Crystallization and morphology of stereocomplexes in nonequimolar mixtures of poly(L-lactic acid) with excess poly(D-lactic acid)

Crystallization of nonequimolar compositions of poly(D-lactic acid) with low-molecular-weight poly(L- lactic acid) (PDLA/LM(w)-PLLA) blends leads to formation of various fractions of stereocomplexed PLA (sc-crystallites) and homocrystallites (PDLA or PLLA). For the PDLA/LM(w)-PLLA blends within the composition window of LM(w)-PLLA content between 30 and 50 wt%, only sc-crystal exists and no homocrystal is present. On the other hand, for PDLA/LM(w)-PLLA blends with excess PDLA, e.g. PDLA/LM(w)-PLLA = 90/10, atomic-force microscopy (AFM) characterization on various stages of crystallization of sc-PLA crystal with PDLA homocrystal shows a repetitive stacking of excess PDLA on pre-formed sc-PLA crystal serving as crystallizing templates. The crystallization initially begins with string-like (fibril-like) PDLA lamellae, followed with PDLA aggregating on sc-PLA crystal into a bead-on-string crystal, then growing to thicker irregularly-shaped dough-like lamellae. Repetitive growth cycle from strings to bead-on-string lamellae continues on top of the dough-like lamellae as new substrates, until ending impingement of the PDLA spherulites. (C) 2011 Elsevier Ltd. All rights reserved.