Homocrystalline mesophase formation and multistage structural transitions in stereocomplexable racemic blends of block copolymers

Polymer crystallization is a multi-stage process and generally takes place through the metastable intermediate structure or mesophase. However, mesopahse is difficult to be experimentally detected during crystallization and its formation mechanism is not well understood. Herein, we find that the formation of mesopahse is facilitated in the crystallization of stereocomplexable block polymers. We use the poly(L-lactic acid)-poly(ethylene glycol)-poly(L-lactic acid) (PLLA-PEG-PLLA) and poly(D-lactic acid)-poly(ethylene glycol)-poly(D-lactic acid) (PDLA-PEG-PDLA) as the stereocomplexable pair and investigate the synergetic effects of copolymer composition and PLLA/PDLA stereocomplexation on the mesophase formation in the PLLA-PEG-PLLA/PDLA-PEG-PDLA 1/1 blends. Intriguingly, the stereocomplexable blend having the high PEG fraction forms the unique homocrystalline mesophase at the low crystallization temperature (T-c, 70-120 degrees C) but the usual alpha-form homocrystals (HCs) at high T-c (120-160 degrees C), except for the stereocomplexes (SCs). However, the crystalline structure of PLLA-PEG-PLLA/PDLA-PEG-PDLA blend with low PEG fraction is similar as the PLLA/PDLA blend; it crystallizes in the mixture of alpha(alpha')-HCs + SCs at T-c = 70-160 degrees C. The homocrystalline mesophase formed in stereocomplexable blend is metastable and reorganizes into the more thermally-stable alpha-HCs and SCs during heating. We propose that the mesophase formation in enantiomeric blend is resulted by the constrained effects induced by the stereocomplexation and microphase separation of block copolymers.