Fractional Crystallization Kinetics and Formation of Metastable β-Form Homocrystals in Poly(L-lactic acid)/Poly(D-lactic acid) Racemic Blends Induced by Precedingly Formed Stereocomplexes

Stereocomplex (SC) crystallization has been an effective way to improve the physical performance of polymer materials. However, SC crystallization of stereocomplexable polymer blends with complementary configurations is much more complicated than the conventional homocrystallization because of the interplay and competition between SC and homo crystallizations. Herein, we used the poly(L-lactic acid)/poly(D-lactic acid) (PLLA/PDLA) racemic mixture as the stereocomplexable blend and investigated its crystallization kinetics, polymorphic structure, lamellae structure, and crystalline phase transition. Unexpectedly, the medium-molecular weight (MMW, similar to 40k) PLLA/PDLA racemic blend exhibits unique fractional crystallization kinetics (i.e., multiple crystallization peaks) during heating because of the constrained effect of the precedingly formed SCs in the early stage of crystallization. The MMW PLLA/PDLA blend forms the metastable beta-form homocrystals (HCs) at a low crystallization temperature (T-c, 75-100 degrees C) but the common alpha(alpha')-HCs at a high T-c (100-140 degrees C), besides SCs. We propose that the formation of beta-HCs stems from (i) the constrained effects induced by precedingly formed SCs and (ii) the conformational matching between beta-HCs and SCs. The beta-HCs formed in the MMW blend have a larger long period than its alpha(alpha') counterpart and transform into the more thermally stable alpha-HCs through melt recrystallization during heating. This study has shed light on the unique fractional, constrained crystallization kinetics and its induced formation of metastable HCs in stereocomplexable polymer blends.