AbstractCellulose is the most abundant natural polymer on earth, which has obtained increasing interest in the field of functional materials development for its renewable, high mechanical performance and environmental benign. In this study, the traditional processing method (wet spinning and film production) of cellulose-based materials was applied by using cellulose solution for 3D printing, which can directly build complex 3D patterns. Herein, a natural cellulose is dissolved in an effective mixed aqueous solution of dimethyl sulfoxide (DMSO) and tetrabutylammonium hydroxide (TBAH). The cellulose solution extrusion was controlled by a modified fused deposition modeling (FDM) 3D printer. During the controlled extrusion 3D printing process, the viscous cellulose solution will gelifies and further solidifies into a predetermined 3D pattern at room temperature in air. Subsequently, a cellulose hydrogel skeleton was obtained, when the 3D pattern was solvent-exchanged with deionized water. Finally, the mechanical and swelling performance of the cellulose hydrogel scaffold was improved by a cross-linking agent treatment method. With treatment of the 3D printed scaffolds in 0.8 wt% cross-linking agent solution, the obtained cellulose hydrogel could absorb 28 g/g water, and the compression strength was 96 kPa. This work provided an efficient way to prepare natural cellulose hydrogel by 3D printing under room temperature.
The determination of molecular weight of natural cellulose remains a challenge nowadays, due to the difficulty in dissolving cellulose. In this work, tetra-n-butylammonium hydroxide (TBAH) and dimethyl sulfoxide (DMSO) aqueous solution (THDS) were used to dissolve cellulose in a few minutes under room temperature into true molecular solutions. That is to say, the cellulose was dissolved in the solution in molecular level, and the viscosity of the solution is linearly dependent on the concentration of cellulose. The relationship between the molecular weight of cellulose and the intrinsic viscosity tested in such dilute solutions has been established in the form of the Mark–Houwink equation, [η]=0.24×DP1.21. The value of 1.21 indicates that the cellulose molecules dissolve in THDS quite well. The cellulose dispersion in the THDS was proved to be in molecular level by atomic force microscope (AFM) and dynamic light scattering (DLS). The reliability of the established Mark–Houwink equation was cross-checked by the gel permeation chromatography (GPC) and traditional copper (II) ethylenediamine (CED) method. No considerate degradation was observed by comparing the intrinsic viscosity and the degree of polymerization (DP) values of the original with and the regenerated cellulose samples. The natural cellulose can be molecularly dispersed in the multiple-component solvent (THDS), and kept stable for a certain period. A time efficient and reliable method has been supplied for determination of the degree of polymerization and the molecular weight of cellulose.
Thermal management materials are obtaining increasing research interest, due to the requirement on energy conservation and environment protection. However, the complex designs and energy-consuming manufacturing processes prohibit their wide spread practical account. 3D printing is an intriguing revolutionary technology in fabricating anisotropic thermal conductive materials because of its inherent virtues on directional additive manufacturing a complicated subject with designed microstructure. We demonstrate the coaxial 3D printing along with directional freezing processes to obtain anisotropic thermal conductive composite aerogel consisting of carbon nanotubes (CNTs) and cellulose nanofibers (CNFs). The as prepared composite aerogel, with the thermal conductive CNTs as inner layer, and the insulate CNFs as outer layer, presented remarkable anisotropic thermal conductivity with 0.025 W/(m·K) in the axial direction and 0.302 W/(m·K) in the radial direction. The Young`s modulus of the CNTs/CNFs composite aerogel was tested to be 10.91 MPa in the axial direction, and 2.62 MPa in the radial direction, respectively. The coaxial 3D printed CNTs/CNFs composite aerogel has great potential application in electronics, especially for those custom-tailored products and the related field.
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