| Cellulose nanocrystals(CNCs),a natural biomass polymer material extracted from cellulose,with the advantages of green and renewable,biocompatible and biodegradable,have great potential for application in green chemicals,food,medicine,new materials and other fields,and are considered as an ideal functional and reinforcing material.Unfortunately,due to the rich hydroxyl groups on the surface of CNCs and the small number of functional groups on the surface,they are easy to be regenerated by hydrogen bonding after drying.Hydrophilic CNCs prepared by one-step sulfuric acid method and ammonium persulfate method at the present stage can introduce a small amount of hydrophilic ionization groups on their surfaces,but the hydrophilic groups are generally lower than 2 mmol/g,which makes it has been facing the problems of difficulty in redispersion and poor nano-functionalization effect,which seriously hinders the high value and industrial application of CNCs.Therefore,in order to solve the above problems,we proposed to use ceric ammonium nitrate and sodium citrate to synergistically initiate the free radical polymerization of acrylic acid monomers in CNCs,and successfully prepared CNCs grafted sodium polyacrylate(SCNCPA)powders in an efficient and controlled manner.The high surface carboxylation(11.5 mmol/g)and polyelectrolyte ligand effect of SCNCPA powder greatly improved its hydrophilic ability and redispersibility(100%),further explored the application potential of SCNCPA in the field of moisture collection and nanocomposite hydrogels,and provided an important research basis for the high-value and functional utilization of CNCs.The main research contents are as follows:(1)Based on the cerium ion-initiated free radical polymerization system,the current challenge of lower grafting rate(20%)based on cerium ion-initiated grafting polymerization of CNCs with PAA was broken through the stabilization effect of cerium ions by the addition of sodium citrate.The effects of the ratio of SCNC to AA,the amount of cerium ammonium nitrate and the amount of sodium citrate on the graft polymerization reaction were systematically investigated by controlling variables.The results showed that the highest grafting rate(204%)and monomer conversion(87%)were achieved when the amount of ceric ammonium nitrate was 1.32%of the monomer dosage and the amount of sodium citrate was 60% of the ceric ammonium nitrate dosage.The stability,hydrophobicity and redispersity of SCNCPA were evaluated,and the carboxyl group content(11.5 mmol/g)on the surface of SCNCPA was calculated.Finally,the changes in the structure and morphology of grafted SCNCPA were analyzed by means of chemical structure and microscopic morphology characterization,the mechanism of stabilization of cerium ions by sodium citrate was explored,and the generalizability of the method to CNCs with different surface functional groups was evaluated.(2)A new moisture collection material SCNCPA based-EF was prepared based on the composite of SCNCPA with highly carboxylated surface and elastic fabric,which solves the problem that the current moisture collection material is highly dependent on the moisture-absorbing inorganic salt loading(>80%).The moisture collection capacity and cycling stability of SCNCPA based-EF were evaluated using a dynamic water adsorption system.The results show that SCNCPA based-EF has excellent moisture collection capacity(1.07 g/g,30% humidity,180 min)compared to representative work in the field of moisture collection,and can achieve a daily water yield of 8.4 L/kg working under simulated low humidity(30%)arid region conditions.by analyzing the liquid rheological characteristics and DSC The mechanism of rapid water uptake and desorption of SCNCPA based-EF was discussed by analyzing the liquid rheological characteristics and the evaporation enthalpy of water.Finally,the application potential of SCNCPA based-EF in the field of air dehumidification and fruit preservation was evaluated.(3)Based on the highly carboxylated surface,SCNCPA is compounded with polyacrylic acid hydrogel to improve its interaction with the hydrogel matrix,breaking the contradictory limitation of strength and toughness when CNC reinforced hydrogel.The effects of SCNC and SCNCPA as well as the addition amount of SCNCPA on the mechanical properties of PAA/SCNCPA-H composite hydrogels were investigated,and the results showed that compared with SCNC,the abundant carboxyl groups and distributed PAA molecular chains on the surface of SCNCPA were more effective in enhancing the interaction force between SCNCPA and PAA gel matrix,including intensive strong hydrogen bonding and entanglement,thus improving its mechanical properties more significantly.Thus,the mechanical properties(strength from 0.43 MPa-1.45 MPa,modulus from 20 k Pa-184 k Pa,fracture toughness from 225.4k J m-2-1120 k J m-2)were more significantly improved.The effects of the ratio of free water and bound water inside the hydrogel on the mechanical properties and freezing resistance of the gel were analyzed by DSC and DMA,resulting in PAA-DH hydrogels with excellent mechanical properties and freezing resistance(-30°C,10-fold tensile retention continuously).Finally,the electrical conductivity and strain sensing properties of the PAA/SCNCPA-H composite hydrogel were evaluated,providing a new idea for the design of next-generation flexible sensors. |
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