Stereocomplex- and homo-crystallization behavior, polymorphism, and thermal properties of enantiomeric random copolymers of l- and d-lactic acids from the melt

Abstract

Enantiomeric stereo random copolymers (SRCs), l-lactic acid-rich poly (lactic acid)s (L -PLAs) and d-lactic acid-rich poly (lactic acid)s (D-PLAs) with major monomer unit fraction down to 85% were synthesized from mixtures of l- and d-lactic acids and their stereocomplex (SC)- and homo-crystallization from the melt were investigated. The obtained results were compared with those previously reported for enantiomeric SRCs,l-lactide-rich poly (l-lactide-co-d-lactide)s and d-lactide-rich poly (l-lactide-co-d-lactide)s synthesized from l- and d-lactides (lactic acid cyclic dimers). Homo-crystallization of L-PLAs occurred for major monomer unit fraction (MMUF) = 96.6–100% and major monomer unit sequence length (MMUSL) = 29.4–63.8 monomer units. The crystalline form transition from δ to α occurred in L-PLAs with MMUF = 98.2 and 100% (MMUSL = 55.6 and 63.8 monomer units) between Tc = 90 and 100 °C and Tc = 100 and 110 °C, respectively, whereas only α-form formation took place in L-PLA with MMUF = 96.6% (MMUSL = 29.4 monomer units). The homo-crystallizability and δ-form crystallizability of L-PLAs and l-lactide-rich poly (l-lactide-co-d-lactide)s were determined by MMUSL rather than MMUF. SC crystallization occurred in L-PLA/D-PLA blends for wider MMUF = 84.4–100% (MMUSL = 6.4–56.4 monomer units) compared to that of homo-crystallization of unblended specimens, i.e., SC crystallization widened crystallizable MMUF and MMUSL ranges. The SC crystallizability was determined by not only MMUF or MMUSL but also by other factors, in contrast with the homo- and δ-form crystallizability. The interplanar distance values of L-PLA decreased with an increase in Tc and transition from δ-form to α-form, whereas those of SC crystallites for L-PLA/D-PLA blends remained unchanged for Tc range of 100–180 °C. It is interesting to note that the incorporated d-monomer units at ca. 3% caused compact packing of L-PLA chains in α-form. The equilibrium melting temperature (Tm0) values of SC crystallites [Tm0(S)] decreased with decreasing MMUF or MMUSL. The Tm0 of homo-crystallites [Tm0(H)] and Tm0(S) were lower for PLAs with lactic acid monomers than those with lactide monomers, when compared at the similar MMUF values. Tm0(H) and Tm0(S) values were determined by MMUSL not by MMUF. In summary, MMUSL was found to play a critical role to determine the crystallizability of unblended L-PLAs and the Tm0 values of unblended L-PLAs and L-PLA/D-PLA blends.