Structure And Property Adjustment And Control Of Double Network Hydrogel Based On Polymer Network Chemical Modification

Hydrogels are three-dimensional polymer network materials with high water content.In order to be used as structural material in tissue engineering and other fields,the mechanical properties of hydrogels are important.Double network(DN)hydrogels are one of the biocompatible hydrogels with high strength and toughness,having attracted great attention in recent years.This paper focus on poly(2-acrylamide-2-methylpropane sulfonic acid)(PAMPS)/poly(acrylamide)(PAM)DN hydrogel to modify the chemical structures of the two networks,including introducing reversible addition-fragmentation transfer(RAFT)polymerization to increase the homogeneity of chain length in DN hydrogel networks;copolymerizing cationic monomer and acrylamide(AM)as the second network with multiple ionic interation formed between networks;and using polyampholyte hydrogel as the first network of DN hydrogel,whereby the structures and properties of DN hydrogels could be adjusted and controlled and a series of modified DN hydrogels with enhanced mechanical properties are prepared.Synchrotron radiation small-angle X-ray scattering(SAXS)has been used to study the microstructure changes of the modified single network(SN)and DN hydrogels,and explain the structure and property adjustment and control mechanisms of modified DN hydrogels by chemical structure modification.The first-network-modified DN hydrogels were synthesized by introducing RAFT agents into PAMPS networks.The effects of the RAFT agent amount and the functional chain segments on the swelling and mechanical properties of DN hydrogels were studied.The modified DN hydrogel has an enhanced toughness and its fracture energy reaches 3.3 MJ·m-3.With the addition of PEG and PAM segments into the first network,the toughness of DN hydrogel was further increased.According to SAXS analysis,larger ordered cross-linked domains were formed in the SN hydrogel after the incorporation of a RAFT agent,offering plenty of space for PAM network to entangle and interpenetrate.The interactions between the two networks were enhanced,which helps dissipating concentrated stress more efficiently.By changing the amounts of chemical cross-linking agent and initiator adding into the second network,the influences of the molecular weights of PAM chains on the structure and properties of PAMPS/PAM hydrogels were studied.With no chemical cross-linking agent,the mechanical properties of synthesized semi-interpenetrating polymer networks hydrogels were improved with decreasing the amount of initiator.Based on this result,RAFT agent was added into the PAM network.With similar molecular weight of PAM,the modified semi-interpenetrating polymer networks hydrogels showed better mechanical properties,indicating that increasing the homogeneity of the PAM chain length is beneficial for improving DN hydrogels’mechnical properties.Because interaction between networks has been proven to be important for mechanical property enhancement of DN hydrogels,ionic bonds were introduced.A co-polymer of a cationic monomer and AM was used as the second network of the modified DN hydrogel.The properties of hydrogel were determined by the amount of cationic monomer.Near the ion-balance point,the hybrid DN hydrogels showed the lowest swelling ratio and prominent phase separation,while the tensile strength was highly enhanced to about 1.3 MPa.According to SAXS analysis,larger compact cross-linked domains were found in the hybrid DN hydrogels,especially near the ion-balance point.As the chemical structure differences of cationic monomers,DN hydrogels had different mechanical properties.P(NaSS-co-DAC)/PAM DN hydrogels were synthesized using an polyampholyte hydrogel copolymerized by sodium p-styrenesulfonate(NaSS)and acryloyloxyethyl trimethylammonium chloride(DAC)as the first network.With the deviation from the ion balance in the first network,P(NaSS-co-DAC)/PAM hydrogels were transformed from SN hydrogel character to DN hydrogel character,and the tensile modulus was decreasing.However,the tensile strength was first declined and then largely enhanced with the structure transformation,and finally went down as ionic complex strength decreased.According to SAXS analysis,tough and strong DN hydrogels were achieved when the sizes of ionic complexes and mobilizable chain segments were about the same.