| Poly(L-lactic acid)(PLLA), as one of the green eco-friendly materials, has received increasing attention in the past decades due to its excellent biocompatibility, biodegradability, and comprehensive mechanical properties. However, PLLA has relatively lower crystallization ability and slower hydrolysis degradation rate due to its rigid chain structure, resulting restricted application in some special occasion. Many studies have proved that nanofillers as heterogeneous nucleation agent can enhance PLLA crystallization ability on one hand; on the other hand, polar nanofillers can modify composites surface hydrophilic property. Therefore, it has important significance for further studying hydrolytic behavior of PLLA on the basis of tuning PLLA crystallization behavior by cooperating nanofillers. In this work, different nanofillers were introduced into PLLA matrix to study the effects of nanofillers on crystallization and hydrolytic behaviors of PLLA. The main results obtained in this work are listed as follows:(1) The isothermal and nonisothermal crystallization behaviors of the neat PLLA and the PLLA/Graphene Oxides sheets (GOs) composites which prepared through two-step method were systematically investigated. It was found that GOs exhibited great role in enhancing the crystallization ability of PLLA. During the isothermal crystallization process, addition of only0.1wt%GOs could greatly accelerate the crystallization of PLLA matrix and the crystallization half time was dramatically decreased; Further increasing the concentration of the GOs could greatly accelerate PLLA crystallization, but high concentration of the GOs was unfavorable for the crystal perfection of PLLA, which could be proved by the decrease of crystal equilibrium melting temperature. During the nonisothermal crystallization process, it was observed that the crystallization ability of PLLA was greatly dependent upon both the concentration of the GOs and the cooling rate:at relatively small cooling rate, the crystallization ability of PLLA increased with increasing concentration of the GOs; at relatively high cooling rate, high concentration of the GOs was required to realize the crystallization of the PLLA. For example, in the case of the cooling rate was increased up to10℃/min, the composites with2.0wt%GOs showed the high crystallinity.(2) One-dimensional carbon nanotubes (CNTs) and two-dimensional organic montmorillonite (OMMT) were introduced into PLLA matrix to prepare two kinds of PLLA composites through melt-compounding method, respectively. And then the effects of CNTs and OMMT on the crystallization behaviors and some physics properties of PLLA were comparatively investigated. The results obtained from crystallization behaviors analysis showed that CNTs exhibited higher nucleation efficiency in improving the melt crystallization of PLLA whether during the isothermal crystallization process or during the nonisothermal crystallization process. However, OMMT was better for promoting cold crystallization of PLLA whether during the DSC heating process or during the annealing process. The rheological properties showed that, although both OMMT and CNTs formed the percolated network structure at relatively higher nanofillers concentrations, the percolated network structure of nanofillers in the PLLA/CNTs composites was denser than that of in the PLLA/OMMT composites, consequently, resulting the different cold crystallization behaviors of PLLA matrix:in the latter composites, the growth of PLLA lamellae was easier than that in the former composites owing to the relatively smaller degree of restriction provided by the loose network structure of OMMT. The dynamic mechanical analysis results showed that the presence of nanofiller could improve the stiffness of the composites dramatically and decrease the loss factors. Specifically, OMMT was the better alternative compared with CNTs in improving the stiffness of the composites. However, the thermal gravimetric analysis showed that CNTs exhibited higher efficiency in improving the thermal stability of PLLA compared with OMMT.(3) It was found that besides the occurrence of chain scission, which resulted in the weight loss and decrease of molecular weight, hydrolytic degradation also induced the change of microstructure in the hydrolyzed PLLA sample when amorphous PLLA was hydrolyzed in alkaline media (pH=13) at different temperature (40,50and60℃). It was proved that the evolution of microstructure was greatly dependent upon the hydrolytic degradation conditions. At relatively low hydrolytic degradation temperature (40and50℃), molecular ordering structure was induced, and the amount of locally ordered structure increased with increasing hydrolytic degradation time. However, there was no crystallization occurred in this condition. At relatively high hydrolytic degradation temperature (60℃), crystallization of PLLA was provoked and a large amount of a’-form PLLA was induced.(4) The hydrolytic experiment showed that the presence of OMMT could greatly accelerate the hydrolytic degradation rate of PLLA. Meanwhile, crystallinity (XC-DSC) of PLLA could be tuned by OMMT during anneal process. Therefore, OMMT had great effect on microstructure evolution of annealed semi-crystalline PLLA. The XC-DSC of neat PLLA sample increased during hydrolysis degradation process (37℃) owing to the reduction of PLLA amorphous region. However, both crystalline region and amorphous region of PLLA/OMMT composites hydrolyzed due to the presence of OMMT, as a result, the Xc of composites decreased.(5) The hydrophilicity of the sample surface increased with increasing content of CNTs, which could further improve the hydrolysis degradation rate, and there was positive correlative between hydrolysis degradation rate and CNTs content. During the annealing process, CNTs exhibited nucleation effect for the cold crystallization of PLLA matrix. However, higher Xc leaded to the decrease the hydrolysis degradation rate. Hence, the hydrolysis degradation rate was greatly influenced by two main factors:at relative lower CNTs concentration, Xc-DSC was the main factor; at relative higher CNTs concentration, CNTs was the main factor in affecting the hydrolysis degradation rate. At the same time, crystallization occurred in the amorphous PLLA during hydrolyzed degradation process (37℃) induced by CNTs.; However, for semi-crystalline PLLA, the crystallinity first increased and then decreased, namely, this was a dynamic process for crystal formation and crystal destroy during the hydrolytic degradation process.(6) Poly(L-lactide)/silica (PLLA/SiO2) composites with good dispersion of SiO2in the PLLA matrix had been prepared through a two-step compounding method. The presence of SiO2particles greatly improved the hydrophilicity of the sample. Largely enhanced hydrolytic degradation ability was obtained for the PLLA/SiO2composites, and the hydrolytic degradation rate depended on the content of SiO2and the hydrolytic degradation temperature:increasing the content of SiO2or enhancing the hydrolytic degradation temperature facilitated the hydrolytic degradation of PLLA matrix. Further results showed that SiO2promoted the reorganization of microstructure of PLLA matrix during the hydrolytic degradation process. At37℃, it promoted the formation of locally ordered structure in the PLLA matrix, while it promoted the crystallization of PLLA matrix at55℃.According to the study in this paper, it was found that PLLA crystallization and hydrolytic behaviors could be improved by cooperating GOs, CNTs, OMMT and SiO2into PLLA matrix. At the same time, these nanofillers could induce PLLA microstructure changed during the hydrolysis degradation process. In this paper, through the systematic characterization and analysis, the effect of nanofillers on the crystallization and hydrolytic degradation behaviors of PLLA was clarified and a model of PLLA microstructure evolution during the hydrolytic degradtation process was proposed. We believe that this work could provide new insights about the crystallization and hydrolytic degradation of PLLA. |
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