Preparation And Mechanism Of Lignin/Polyvinyl Alcohol Functional Composites

Recently,the problem of"white pollution"caused by waste plastics poses a great threat to the ecological environment and human health.The development of biodegradable polymer materials is one of the effective ways to alleviate these problems.Polyvinyl alcohol(PVA)is a kind of biodegradable polymer with good performance,which has been used in various fields.However,due to the large amount of hydroxyl groups in the main chain of PVA,the melting temperature of PVA is very close to the decomposition temperature,which is not conducive to the melting process.The melting point can be reduced by chemical modification or blending modification,but the mechanical properties of PVA are usually seriously reduced.Besides,PVA has poor water resistance and single function,which greatly limits its application.Adding modifier to PVA can improve its performance,increase its function and expand its application areas.Industrial lignin has many advantages,such as wide source,low price,non-toxic,degradable,anti-oxidation,UV-shielding and so on.It is of great economic and environmental value to use lignin to modify PVA to prepare biodegradable composites.However,strong hydrogen bonding in lignin causes the poor dispersion in PVA matrix,resulting in weak mechanical properties of lignin/PVA composites.To solve these problems,this work creatively proposed imitating natural spider silk in PVA matrix to construct nano microphase separation structure based on lignin amphiphilic structure.At the same time,dense interface dynamic hydrogen bonding formed between the abundant oxygen-containing polar functional groups of lignin and PVA matrix,which improved the interface force,solving the problem of poor dispersion and compatibility between lignin and PVA.Firstly,LA/PVA nanocomposite films with high strength and toughness were prepared by solution blending with lignosulfonic acid(LA).The tensile strength reached 98.2 MPa and the tensile relative toughness reached 172±5 J/g,which was the highest specific toughness PVA nanocomposite film reported at that time.The results of mechanism study showed that LA with amphiphilic structure formed multilayer nano/microphase separation structure in PVA matrix.During the external force strenching,LA secondary nanospheres were gradually broken and dispersed with the increase of strain,resulting in the strain induced dispersion.In this process,energy was dissipated between LA primary nanoparticles and PVA matrix because of the exisitence of large numbers of interface dynamic hydrogen bonds.Meanwhile,LA molecules effectively restrain the amorphous segments of PVA through 3D meshing effect and promote the elongation and orientation of the segments,realizing the simultaneous reinforcement and toughening of the composites.The introduction of LA can significantly reduce the melting temperature of PVA nanocomposites,improve their thermal stability and provide a larger processing window for the melting processing of PVA.Besides,LA endued PVA nanocomposite film excellent UV-shielding performance.Next,to solve the problem of poor water vapor barrier of PVA,hydrophilic nano micelles(LNM)was obtained by a simple hydrophobic self-assembly method,and then it was introduced into PVA to prepare high barrier LNM/PVA nanocomposite films.The"tortuosity"of water vapor transmission path in PVA matrix increases resulting from the formation of nano microphase separation structure,the construction of hydrogen bond interaction between LNM and PVA,the increase of surface micro nano roughness of composite film and the increase of crystallinity of PVA.Thus,the water vapor barrier performance of PVA composite films enhanced obviously.To simplify the modification process of lignin,cellulose-containing lignin was obtained by simple fibrillation of the bark.The transparent LCNF/PVA composite films were successfully prepared by introducing TEMPO oxidized lignocellulose(LCNF)into PVA.LCNF dispersed well in PVA matrix,and hydrogen bond interaction formed between LCNF and PVA.The tensile strength and Young's modulus of PVA composite film were 77.3 MPa and 2.82 GPa,respectively.In addition,the lignin in LCNF also improved the water vapor barrier performance,thermal stability and UV-shielding performance for PVA.On the basis of using lignin to construct nano microphase separation and interfacial hydrogen bonding to achieve strengthening and toughening,a high-efficiency antibacterial agent TA@LS-Ag was prepared by a simple microwave-assisted reduction method with LS as dispersion carrier and tannic acid(TA)as reducing agent by using the adsorption and dispersion function of LS for silver ions.Then,the antibacterial agent was introduced into PVA to prepare TA@LS-Ag/PVA antibacterial composite film.TA@LS-Ag antibacterial agents shows excellent antibacterial activity with only 0.32 wt%loading of silver.The PVA composite films with 1?10 wt%antibacterial agent perform good antibacterial activity against E.coli and S.aureus at very low silver content(0.032?0.32 wt‰).TA@LS-Ag also endows the composites good antioxidant activity and UV-shielding properties.In addition,the polar functional groups such as phenolic hydroxyl groups in LS and TA molecules form strong interfacial hydrogen bonding with PVA,which makes the composite film also show excellent mechanical properties.The tensile strength of the composite film reached up to131.6 MPa,and the fracture toughness was up to 173 J/g,which is far higher than that of natural spider silk.To widen the application field of lignin/PVA composites,PVA composite hydrogel was developed by a biomimetic strategy.LS/PVA composite hydrogel was prepared by a facile freezing thawing/solvent exchange method.After immersed it in silver nitrate and green reducing agent citric acid solution successively,the highly conductive antibacterial conductive hydrogel Ag~0@LS/PVA with tight inner and loose microstructure was successfully prepared.Attributed to the fracture and reconstitution of hydrogen bond interaction between LS and PVA in hydrogel,the rupture and separation of silver nanoparticle clusters,the unwrapping of the surface dense hydrogel network and the slipping of silver nanoparticles on the hydrogel network,PVA composite hydrogel showed excellent mechanical properties.The tensile strength of the hydrogel was as high as 8.49 MPa and the elongation at break was 1285%.The abundant silver nanoparticles endued good conductivity to the hydrogel,and its conductivity reached up to 7.1 S/m.The hydrogel exhibited stable response and high sensitivity for deformation and environmental temperature changes.Besisdes,the abundant silver nanoparticles gave composite hydrogel high antibacterial activity,which is expected to be used in electronic skin,intelligent devices and other fields.In this thesis,the structural and functional characteristics of lignin were studied systematically.Lignin was used to prepare multifunctional biodegradable lignin/PVA composites.It can not only promote the high value utilization of lignin as an important biomass resource,but also promote the greening of polymer materials,which has a certain guiding significance for the reinforcement,toughening and functional modification of biodegradable polymer materials.