Synthesis, stereocomplex crystallization, homo-crystallization, and thermal properties and degradation of enantiomeric aromatic poly(lactic acid)s, poly(mandelic acid)s

Abstract

Enantiomeric aromatic poly(lactic acid)s, i.e., poly(l-mandelic acid) (PLMA) and poly(d-mandelic acid) (PDMA), with different number-average molecular weight (Mn) values were synthesized by polycondensation and their stereocomplex (SC) crystallization and homo-crystallization by solvent-evaporation, as well as thermal properties and degradation were investigated in detail. The SC crystallization was confirmed by wide-angle X-ray diffraction and differential scanning calorimetry. However, Fourier-transfer infrared spectroscopy could not identify SC crystallization, excluding the lowered peak width of the carbonyl group. Predominant SC crystallization was observed for high molecular weight PLMA/PDMA (Mn = 1.7 × 104 and 1.5 × 104 g mol–1, respectively) blends and low molecular weight PLMA/PDMA (Mn = 5.8 × 103 and 7.2 × 103 g mol–1) blends, except for low molecular weight PLMA/PDMA blend with a PLMA fraction of 75%, where both SC crystallization and homo-crystallization occurred. This can be explained by the lower crystallizability of homo-crystallites at high Mn values compared to that of SC crystallites. The glass transition temperatures of poly(mandelic acid)s (87 °C − 112 °C) were higher than those previously reported for poly(lactic acid)s (PLAs) (approximately 60 °C), poly(2-hydroxybutanoic acid)s [P(2HB)s] (24 °C − 44 °C), and poly(phenyllactic acid) (32 °C − 47 °C). In marked contrast with the results previously reported for enantiomeric PLAs and P(2HB)s, melting temperatures of SC crystallites of PMAs (105 °C – 127 °C) were lower than those of homo-crystallites (162 °C − 180 °C). The thermal degradation temperatures at 10% weight loss for unblended PLMA, PDMA, and their (50/50) blend (290 °C − 313 °C) were much higher than those reported for unblended enantiomeric PLAs, and their (50/50) blend (218 °C − 243 °C) and similar to those reported for enantiomeric P(2HB)s, and their (50/50) blend (300 °C − 330 °C). The thermal degradation profiles and temperatures of unblended PLMA, PDMA, and their blend were similar with each other, whereas the activation energy for thermal degradation (ΔEtd) of PLMA/PDMA blend (136.7− 163.0 kJ mol–1) was between those of unblended PLMA (117.7 − 154.4 kJ mol–1) and unblended PDMA (196.3 − 226.6 kJ mol–1). These results contrast with those previously reported for enantiomeric PLAs and P(2HB)s, wherein ΔEtd values were increased by enantiomeric polymer blending. Furthermore, the crystallizability of aromatic poly(mandelic acid) and poly(phenyllactic acid) was compared.