functionality

The development of the first star-structured poly(l-lactides) (s-PLLAs) with dual functionality as both nucleating agent and plasticizer in polylactide (PLA) blends is described. Mechanisms controlling this functionality are deduced. Blends of PLA containing s-PLLAs show significant improvements in thermal and mechanical properties. The s-PLLAs are made by using appropriate saccharides, e.g., methyl-α-d-glucopyranoside and β-cyclodextrin, as star-shaped core macroinitiators for l-lactide polymerization. Varying the l-lactide mole ratio gives control of the degree of polymerization (DP_n) of PLLA branches from 5 to 30. Blending ≈1 wt% of the new s-PLLAs with PLA resins produces a drastic decrease in the glass transition temperature (T_g) (from 57 °C to 22 °C), and an increase in the elongation at break (30–40%) confirms significant increases in chain mobility and plasticity. A concomitant reduction of crystallization temperature (from 127 °C to 81 °C) as well as a higher crystallization rate (a factor of four increase) implies rapid nucleation of crystalline domains. With only 1 wt% of s-PLLA in the blend of PLA film, the elongation at break of PLA increases eightfold. In addition to the dual functionality, the PLLA branches also provide miscibility with PLA in the blend (as confirmed by a single T_g, combined with estimates from the Fox equation and solubility parameters).