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

POLYMER

卷:228

Article

Tsuji, Hideto¹  Ohsada, Kaito¹  Arakawa, Yuki¹ 

[1] Toyohashi Univ Technol, Grad Sch Engn, Dept Appl Chem & Life Sci, Tempaku Cho, Toyohashi, Aichi 4418580, Japan

关键词: Crystallization;    Stereo random copolymers;    Poly(lactic acid);    Stereocomplex;    Polymorphism;   

DOI  :  10.1016/j.polymer.2021.123954

来源: Elsevier

【 摘 要 】

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 Land 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 (Llactide-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 8 to alpha occurred in L-PLAs with MMUF = 98.2 and 100% (MMUSL = 55.6 and 63.8 monomer units) between Tc = 90 and 100 degrees C and Tc = 100 and 110 degrees C, respectively, whereas only alpha-form formation took place in L-PLA with MMUF = 96.6% (MMUSL = 29.4 monomer units). The homo-crystallizability and 8-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 8-form crystallizability. The interplanar distance values of LPLA decreased with an increase in Tc and transition from 8-form to alpha-form, whereas those of SC crystallites for LPLA/D-PLA blends remained unchanged for Tc range of 100-180 degrees C. It is interesting to note that the incorporated D-monomer units at ca. 3% caused compact packing of L-PLA chains in alpha-form. The equilibrium melting temperature (Tm0 ) values of SC crystallites [Tm0 (S)] decreased with decreasing MMUF or MMUSL. The Tm0 of homocrystallites [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 LPLAs and the Tm0 values of unblended L-PLAs and L-PLA/D-PLA blends.

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