| Cellulose has been recognized as the most productive natural polymer with sustainability.Owing to the broaden resource and low cost,it exhibits features of renewability,biodegradability,hydrophily,flexibility,and so on.Compared with most other synthetic polymers,cellulose cannot be directly molded by melting process due to its particularity of strong and high-density hydrogen-bonding in the structures.It seems that the solution route is almost the only way to process and mold the non-derived cellulosic materials.Therefore,the structures and mechanical performances of the regenerated cellulose are greatly affected by the solvent system and the regeneration conditions.To improve the value of regenerated cellulosic materials,as well as to promote the industrialization process,it is necessary to investigate the relationship between the structure and performance of the regenerated cellulose materials during the coagulation process.The alkyl quaternary ammonium base-water solvent system has been developed as a new environmentally friendly solvent for cellulose,and is expected to play an important role in achieving the high-performance cellulose materials.Currently,there is a lack of research on the regeneration process of cellulose solution prepared by this solvent system,the formation mechanism of regenerated cellulose structure and the mechanism of performance regulation,which restricts the development of the direct molding theory and engineering technology of natural cellulose based on this solvent system.In this dissertation,the regenerated cellulose materials have been identified as the research object.The structural and functional regulation of the regeneration cellulose in coagulation baths with different polarities has been deeply and systematically studied.Through the combination of experimental research and simulation calculation,the influence of the polarities of the coagulation baths on the regeneration process is explored,revealing the mechanisms of assembly regulation,structure formation,and mechanical property enhancement.Further research on its functional controlling mechanism in fibers and films,provides an effective approach to the precise regulation and high-value utilization of the structure and properties of regenerated cellulose materials.(1)The effect of coagulation polarity on the assembly of cellulose molecules has been studied and the results reveal a new mechanism for the regulation of cellulose supramolecular assembly based on hydrogen bonding or van der Waals interactions.Natural cellulose@tetrabutylammonium hydroxide/urea-water solution is prepared,and the 2D lamellae of regenerated cellulose nanosheets forms in the strong-polar environment of a water bath by experimental optimization.The average area of regenerated cellulose nanosheets is increased from 5.5μm~2 to 84.6μm~2 as the assembly time increases,and the crystallinity up to91.6%can be obtained.By modulating the polarity of the coagulation bath(from high polarities water,dimethyl sulfoxide reduces to moderately and low polarities of acetone,tetrahydrofuran and cyclohexane),the prepared regenerated cellulose nanosheets exhibit a gradual increase in water contact angle,which can be changed from 11.5°,8.6°to 39.6°,40.4°and 61.3°,indicating the change of exposing surface from hydrophilic to hydrophobic.(2)The molecular interactions between the coagulation bath and the cellulose molecules,as well as the supramolecular assembly behavior of cellulose and the structural regulation of regenerated cellulose nanosheets,have been studied by Molecular Dynamics(MD)simulation.In the strong-polar coagulation baths(e.g.water,dimethyl sulfoxide),the cellulose-cellulose intermolecular hydrogen bonding has been shielded by the strong coagulator-cellulose hydrogen bonding interactions,so that cellulose is forced to assemble along the axial direction of cellulose with van der Waals interaction.In the medium-polar coagulation baths(e.g.acetone,tetrahydrofuran),intramolecular hydrogen bonding of cellulose can be destroyed by the coagulator,resulting in a flexible cellulose chain and inhibiting the stack of the anti-parallel chain of cellulose II.As a result,cellulose chains tend to twist and assemble along the equatorial direction of the cellulose.In the weak-polar coagulation baths(e.g.cyclohexane),the assembly of cellulose is dominated by the intermolecular hydrogen bonding as the solvent is unable to form hydrogen bonding interactions with the cellulose.Cellulose can assemble along the equatorial direction of cellulose within a certain angle.(3)In-situ assembly of regenerated cellulose nanosheets during the wet-spinning of cellulose,as well as the corresponding strengthening mechanism have been studied.The best mechanical performance of the regenerated cellulose fibers has been optimized by the orthogonal experiments.The results show that tensile strength of 167.9±11.6 MPa can be obtained for fibers regenerated in tetrahydrofuran with a temperature of 5~oC,stretched at a speed of 50 rpm/min,and neutralized with an aqueous solution of 10 wt%sulphuric acid.Further research shows that the formation of regenerated cellulose nanosheets structure can be facilitated by the low temperature of the coagulation bath.And with the formation of regenerated cellulose nanosheets,the crystallinity and molecular orientation of regenerated cellulose fibers can be reached up to 95.3%and 0.66 respectively.(4)The gelation behavior of cellulose solution in coagulation baths with different polarities,the formation of regenerated cellulose nanosheets,regulation of the structure and performance of gel,and the final regenerated cellulose films have been studied.Toughening mechanism of cellulose film based on regenerated cellulose nanosheets has been discussed.In the coagulation bath of cyclohexane,the gelation can hardly be achieved due to the difficulty in the double diffusion of coagulator-solvent on the phase interface between the cellulose solution and the coagulation bath.As a result,the film cannot be formed.In the coagulation bath of tetrahydrofuran,a gentle gelation process occurs,resulting in a finely gelation network structure with fewer defects.However,there is a rapid formation of large-sized regenerated cellulose nanosheets in the water bath.Through the synergistic effect of the regenerated cellulose nanosheets and gel structure,an optimum elongation at break of 16.1%±1.6%can be received.This is higher than that of films prepared in the coagulation bath of tetrahydrofuran(9.7%±0.7%).(5)The cellulose-based hydrogel spheres are constructed by the electrospinning regeneration technology,and the cellulose/polypyrrole composite hydrogel sphere adsorption material with high adsorption performance is developed by in-situ polymerization of pyrrole and its adsorption mechanism is further studied.The results show that the composite hydrogel spheres have long-term structural and functional stability.The maximum adsorption capacity of the hybrid spheres(520.2 mg/g)is 12 times greater than that of the unmodified hydrogel sphere.Combining experiments and theoretical calculations,the mechanism of the efficient adsorption of EY can be attributed to the multiple interactions(hydrogen bonds,π-πstacking,and electrostatic interactions)of the composite hydrogel sphere with EY molecules.In addition,the composite hydrogel sphere exhibits good cyclic adsorption stability,as the recycling experiments show that about 75.0%of the initial adsorption capacity of the composite hydrogel sphere could be maintained after six adsorption-desorption cycles. |
PDF Full Text Request