Stereocomplex formation between poly(L-lactic acid) and poly(D-lactic acid) with disproportionately low and high molecular weights from the melt

Poly(L‐lactic acid) (PLLA) and poly(D‐lactic acid) (PDLA) with very different weight‐average molecular weights (Mw) of 4.0 × 103 and 7.0 × 105 g mol−1 (Mw(PDLA)/Mw(PLLA) = 175) were blended at different PDLA weight ratios (XD = PDLA weight/blend weight) and their crystallization from the melt was investigated. The presence of low molecular weight PLLA facilitated the stereocomplexation and thereby lowered the cold crystallization temperature (Tcc) for non‐isothermal crystallization during heating and elevated the radial growth rate of spherulites (G) for isothermal crystallization, irrespective of XD. The orientation of lamellae in the spherulites was higher for the neat PLLA, PDLA and an equimolar blend than for the non‐equimolar blends. It was found that the orientation of lamellae in the blends was maintained by the stereocomplex (SC) crystallites. Although the G values are expected to decrease with an increase in XD or the content of high‐molecular‐weight PDLA with lower chain mobility compared with that of low‐molecular‐weight PLLA, G was highest at XD = 0.5 where the maximum amount of SC crystallites was formed and the G values were very similar for XD = 0.4 and XD = 0.6 with the same enantiomeric excess. This means that the effect of SC crystallites overwhelmed that of chain mobility. The nucleating mechanisms of SC crystallites were identical for XD = 0.1–0.5 in the Tc range 130–180 °C. Copyright © 2011 Society of Chemical Industry The presence of low‐molecular‐weight poly(L‐lactic acid) facilitates stereocomplexation, which dominates and accelerates crystallization. The nucleating mechanisms were identical for poly(D‐lactic acid) fractions of 10–50%.