424772 Enhanced Stereocomplex Crystallization of High-Molecular-Weight Poly (lactic acid)

Wednesday, November 11, 2015: 5:30 PM
251C (Salt Palace Convention Center)
Lili Han, Pengju Pan, Guorong Shan and Yongzhong Bao, State Key Laboratory of Chemical Engineering, College of Biological and Chemical Engineering, Zhejiang University, Hangzhou, China

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. 

[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.

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