Preparation Of All-cellulose Nanocomposite Film Based On Controlled Dissolution And Its Crosslinking Reinforcement

Cellulose is the most abundant and widely sourced natural biopolymer on the earth.The rich hydrogen bond network between and within the cellulose molecular chains makes the cellulose organized in a multi-layered structure,nanocellulose can be separated from multi-layered structure by physical,chemical,enzymatic treatment and other methods.Compared with cellulose raw materials,nanocellulose has a high specific surface area and high crystallinity,showing excellent mechanical properties.Compared with traditional nanofillers,nanocellulose has the advantages of wide spread,renewable,biodegradable,good biocompatibility,and low cost.At present,nanocellulose has shown good application prospects in food,biomedical applications,energy and environmental and composite materials.However,when nanocellulose is used as a reinforcement to strengthen the polymer matrix,the uniform dispersion of fillers is a big challenge.All-cellulose nanocomposites(ACNCs)prepared by controlling the cellulose dissolution and molding process can avoid the complex extraction process and dispersion problems of nanocellulose.The dissolution process is controlled to realize the one-step preparation of ACNCs.At present,the process conditions reported in the literature for the preparation of ACNCs are harsh,high energy consumption,and cannot provide strong evidence for the presence of nanocellulose in ACNCs.Based on this,this thesis uses tetra-butyl ammonium hydroxide(TBAH),dimethyl sulfoxide(DMSO),and water as a complex solvent system(hereinafter abbreviated as TBAH/H₂O/DMSO solvent)based on the previous research work of the research group.The derivatization process confirmed the existence of nanocellulose in the cellulose solution,we also studied the effect of dissolution time on the size change of cellulose and the structure and properties of ACNCs,and studied the effect of crosslinking agent with different chain length on the properties of ACNCs.The specific research work and main research results of this paper include:(1)The dissolution process of cellulose in the TBAH/H₂O/DMSO solvent system was studied,the possibility of preparing cellulose nanocrystals by controlled dissolution of cellulose was explored,and the mechanism of preparing cellulose nanocrystals by controlled dissolution was revealed.The etherification reaction of the cellulose solution by2,3-epoxy propyltrimethylammonium chloride(Glycidyl trimethyl ammonium chloride,GTMAC)introduced positively-charged quaternary ammonium groups to the surface of the cellulose,successfully achieving separation of the micron-scaled,nano-scaled and well-dissolved cellulose.TEM characterization confirmed the existence of cellulose nanocrystals(Cellulose nanocrystals,CNCs),XRD confirmed that the crystal form of the derivatized CNCs and raw materials are the same.It means that CNCs were obtained by dissolving cellulose raw materials into the nanometer scale.Further analysis and comparison of the changes in the characteristic diffraction peaks of raw materials and CNCs revealed the controllable dissolution mechanism of TBAH/H₂O/DMSO complex solvent system for preparing CNCs by cellulose.When cellulose is dissolved,TBAH complex solvent can controllably dissolve"amorphous regions"and""Low crystalline area",the crystal ine area remains in situ to obtain CNCs.(2)ACNCs were prepared by controlling the dissolution regeneration process,and the influence of dissolution time on the structure and mechanical properties of ACNCs was studied.The cellulose solution containing nanocrystals was regenerated,desolventized and dried to successfully prepare all-cellulose nanocomposite films.Cellulose nanocrystals were difficult to dissolve,but in the long-term(within 360 min)dissolution process,its size was still decreasing.The length decreased from 168.6 nm to 89.0 nm,and the width decreased from 18.9 nm to 13.3 nm,the aspect ratio was reduced from 9.6 to 6.6.XRD was used to characterize the crystal structure of ACNCs.It was found that a new cellulose II crystal form was formed after the cellulose solution was regenerated.With the extension of the cellulose dissolution time,the total crystallinity of the obtained ACNCs showed a rapid decrease in the initial stage,and then tended to be gentle.The composition of cellulose I and II crystal forms in ACNCs also showed a rapid decrease in the initial stage,and then tended to be gentle.The mechanical properties test showed that with the extension of the cellulose dissolution time,the tensile strength of ACNCs increased first and then decreased.When the dissolution time was 180 min,ACNCs had the best mechanical properties,and the tensile strength was 115.26 MPa.With the extension of the dissolution time,the micron-scaled cellulose on the stretched section disappeared and the section roughness increased,indicating that extending the dissolution time is beneficial to improve the interaction between the cellulose matrix and the undissolved reinforcement.The study also found that the mechanical properties of ACNCs first increased and then decreased with the dissolution time.This is because during the dissolution process,the content and size of CNCs gradually decrease with the dissolution time,and the enhancement effect of CNCs gradually weakens.Therefore,for the preparation of high-strength ACNCs,not only the interface adhesion between the matrix and the reinforcement must be considered,but also the influence of the content and size of the reinforcement on the mechanical properties should be taken into consideration.(3)Epichlorohydrin(ECH)with the shortest chain length,Ethylene glycol diglycidyl ether(EGDE)with medium chain length,and polyethylene glycol diglycidyl ether with the longest chain(Poly(ethylene glycol)diglycidyl ether,PEGDE)weres used as cross-linking agents,which further enhanced the mechanical properties of ACNCs through chemical cross-linking,and the influence of cellulose cross-linking agent content and chain length on ACNCs was studied.Characterization by infrared spectroscopy proved the successful cross-linking of the cellulose film.For the three cross-linking agents,as the content of the cross-linking agent increased,the tensile strength and elongation at break of the cellulose composite film increased first and then decreased.With the increase of the molecular chain length of the crosslinking agent,the best tensile strength and elongation at break of ACNCs gradually increase,indicating that increasing the chain length of the crosslinking agent to a certain extent is beneficial to the improvement of the strength and toughness of cellulose materials,appropriately increasing the chain length of the cross-linking agent can reduce the negative effect of the cross-linking agent on the movement of the cellulose molecular segment and improve the mechanical properties of the composite material.