Study On The Hydrolytic Degradation Behavior And Microstructure Changes Of Polylactide Induced By Graphene Oxide

Nowadays,the fossil resources are being exhausted gradually,the sources of the synthetic polymers is faced with a great challenge.In addition,the environment pollution from the deposit of non-biodegradation synthetic polymers is becoming more and more serious.As one of the green biodegradable polymers,poly(lactic acid)(PLA)has attracted increasing attention due to its excellent biocompatibility and favorable mechanical properties.PLA can be degraded in various forms,but hydrolysis is still the most common form for the degradation PLA.Thus,it really makes sence for the further research and controllable regulation of the hydrolytic degradation behavior of PLA.The most direct method for controlling the hydrolytic degradation property of PLA lies on the modification of its molecular structure through copolymerization.Compared with the tedious procedure for copolymerization,the introduction of other component or nanofiller is a more effective and feasible method for adjusting the degradation behavior of PLA.However,most of the PLA based blends can not achieve complete degradation,besides high content of nanofiller will affect the mechanical properties of PLA to a certain extent.Therefore,it is of great importance to choose the appropriate nanofiller and other component to obtain PLA based composites with outstanding comprehensive performances and controllable degradation property.The main attention of this paper is focused on poly(L-lactic acid)(PLLA).Firstly,the microstructure of PLLA is adjusted through the introduction of graphene oxide(GO),and then the hydrolytic degradation behavior of PLLA/GO composites in three different mediums and the corresponding microstructure evolution are systematically investigated.After that,the crystalline structures of PLLA are tuned through annealing,and the effect of the filler content,crystallinity and crystalline structure on the hydrolytic degradation behavior of PLLA are comparatively studied.At last,polyethylene glycol(PEG)and GO are together introduced into PLLA to study their impact on the hydrolytic degradation behavior and the corresponding microstructure evolution of PLLA,additionally,the effect of the molar mass of PEG on the hydrolytic degradation behavior of PLLA is also explored.The main conclusions in this research are listed as follows:(1)The PLLA/GO composites are prepared through solution blending,and then the hydrolytic degradation behavior of PLLA/GO composites in three different mediums(including the alkaline solution with pH value of 12,the acid solution with pH value of 2,the deionized water with pH value of 7)and the corresponding microstructure evolution are systematically investigated.The result of the contact angle measurement shows the improved hydrophilicity of PLLA/GO composites compared with pure PLLA.The hydrolytic degradation of the PLLA matrix is greatly accelerated through the introduction of GO,pure PLLA and the PLLA/GO composites exhibite the largest degradation rate in alkaline solution,and the presence of GO do not change the erosion mechanisms of PLLA in all mediums.Hydrolysis induces the molecular ordering and crystallization of PLLA matrix especially in alkaline solution.The existence of GO that exhibites perfect nucleation effects can promote the crystallization of PLLA in the hydrolytic degradation process,while the newly-formed crystals will suffer from corrosion due to the enhancement in degradation ability of PLLA/GO composites.(2)The pure PLLA sample and PLLA/GOO.5 composite are prepared through solution blending.In order to investigate the effect of condensed structure on the hydrolytic degradation behavior of PLLA,samples are being annealed to control its crystalline structures(including crystallinity and crystal form),and then,the effect of the filler content,crystallinity and crystalline structure on the hydrolytic,degradation behavior of PLLA are comparatively studied.The results show that the increase in crystallinity and perfection degree of crystal structure will have a significant inhibitory effect on the decrease of the molecular weight of PLLA.In addition,the mass loss of samples and the dissolution rate of hydrolysate gradually decrease with the increase in crystallinity and perfection degree of crystal structure.The penetration of water molecules becomes even more difficult due to the tighter arrangement of PLLA molecules in the more perfect crystalline region.The spherulite exhibits higher stability in the hydrolytic degradation process compared with rod-like crystal.The presence of GO promotes the reduction of the molecular weight of PLLA and increases the mass loss of samples during the degradation process.Changes in crystallinity and hydrophilicity have a significant effect on the hydrolytic degradation property of PLLA,while the effect of crystal structure is a little bit weak.(3)PEG and GO are introduced into PLLA to prepare PLLA/PEG blends and PLLA/PEG/GO composites through melt blending.The results demonstrate that the hydrolysis rate of PLLA is greatly accelerated with the introduction of PEG and GO.Futher analysis shows that the hydrophilicity of samples is significantly improved with the introduction of PEG and GO.On the one hand,the penetration of water molecules is greatly improved.On the other hand,the dissolution of PEG and the interface around GO provide more sites for the hydrolysis reaction,causing a more quickly decrease in the molecular weight of PLLA.The dissolution of PEG and digestion of the hydrolysate are accelerated by the introduction of GO,mass loss of the PLLA/PEG/GO ternary composites during hydrolytic degradation process increases with the increase of GO content.The rod-like crystal can be generated from the degradation in neutral environment in the presence of PEG,while the newly-formed rod-like crystal is easy to be further degraded.Besides,the molecular weight of PEG has certain efifect on the hydrolysis rate of PLLA,PEG with low molecular weight shows better promotion to the hydrolysis of PLLA.