Surface Thiolation Modification Of Nanocrystalline Cellulose And Its Modified Hydrogel Materials

Hydrogels are a spectacular form of soft matter with water-swollen,cross-linked polymeric structures that respond to external perturbations.Many synthetic hydrogels are known to be fragile and brittle,limiting their biological applications,thus,it is necessary to strengthen and toughen the hydrogels.Nanocrystalline cellulose(NCC)are attractive as reinforcing nano-fillers due to their abundance,renewability,low price,biocompatibility,low weight,and ultrahigh tensile strength and high modulus.However,due to the large amount of hydrophilic hydroxyl groups on NCC surface,NCC have the tendency to self-aggregate and possess bad compatibility with hydrophobic polymeric matrix,reducing its enhancement effect.Taking advantage of surface modification to regulate and control the physiochemical properties of NCC,reducing the tendency of self-aggregation,and improving the dispersion and compatibility in apolar polymeric matrix.The disulfide bonds originating from the thiol groups oxidation are a form of dynamic covalent bonds.Introducing the thiol groups into NCC surface,then forming the NCC network by oxidation between thiol groups.Based on above description,the primary idea of this paper is to conduct thiolation modification on NCC surface,and use modified NCC as the reinforcing fillers to compound with hydrogels to prepare double network hydrogels under certain oxidizing conditions.The main contents of this paper are as follows:(1)Unmodified NCC with different concentration gradients are combined with poly(hydroxyethyl methacrylate)(PHEMA)hydrogels to obtain nanocomposite hydrogels,and their mechanical properties are characterized;(2)The thiol groups are introduced onto the surface of the NCC by the different modification methods,and the morphology,crystallinity and the content of the thiol groups introduced on the surface of the NCC are characterized.(3)The NCC are subjected to Schiff base reaction to introduce thiol groups,and then modified NCC with different concentrations gradients are mixed with PHEMA hydrogels to prepare double-network hydrogels,and their mechanical properties are characterized.The main achievements of this paper include:(1)Enhanced PHEMA hydrogels with unmodified NCC.The results show that the tensile strength and elongation at break of NCC/PHEMA nanocomposite hydrogels are higher than 114.3 % and 85.5 %,respectively,in comparison with those of pure PHEMA hydrogels when the mass concentration of NCC is 1 %.At the same time,NCC/PHEMA nanocomposite hydrogels show good elastic behavior under low tensile rate.With the increase of tensile rate,NCC/PHEMA nanocomposite hydrogels show lag phenomenon,hysteresis loop area gradually increases,fulfilling the transformation from the elasticity to viscoelasticity.(2)Four modification methods are successfully utilized to prepare thiol modified NCC(NCCm),among them,the experimental conditions of Schiff base reaction are mild,the morphology and crystallinity of the products can be better maintained,and the thiol groups content on NCC surface is 0.348 mmol/g,these merits make it to the relative optimum thiolation modification method.(3)The double-network hydrogels with improved mechanical properties are obtained by the combination of thiolated NCCm modified by Schiff base reaction with PHEMA hydrogels.The results of tensile tests show that the tensile strength and elongation at break of NCCm/PHEMA double-network hydrogel increased by278.1 % and 216.7 %,respectively,comparing with those of pure PHEMA hydrogels when the mass concentration of NCCm is 2 %.In this paper,the high value utilization of renewable NCC and its modified hydrogel materials as the goals,enriching the surface chemical modification methods of NCC,at the same time,making use of the excellent characteristics of high strength,high modulus of NCC,the modified hydrogel materials with excellent mechanical properties are prepared by combination NCC with hydrogel materials.