(617i) Enhanced Stereocomplex Crystallization of High-Molecular-Weight Poly (lactic acid)

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.^[1] 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.^[2] 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.^[3] 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. 

References
[1] Pan PJ, Han LL, Shan GR, Bao YZ. Heating and Annealing Induced Structural Reorganization and Embrittlement of Solution-Crystallized Poly(L-lactic acid). Macromolecules. 2014, 47(22): 8126-8130.
[2] Pan PJ, Han LL, Bao JN, Xie Q, Shan GR, Bao YZ. Competitive Stereocomplexation, Homocrystallization, and Polymorphic Crystalline Transition in Poly(L-lactic acid)/Poly(D-lactic acid) Racemic Blends: Molecular Weight Effects. J Phys Chem B. 2015, accept.
[3] Han LL, Pan PJ, Shan GR, Bao YZ. Stereocomplex crystallization of high-molecular-weight poly(l-lactic acid)/poly(d-lactic acid) racemic blends promoted by a selective nucleator. Polymer. 2015, 63: 144-153.