Ternary stereocomplex crystallization of poly(l-2-hydroxybutanoic acid), poly(d-2-hydroxybutanoic acid), and poly(d-lactic acid) from the melt

Ternary stereocomplex (TSC) crystallization of optically active polyesters, l-configured poly(l-2-hydroxybutanoic acid) [P(l-2HB)], d-configured poly(d-2-hydroxybutanoic acid) [P(d-2HB)] and poly(d-lactic acid) (PDLA) from the melt is reported. Ternary P(l-2HB)/P(d-2HB)/PDLA blends were prepared by solution-casting to have a fixed content (50 mol%) of l-configured P(l-2HB) and different contents of d-configured P(d-2HB) relative to those of PDLA and stereocomplex crystallization in the blends from the melt was investigated using wide-angle X-ray diffractometry and differential scanning calorimetry. For all the crystallization temperature (Tc) range of 60-180 °C, the interplane distances of melt-crystallized blends estimated from the 2θ values at around 11° and 22° and the melting temperatures of stereocomplex increased continuously with increasing the P(d-2HB) content relative to that of PDLA in the blends. This result indicates that the crystallization of ternary stereocomplex and the continuous changes of stereocomplex lattice dimensions, irrespective of Tc. The equilibrium melting temperature values of TSC crystallites increased continuously with increasing the content of P(d-2HB) relative to that of PDLA. For all the Tc, the higher crystallinity values of SC crystallites in the ternary blends compared to those of the binary blends strongly suggested the relatively random incorporation or no strict selection of d-configured P(d-2HB) and PDLA, resulting in higher crystallinity of ternary stereocomplex in the limited period of time of isothermal crystallization compared to those of binary P(l-2HB)/PDLA hetero-stereocomplex in P(l-2HB)/PDLA blends and P(l-2HB)/P(d-2HB) homo-stereocomplex in P(l-2HB)/P(d-2HB) blends.