Poly (lactic acid) (PLA) has attracted much attention due to its biodegradability, biocompatibility , good processibility and eco-friendly. It has been widely used in the biomedical and commodity applications for replacement of the conventional oil-based thermoplastics. However, the PLA material has low thermal-resistant temperature, which has limited its wide application. Stereocomplexed PLA (sc-PLA) has a melt temperature of 230 ºC, which is 50 ºC higher than the conventional PLA material. Therefore, the stereocomplex crystallization of poly (L-lactic acid) (PLLA) and poly (D-lactic acid) (PDLA) has been considered as a promising method to improve the thermal and mechanical properties of PLA-based materials. However, the PLLA/PDLA stereocomplex crystallization just took place in the common enantiomeric blends with the low molecular weights. For the high molecular weight (HMW) PLLA/PDLA blends, only homo-crystals with lower melting point were formed. In our previous work, a phosphate nucleating agent i.e. zinc phenylphosphonate (PPZn) was used to promote the sc crystallization and suppress the hc crystallization for HMW PLLA/PDLA blend. In this study, we proposed three methods, i.e., block copolymerization via controlled two-step ring-opening polymerization (ROP), block copolymerization via the combination of ROP and click-chemistry, and the PLLA-PEG-PLLA/PDLA-PEG-PDLA triblock blending to enhance the stereocomplex crystallization of HMW polylactide.
The linear and star-shaped stereo-diblock copolymers of PLLA-b-PDLA with the high molecular weight (Mn>100k) were synthesized by the controlled ring-opening polymerization of L-lactide and D-lactide. DSC and WAXD results indicated that the stable stereocomplex crystals were formed in the PLLA-b-PDLA copolymers under all the crystallization conditions investigated. The PLLA-b-PDLA copolymers exhibited a high melting point of around 230 ºC. As the crystallization temperature increases, the degree of crystallization and lamellar thickness of PLLA-b-PDLA copolymer increased. The as-prepared stereo-diblock copolymers had higher storage modulus than the PLLA homopolymers. On the other hand, the stereo-diblock PLLA-b-PDLA copolymers with non-equivalent D/L ratio (~ 100k) were prepared by the combination of ROP of L- and D-lactide and click-chemistry. FT-IR and 1H-NMR results confirmed the successful synthesis of all PLLA-b-PDLA copolymers. Vast stereocomplex crystals were formed under nonisothermal and isothermal crystallization investigated by DSC and WAXD even when the L/D ratio was 8/2. Lamellar morphology was also investigated by SAXS.
Furthermore, the enhanced stereocomplex crystallization induced by the incorporation of PEG was also studied through the blending of PLLA-PEG-PLLA and PDLA-PEG-PDLA. The sc crystals content increased with the increasing PEG length as investigated by DSC.
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