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

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 (M-n) 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 (M-n = 1.7 x 10(4) and 1.5 x 10(4) g mol(-1), respectively) blends and low molecular weight PLMA/PDMA (M-n = 5.8 x 103 and 7.2 x 10(3) 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 degrees C - 112 degrees C) were higher than those previously reported for poly(lactic acid)s (PLAs) (approximately 60 degrees C), poly(2-hydroxybutanoic acid)s [P(2HB)s] (24 degrees C - 44 degrees C), and poly(phenyllactic acid) (32 degrees C - 47 degrees C). In marked contrast with the results previously reported for enantiomeric PLAs and P(2HB)s, melting temperatures of SC crystallites of PMAs (105 degrees C - 127 degrees C) were lower than those of homo-crystallites (162 degrees C - 180 degrees C). The thermal degradation temperatures at 10% weight loss for unblended PLMA, PDMA, and their (50/50) blend (290 degrees C - 313 degrees C) were much higher than those reported for unblended enantiomeric PLAs, and their (50/50) blend (218 degrees C - 243 degrees C) and similar to those reported for enantiomeric P(2HB)s, and their (50/50) blend (300 degrees C - 330 degrees 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 (Delta E-td) 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 Delta E-td values were increased by enantiomeric polymer blending. Furthermore, the crystallizability of aromatic poly(mandelic acid) and poly(phenyllactic acid) was compared. (C) 2021 Elsevier Ltd. All rights reserved.