Study On Structure Evolution And Hydrolytic Degradation Behaviors Of Poly(L-Lactic Acid) Induced By Dispersed Phase

Poly(L-lactic acid) (PLLA) has received increasing attention and has great potential in agricultural film, packaging, disposable tableware and biomedicine due to its so many advantages such as excellent mechanical properties, biodegradability, biocompatibility, high transparency, easy processing and wide raw material sources. As one kind of polyester, PLLA can be completely degraded through hydrolytic degradation and enzymolysis of the two processes. However, the hydrolysis rate of pure PLLA is slow compared to other poly(a-ester) such as poly(glycolic acid) (PGA), polydioxanone (PDS), etc. Melt blending or composite is a convenient and effective method to improve hydrolysis degradability of PLLA matrix.In this work, poly(ethylene glycol) (PEG), poly(butylene succinate) (PBS) as well as PBS and SiO2 were introduced into PLLA to prepare PEG plasticized PLLA, PLLA/PBS blends and PLLA/PBS/SiO2 ternary composites. The microstructure and morphologies of samples before and after hydrolysis were investigated through differential scanning calorimetry (DSC), wide angle X-ray diffraction (WAXD), fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). The hydrophilicity of samples was evaluated by contact angle measurement. The structure evolution and hydrolytic degradation behavior of PLLA plasticized by PEG as well as that of PLLA and PLLA/SiO2 induced by PBS were researched systemically from the point of view of the variations on microstructure, morphologies and hydrophilicity of samples. Besides, the effect of PBS on the mechanical properties of PLLA/SiO2 composites was also investigated. The main results obtained from above research are listed as follows:(1) The hydrolytic degradation behavior of pure PLLA and PEG plasticized PLLA was investigated at 60℃ in alkaline solution (pH=12). The results demonstrated that the plasticized PLLA samples exhibited accelerated hydrolytic degradation compared with pure PLLA due to relatively high hydrophilicity of the former and dissolution of PEG during hydrolysis process, and the hydrolytic degradation was also dependent upon the PEG content. The more the PEG content was, the faster the hydrolysis rate was. The results of microstructure characterization indicated that PEG improved the movement ability of PLLA molecular chain and chain segment and induced crystallization and ordering of PLLA matrix during processing and hydrolysis. Further studies indicated that PEG is only able to induce rapid crystallization of PLLA matrix in water atmosphere at a relatively high temperature. The results of morphologies characterization showed that the hydrolytic degradation of plasticized PLLA in alkaline solution obeys both surface-erosion mechanism and bulk-erosion mechanism which is attributed to the dissolution of PEG during hydrolysis. Besides, the erosion of surface part was more serious than that of inner part.(2) The hydrolytic degradation behavior of pure PLLA and PLLA/PBS blends was investigated at 37℃ in alkaline solution (pH=13). The results demonstrated that the presence of PBS accelerated hydrolytic degradation of PLLA matrix due to increasing hydrophilicity and the presence of interfaces. Specifically, the higher the content of PBS was, the bigger the weight loss per unit area of sample was. The results of morphology characterization showed that the hydrolysis mechanism of PLLA/PBS blends is surface-erosion mechanism in alkaline solution and the degradation occurs mainly in the interface region between PLLA and PBS. Besides, the erosion of PLLA was more serious than that of PBS. The results of microstructure characterization indicated that the presence of PBS induced crystallization of PLLA during neither processing nor hydrolysis.(3) The hydrolytic degradation behavior of PLLA/SiO2 and PLLA/PBS/SiO2 was investigated at 50℃ in alkaline solution (pH=12). The results demonstrated that the presence of PBS accelerated hydrolytic degradation of PLLA/SiO2 due to increasing hydrophilicity and the presence of interfaces. The results of microstructure and morphologies characterization indicated that the presence of PBS weakened crystallizability of PLLA matrix in composites due to the selective distribution of SiO2 induced by PBS. The results of morphologies characterization showed that the hydrolytic degradation of both PLLA/SiO2 and PLLA/PBS/SiO2 obeys surface-erosion mechanism. The results of mechanics performance testing indicated that the introduction of PBS can improve non-notched impact strength of PLLA/SiO2 while reducing tensile strength and Young’s modulus of PLLA/SiO2. In addition, the ductility of PLLA/SiO2 would not be influenced by introducing PBS.In this work, the effect of dispersed phase on crystallizability and hydrolyzability of PLLA matrix has been investigated. The structure evolution and hydrolysis process of PLLA induced by dispersed phase have been elaborated. The variation of microstructure, morphologies and hydrolysis behavior of PLLA-based materials has been mastered. Above all, this work can offer academic support for design and exploitation of hydrolysis-controlled high-performance PLLA-based materials.