Study On The Enhancement Of Hydrolysis Resistance Of Poly(L-lactic Acid) And Its Composites Through Tailoring The Interfacial Interaction

As people pay more and more attention to energy and ecological environment,develop and use of bio-based and biodegradable materials that are renewable and disappear without leaving marks are becoming very important.Among them,poly(L-lactic acid)(PLLA)has attracted much attention from the industry and researchers because of its excellent processing properties,mechanical properties,optical properties and biocompatibility.At present,PLLA products developed by the industry are mostly non-durable consumer goods,such as packaging,short-term medical supplies and service supplies.It is not enough to be used as a durable consumer in the fields of electrical appliances and automobiles.The biggest reason is that PLLA is prone to hydrolytic degradation.The hydrolytic degradation of PLLA causes its molecular weight to decrease,thereby lowering its mechanical properties such as tensile strength and seriously affecting the service life of the PLLA-based articles.Therefore,improving the hydrolysis resistance of PLLA and its products is of great significance for expanding its application in the field of durable consumer goods.However,chemical modification of PLLA is often difficult and costly.Therefore,preparation of hydrolysis-resistant PLLA-based composites by means of formulation design and structural regulation has become a valuable research topic.The research object of this thesis is Poly(L-lactic acid,PLLA).First,poly(methyl methacrylate)(PMMA)was added to PLLA.The hydrolysis behavior of PLLA is regulated by PMMA,which have strong interation with PLLA and can not be hydrolyzed.The crystal structure of PLLA/PMMA blend was controlled by annealing treatment.The effects of PMMA content in the blend and the crystallinity of PLLA on its hydrolysis behavior were compared.On this basis,PMMA was introduced into PLLA/carbon nanotubes(CNTs)composites.The effects of PMMA content and the interaction between the components of composites on the crystallization behavior and hydrolysis behavior of PLLA were studied.And the hydrolysis mechanism of PLLA/CNT and PLLA/PMMA/CNT composites was clarified.At last,the interaction between carbon nanofibers(CNFs)and PLLA is regulated by oxidative modification and graft modification of CNFs.The effects of the interaction between modified CNFs and PLLA on the crystallization behavior and hydrolysis behavior of PLLA were investigated.And their hydrolysis mechanism in alkaline environment was elucidated.The main conclusions in this research are listed as follows:(1)In order to compare the effect between the content of PMMA and the crystallinity of PLLA on the hydrolysis performance of PLLA.The PLLA/PMMA blends were prepared by melt blending,and the samples with different crystal structures were obtained by quenching and annealing treatment.Then the hydrolysis behavior and microstructure changes of PLLA/PMMA blends were studied under alkaline conditions.The results show that the addition of PMMA inhibits the segmental motion of PLLA,making its crystallization more difficult.The crystallinity of PLLA after annealing treatment was 19.4%.After adding 10 wt% PMMA,the crystallinity of PLLA in the blends was only 2.4%.PMMA exhibits two mechanisms of action in the blends.On the one hand,the addition of non-hydrolyzed PMMA can inhibit the hydrolysis of PLLA by diluting the concentration of the molecular chain of PLLA.On the other hand,it inhibits the crystallization of PLLA,and the crystallinity after annealing of PLLA is lowered,that makes PLLA not resistant to hydrolysis.When the content of PMMA is less than or equal to 5 wt%,the decrease of crystallinity of PLLA dominates the hydrolysis of the blended materials,and the hydrolysis resistance of the blended materials reduced.However,when the content of PMMA is more than 10 wt%,the influence of content of PMMA on the hydrolysis of the blended materials is dominant,and the hydrolysis resistance of the blended material is enhanced.Since the hydrolysis temperature is much lower than the glass transition temperature of PLLA,the blended materials hardly undergoes structural changes during the hydrolysis process.In addition,PLLA and PLLA/PMMA blends still follow the surface erosion mechanism in alkaline environments.(2)To explore the effect of PMMA on the hydrolysis behavior of PLLA/CNT composites.The PLLA/CNT and PLLA/PMMA/CNT composites were prepared by melt blending.The effects of PMMA addition on the dispersion of CNTs,the crystallization behavior and hydrolysis behavior of PLLA/CNT composites were investigated.The results show that the addition of PMMA promotes the dispersion of CNTs in PLLA.Even though the composite contains 1 wt% of CNTs that can act as heterogeneous nucleating agents of PLLA,PMMA still significantly inhibits cold crystallization and isothermal crystallization of PLLA.In addition,the addition of PMMA also significantly improved the hydrolysis resistance of PLLA/CNT composites.The interfacial energy of PLLA,PMMA and CNTs in the literature were reviewed and the interaction between them was calculated.It was found that the interaction between PMMA and CNTs was stronger than the interaction between PLLA and CNTs.CNTs tend to adsorb more PMMA molecular chains.This not only reduces the heterogeneous nucleation of CNTs to the isothermal crystallization of PLLA,but also makes it difficult for the PLLA/CNT composite to be hydrolyzed at the interface between the CNTs and the matrix after the PMMA is added.(3)To investigate the effect of the interaction between CNFs and PLLA on the hydrolysis behavior of PLLA.CNFs was treated by oxidation modification and graft modification.Then,untreated Carbon nanofibers(raw-Carbon nanofibers,r-CNFs),oxidized modified Carbon nanofibers(functional-Carbon nanofibers,f-CNFs)and Carbon nanofibers grafted PEG(grafted-Carbon nanofibers,g-CNFs)were separately introduced into PLLA matrix and the effects of these three different CNFs on the crystallization behavior and hydrolysis behavior of PLLA were investigated.The results showed that about 12 wt% of oxygen-containing functional groups were introduced on the surface of f-CNFs by oxidation modification,and about 39 wt% of polyethylene glycol(PEG)was grafted on the surface of g-CNFs.The hydrophilicity of f-CNFs and g-CNFs were significantly enhanced.The interaction between surface-inert r-CNFs and PLLA is the weakest.The interaction between f-CNFs and PLLA is much smaller than the interaction between g-CNFs and PLLA.However,they have different effects on the crystallization behavior and hydrolysis behavior of PLLA.f-CNFs can promote the crystallization of PLLA more than r-CNFs,but its hydrophilicity makes PLLA less resistant to hydrolysis.While in PLLA/g-CNF composites,the surface of g-CNFs is linked to a large amount of PEG through ester bonds,making PLLA difficult to crystallize on its surface,and PLLA is more resistant to hydrolysis due to the strong interaction between PEG and PLLA.