Preparation And Characterization Of Chitosan-based Hydrogels Reinforced By Cellulose Nanowhiskers Through Schiff Base Reaction

Cellulose,the most abundant renewable nature polymer produced in the biosphere,is widely distributed in higher plants.Cellulose plays an essential role in maintaining the structure of plants cell walls.Cellulose nanowhiskers?CNWs?is nanometer-sized single crystals of cellulose extracted from nature cellulose or marine animals.In recent years,CNWs have garnered more and more attention due to their exceptional mechanical strength,high aspect ratio and large specific surface area.CNWs have become a hot research as reinforcing agents in bio-based nanocomposite.In this work,oxidized cellulose nanowhiskers?OCNs?were used as nano-reinforced phase and potential cross-linking agent,chitosan?CS?was used as polymer matrix to build chemically crosslinked hydrogel and semi-interpenetrating network hydrogel,provides a new method to improve mechanical properties of chitosan hydrogel.The synthesis mechanism and physicochemical properties of hydrogels has been studied.The mechanical strength of hydrogel has also been discussed.CNWs were first obtained by sulfuric acid hydrolysis of bamboo pulp cellulose and then oxidized by sodium periodate with three different oxidation level?OCN1,OCN2 and OCN3?.OCNs and CNWs were characterized using SEM,laser nanometer measurement,FT-IR,TGA,elemental analysis and XRD.FT-IR analysis found the characteristic C=O stretching vibration band of the carbonyl group appeared at 1798 cm^-1 and the peak increased from a small shoulder to a distinct peak with increasing oxidation level.Besides that,hydroxyl peaks at 3400 cm^-1 are also appropriately narrow.The spectral data confirmed CNWs was successfully oxidized and the number of aldehyde groups was confirmed by elemental analysis.The increase in sodium periodate for oxidative treatment of CNWs resulted in a corresponding increase in aldehyde content of OCNs,from 0.11 to 2.76 mmol/g.XRD found the internal structure of the OCNs were not destroyed by sodium periodate.The crystallinity of OCNs was reduced,from 76.51%to71.79%;TGA found thermal stability of OCNs were improved.The oxidation of CNWs by sodium periodate can form aldehyde groups,which can undergo crosslinking reactions and further modification to extend the range of applications of CNWs.Chemically cross-linked hydrogel?CS/OCNs?were obtained through covalent linkage between OCNs and CS.A series of experiments were performed to evaluate the effect of oxidation level of CNWs on the swelling behavior,morphology,viscoelastic properties and mechanical properties of the cross-linked hydrogels.Chemical cross-links,hydrogen bonds as well as nanofiller reinforcement between the two materials played a major role in determining the properties of the hydrogels.The covalent linkage between OCNs and CS was confirmed by FT-IR spectroscopy.Swelling test results showed that the incorporation of OCNs decreased the water absorbability of the hydrogels due to the increase in cross-linking degree.Viscoelastic properties of the CS/OCN3 hydrogel was increased by 535%in storage modulus compared to CS hydrogel,from 5.02 kPa to 26.9 kPa.Compressive strength of 42.3 kPa at 50%strain was observed from the cross-linked hydrogels,compared with 4.95 kPa of the pure chitosan hydrogel.A semi-interpenetrating network chitosan based nanocomposite hydrogel?CS/PVP/CNWs?with superior mechanical properties were prepared using CS,OCNs and polyvinyl pyrrolidone?PVP?.An increased chemical cross-linking degree?6⁴0%?of hydrogels was found with the increased oxidation level of CNWs?aldehyde contents,0.11².76 mmol/g?.The effects of the oxidation level of CNWs on the properties and structures of the hydrogels were studied.The morphology results showed that the reaction with OCNs increased the hydrogels network density and the porous structure showed more compact and regular in shape.Besides,the change in the morphology properties of hydrogels has an influence on hydrogels swelling behavior,exhibiting relatively reduced water content and swelling ratio.The rheological measurements exhibited 263%and 394%improvements in storage modulus and loss modulus,respectively,and the compressive strength confirmed an increasement by about 340%,were observed from the semi-interpenetrating network chitosan hydrogels,compared with physically cross-linked hydrogels.Moreover,the thermal experiments showed that the chemical cross-linking slightly increase the resistance toward thermal degradation of the hydrogels.