Regulating the Crystallization Behaviors of High-molecular-weight Poly(lactic acid) Stereocomplex System Based on the Structure of Poly(ethylene glycol)

We designed and synthesized comb-like block copolymer poly[poly(ethylene glycol) methyl ether acrylate] (PPEGA) and poly(ethylene glycol)-b-PPEGA (PEG-b-PPEGA). By mixing with the high-molecularweight poly(L-lactic acid) (PLLA) and poly(D-lactic acid) (PDLA) blends, the effect of various PEG structures on the crystallization of PLA stereocomplex system (sc-PLA) was explored. The results measured from optical microscopy (OM) show that homopolymer PEGA and PEG and copolymer PEG-b-PPEGA are completely miscible with sc-PLA, whereas PPEGA is immiscible. The non-isothermal and isothermal crystallization data for all these blends by means of differential scanning calorimetry (DSC) indicate that both PEGA and PEG could promote the formation of stereocomplex crystallites (SCs), but reduce the crystallinity of homochiral crystallites (HCs). However, introducing PPEGA and PEG-b-PPEGA could simultaneously enhance the crystallinity of HCs and SCs. Besides, the melting points of SCs in sc-PLA/PPEGA and sc-PLA/PEG-b-PPEGA blends increase with the increasing content of these two copolymers. These might be ascribed to the local interfacial interactions between PLA and the branched-chains PEGA. From polarized optical microscopy (POM), we could find that the introduction of these four polymers could significantly increase the spherulitic growth rates but reduce the number of nuclei, indicating that the mobility of PLA chains play a key role on the increased crystallinity of SCs. X-ray analysis further revealed that miscible PEGA could induce higher long spacing of SCs in sc-PLA/PEGA blends than that in sc-PLA/PPEGA blends and comb-like PPEGA has a significant impact on the thickening of SCs, resulting in a higher melting point of SCs. In addition, although the block copolymer PEG-b-PPEGA formed by PEG and PPEGA promotes the miscibility of PPEGA with PLA, PPEGA would adversely affect the interaction between PEG and PLA molecular chains during crystallization, and thus the synergistic effect of PEG and PPEGA on promoting crystallization is weakened. [GRAPHICS] .

Automated summary

In this study, different PEG and PPEGA block copolymers were synthesized to explore their effects on the crystallization of PLA stereocomplex system. The results showed that PEGA and PEG promote the formation of SCs but reduce the crystallinity of HCs, while introducing PPEGA and PEG-b-PPEGA enhances the crystallinity of both HCs and SCs. Additionally, the melting points of SCs in sc-PLA/PPEGA and sc-PLA/PEG-b-PPEGA blends increase with the increasing content of these copolymers.