Stereocomplex Formation between Enantiomeric Alternating Lactic Acid-Based Copolymers as a Versatile Method for the Preparation of High Performance Biobased Biodegradable Materials

Abstract

Enantiomeric alternating lactic acid-based copolymers, i.e., poly(l-lactic acid-alt-glycolic acid) [P(LLA-alt-GA)] and poly(d-lactic acid-alt-glycolic acid) [P(DLA-alt-GA)], with the number-average molecular weights (Mn) of 5 × 103 g mol–1 were synthesized, and their stereocomplex (SC) formation was reported for the first time. Wide-angle X-ray diffractometry indicated that the SC crystalline modification of P(LLA-alt-GA) and P(DLA-alt-GA) is different from those reported for enantiomeric homopolymers of poly(l-lactic acid) (PLLA) and poly(d-lactic acid) (PDLA). The SC crystallizability of copolymer blends was much higher than the homocrystallizability of neat copolymers, as evidenced by the higher crystallinity and melting enthalpy values. The melting temperature values of the stereocomplexed blends (187.6 and 187.8 °C) were much higher than those of the neat copolymers (74.6–83.2 °C) and that reported for poly(l-lactic acid-alt-6-hydroxycaproic acid)/poly(d-lactic acid-alt-6-hydroxycaproic acid) SC (45.7 °C) but slightly lower than those reported for PLLA and PDLA homopolymer blends with the similar average Mn values of 2.9 × 103and 7.0 × 103 g mol–1 (196.6 and 224.2 °C, respectively). The P(LLA-alt-GA)/P(DLA-alt-GA) blends are expected to have higher thermal stability compared to that of neat P(LLA-alt-GA) and P(DLA-alt-GA), since the melting temperature values of the former were higher than those of the latter. The radial growth rates of spherulites of SC crystallites in the blends were in the range 0.49–9.4 μm min–1, with which the maximum value was much lower, by 2 orders, than those reported for enantiomeric PLLA and PDLA homopolymer blends with the similar average Mn values of 2.9 × 103 and 7.0 × 103 g mol–1 (247.7 and 120.8 μm min–1, respectively). SC formation did not largely affect the FTIR peak shape and frequency, suggesting that the helical structure of the alternating copolymer chains was not so largely altered by SC formation. The sufficiently high melting temperature values of P(LLA-alt-GA)/P(DLA-alt-GA) blends compared to those of the neat polymers as well as the reported high mechanical performance and high thermal stability reported for stereocomplexed PLLA/PDLA blends compared to the neat polymers strongly suggest that SC crystallization between enantiomeric alternating lactic acid-based copolymers with various counterparts of lactic acid units is a versatile method for the preparation of high performance biobased biodegradable materials with diverse physical properties and biodegradability.