Poly(D-lactic acid) as a rheological modifier of poly(L-lactic acid): Shear and biaxial extensional flow behavior

The effect of addition of a small amount of poly(D-lactic acid) (PDLA) on the melt rheology of poly(L-lactic acid) (PLLA) was investigated. PDLA added to PLLA produces a stereocomplex which stays unmelted even above T-m of PLLA. Because of the imbalanced contents of component polymers, the stereocomplex does not grow into a large crystallite but rather acts as a crosslinking point of PLLA chains resulting in the apparent introduction of the long chain branching as well as in the apparent increase in molecular weight. Such changes in the molecular structure give rise to the change in the melt rheology depending on the content and the molecular weight of PDLA. The addition of low M, PDLA significantly affects the shear rheology of PLLA melts while high M, PDLA does not give such a significant effect. However, the addition of PDLA with high and low M-w's gives a strong strain hardening character to PLLA melt even at a very low PDLA content. (C) 2004 The Society of Rheology.