Research On The Preparation And Strengthening Mechanism Of Silica Nanocomposite Hydrogels

Hydrogels are soft materials composed of water and polymer networks,which have important application prospects in mechanical devices,smart materials,drug delivery,tissue engineering,superabsorbent,and other fields.The conventional polyacrylamide-based hydrogels have low strength and toughness,which limit their application in many fields.Therefore,various strengthening mechanisms have been developed to prepare high strength hydrogels.The mechanisms maintain high strength of hydrogels are mainly based on increasing the crosslinking density of polymer networks,including interpenetrating double networks(DN)structure,nanohybrid crosslinkers,hydrophobic association,and combining multiple strengthening mechanisms.In this paper,several novel nanocomposite high strength hydrogels were designed and developed by using nano-silica or chemically modified nano-silica as nanohybrid crosslinkers,combined with hydrophobic association,DN structure,and other strengthening mechanisms.The synergistic effects of silica nanohybrid crosslinkers and various strengthening mechanisms in hydrogels were investigated in detail.The main works of were as follows:The P(HPAM/HMAM/SiO2)hydrogel was prepared by adding hydrophilic nano-silica with the mass fraction of 0.1 wt%～1.0 wt%and the average particle size of 50 nm into the conventional acrylamide-based hydrogel system.The stability time of hydrogel at 45℃ was extended from 30 days to 90 days and the dehydration rate decreased by 28%on the 150th day.According to the sydansk strength code,the maximum hydrogel strength was increased from code "F" to codes "G" and "H".The strengthening mechanism was that the hydrogen bond was formed between the silanol group on the surface of nano-silica and water molecules and the amide group on the polymer chain,which increased the crosslinking density and the proportion of bound water in the hydrogel,effectively improving the strength and stability of hydrogel.MD simulation result showed that nano-silica reduced the total potential energy of the hydrogel system,enhanced the intermolecular interaction in the system,making the hydrogel system more stable.In order to prevent the agglomeration of nano-silica,hydrophobic modified nano-silica(M-SiO2)were prepared by "silane coupling method".Using M-SiO2 as the nanohybrid crosslinker,P(NHAM/AA/AMPS/M-SiO2)hydrogel was prepared by free radical copolymerization with monomers of NHAM,AA and AMPS.The maximum compressive stress of the hydrogel was 2.15 times and 1.51 times of P(NHAM/AA/AMPS)hydrogel and P(NHAM/AA/AMPS/SiO2)hydrogel,respectively.In the tensile test,the tensile fracture length of the hydrogel was 24.5 cm,which was 5.4 times of the initial length.The microscopic analysis showed that M-SiO2 had good dispersion in the hydrogel,and enhanced the crosslink density of the hydrogel,making hydrogel had a more compact the 3D network structure and excellent mechanical strength due to its hydrophobic association.The vinyl modified nano-silica(TM-SiO2)were prepared as a nanohybrid crosslinker with triethoxyvinylsilane as modifier.The TM-SiO2/PAM/PAA DN hydrogel was prepared by two-step free radical polymerization.When the content of TM-SiO2 increased from 0.2 wt%to 0.8 wt%,the tensile fracture stress of hydrogel increased from 0.26 MPa to 0.43 MPa and the compressive stress increased from 4.93 MPa to 7.01 MPa at 70%strain.The strengthening mechanism was that the crosslinking between PAM network and PAA network increased the crosslinking density of hydrogel.TM-SiO2 could form stronger chemical bonds with the polymer matrix,giving the hydrogel a stronger network structure and excellent mechanical strength.MD simulation results showed that the DN structure of PAA and PAM networks,and TM-SiO2 enhanced the interaction between molecules in hydrogel system,which endowed the network structure more stable.Further,TM-SiO2 as crosslinker and emulsifier,which were copolymerized with AM,AMPS and sodium alginate(SA)by microemulsion polymerization to prepare P(AM/AMPS/SA/TM-SiO2)gel microspheres.Compared with P(AM/AMPS/SA)gel microspheres,TM-SiO2 significantly improved the temperature resistance,salt resistance,acid and alkaline resistance of the microspheres.At the same shear time,the higher the content of TM-SiO2,the more complete the morphology of the microspheres.The microspheres were used as adsorbent,which could selectively remove cationic dyes from dye wastewater with a removal rate of up to 97.85%.In the cyclic adsorption experiment,the microspheres remained intact in shape and high adsorption capacity after five adsorption-desorption cycles.The results showed that TM-SiO2 increased the mechanical strength of the microspheres and improved their recyclability.In a word,nano-silica,hydrophobic modified nano-silica and polymerizable alkenyl-modified nano-silica were introduced into the hydrogel matrix through hydrogen bonding,hydrophobic association and chemical bond respectively as nanohybrid crosslinker,and combined with other strengthening mechanisms could further improved the strength and performance of the hydrogel.