Fabrication Of Strong And Tough Cellulose Nanofibril Films And Their Strengthening And Toughening Mechanism

The development of sustainable,biodegradable plastic alternatives has become a hot research topic in recent years.Cellulose nanofiber(CNF)films are expected to partially replace plastics in many fields such as food packaging,flexible electronics,and biomedical applications for their characteristics of transparency,high strength,and stable performance.However,compared with conventional plastics,CNF films have higher strength(100-300 MPa)but lower toughness(<20 MJ/m~3),especially their toughness is generally lower than most petroleum-based polymer films(25-100 MJ/m~3),limiting their industrial application.The production of CNF films with both high strength and high toughness can accelerate the replacement of some plastic films,which is important for slowing down the consumption of petroleum resources and reducing the burden on the environment,and contributes to the national green development strategy.In this thesis,two strategies of internal toughening mechanism(optimization of CNF structure)and external toughening mechanism(addition of toughening agent)were proposed to prepare CNF films with both high strength and high toughness.Firstly,using spruce as raw material,we developed a hydrothermal pretreatment coupled with dilute alkali-sulfate pulping process and optimized disintegration method to preserve the maximum degree of polymerization and achieve enhanced toughening of CNF films from the perspective of internal toughening mechanism.Further,based on the external toughening mechanism,lignosulfonic acid(LA)was introduced into the CNF dispersion as a toughening agent to construct an additional energy dissipation system to achieve the simultaneous enhancement of strength and toughness of CNF films.Based on the above two strategies,deep eutectic solvent(DES)pulping technology was used to obtain lignin-containing cellulose nanofibers(D-LCNF)rich in the amorphous region,and the prepared films had significantly improved toughness.The toughening mechanism of D-LCNF films was investigated by FTIR,XRD and AFM,and the results showed that the high toughness of the films was attributed to the abundant amorphous regions and the partially retained lignin.The results of this thesis will realize the toughening of CNF films and promote the development of natural polymer materials in plastic substitution,which has important theoretical significance and practical application value.The main research contents are as follows:(1)Aiming at the problem of severe deterioration of cellulose polymerization by the traditional sulfate method(Kraft),a hydrothermal pretreatment coupled with dilute alkali sulfate process was designed based on the degradation mechanism of cellulose in the cooking process,and high degree of polymerization of wood fibers was successfully prepared under the condition of low alkali usage.Hydrothermal pretreatment removes part of the hemicellulose and raises the pore volume in the wood chips from 0.0012 cm~3/g to 0.0023 cm~3/g,which improves the mass transfer rate of the reaction solution during the pulping process.On the basis of hydrothermal pretreatment,the effect of the alkalinity on the degree of polymerization of wood fibers was investigated,and the results showed that the alkalinity was inversely proportional to the degree of polymerization of wood fibers.When the alkalinity is 10%,the degree of polymerization of the bleached wood fibers can reach up to 2200,which is more than twice as high as that of commercial wood fibers(～1000).The high degree of polymerization of wood fibers lays the raw material basis for the subsequent preparation of CNF with high degree of polymerization.(2)In response to the drastic degradation of cellulose macromolecule structure during conventional disintegration process,CNF-10 with a high degree of polymerization up to 1333,which is comparable to commercial wood pulp,was successfully prepared by optimizing the carboxymethylation pretreatment and high-shear blending process.The aspect ratio of CNF-10is close to～1000,which allows CNF to form a more complex entanglement network,giving the film excellent folding resistance,tensile strength,and toughness.With the increase of degree of polymerization,the highest degree of polymerization(1333)of CNF-10 films reached～4500folding times,which is more than 4 times of the lowest degree of polymerization(661)of CNF-C films;the tensile strength of CNF-10 films reached 238 MPa,elongation at break was 14.6%,and toughness reached 21.1 MJ/m~3,which was 40%higher in tensile strength and 129%higher in toughness than CNF-C film.The enhancement of the aspect ratio of CNF with the degree of polymerization is the main reinforcing and toughening mechanism of the films.(3)Inspired by the natural components of wood,lignosulfonic acid(LA)was added to CNF films to build an additional energy dissipation system to achieve simultaneous improvement of film strength and toughness.The effect of LA addition on the strength and toughness of CNF films was investigated.Compared with pure CNF films,the strength and toughness of CNF/LA composite films increased simultaneously when the LA content was lower than 2.5 wt%,in which the toughness of CNF/LA-1 films prepared by doping 1 wt%of LA increased from 11.0 MJ/m~3 to 23.6 MJ/m~3,an increase of more than 2 times;the strength increased from 198 MPa to 249 MPa,which is 26%.In addition,the interaction forces between LA and CNF and the structural changes of LA aggregates in CNF films during stretching were investigated by rheometer,QCM-D,AFM,XRD,etc.The results showed that the toughening mechanism of CNF/LA composite films lies in the mutual slip of adjacent CNFs during stretching and the energy dissipation during the deformation and transfer of LA aggregates.(4)Based on the above two toughening strategies of CNF optimization and lignin addition,the microwave-assisted choline chloride/lactic acid DES pulping technology,combined with carboxymethylation and high-shear blending process,was further adopted to successfully prepare D-LCNF with internal toughening component(lignin),and its film toughness was significantly improved due to the retention of internal lignin and rich amorphous region in CNFs,and the toughness of D-LCNF5 film with 5%lignin content was 28.3 MJ/m~3,which is 1.7 times higher than that of the film prepared by Kraft pulping technology.The toughening mechanism of D-LCNF films was investigated,showing that the structural changes of CNF during DES treatment were the main reason for the enhancement of film toughness:strong hydrogen bonding between choline chloride and cellulose could make some crystalline regions of cellulose swell,and the grain size was reduced from 5.9 nm to 3.2 nm compared with Kraft pulping;more hydroxyl groups were exposed after swelling to undergo esterification with lactic acid,which in turn weakened the hydrogen bonding between molecular chains,increases more amorphous regions,and facilitates toughening.Further microstructural characterization showed that the internally retained lignin could be uniformly distributed in the D-LCNF network,which could also facilitate the effective dispersion of CNF and enhance interfacial interactions,thus improving the toughness of the film.