Study On Fully Chemically Cross-linked Double Network Hydrogel Based On Polystyrene

Hydrogel,as an excellent soft material,is widely used in the fields of biocompatible sensors,soft robots,skin care materials and so on.The network structure inside the hydrogel can hold a large amount of water,which improves the biocompatibility of the hydrogel and reduces the mechanical properties of the hydrogel.Among them,hydrogels crosslinked by covalent bonds show the characteristics of fragility and less energy consumption.Researchers have proposed many design methods to improve the properties of hydrogels,such as introducing energy dissipation units,improving cross-linking density and uniformity.In this paper,carboxyl-modified polystyrene and amino-modified polyethylene glycol are covalently crosslinked as energy dissipation units,and then form a double-network hydrogel with polyacrylamide network to improve the crosslinking density and realize the high mechanical properties and rapid recovery of covalently crosslinked hydrogel.As a typical hydrophobic polymer,polystyrene(PS)can be stretched from an irregularly curled state to a chain state under the action of external force.When the external force is removed,the molecular chain can be restored to an irregularly curled state under the action of entropy.Single molecular force spectroscopy(SMFS)based on atomic force microscope(AFM)can characterize the hydrophobic force of PS chain,which provides feasibility for the experimental scheme.In this paper,PS model based on SMFS technology is introduced into the cross-linking network of hydrogel to study the mechanical properties of PS hydrogel.In free radical polymerization,the reversible addition-fragmentation chain transfer(RAFT)polymerization method can make the polymer chain growth and the addition of dithioester compounds at the same time.The target product,carboxyl-functionalized polystyrene(CPS),can be obtained by RAFT polymerization.The results of 1H NMR show that the synthesized CPS has carboxyl functional groups at both ends.Subsequently,CPS molecules are modified on the substrate,and under the action of hydrophobic driving force,it will collapse and condense into dense nanospheres.Using AFM,CPS nanospheres can be unfolded,and then the SMFS curve of CPS can be obtained.It shows that CPS nanospheres can be stretched from spherical to long-chain shape under the action of external force,and CPS will return from longchain shape to spherical shape after the external force is removed.This process shows that CPS can be deformed under the action of external force,and it can be restored when the external force is removed.The covalent cross-linking network prepared by CPS and 4-armed-PEG-NH2 has a compressive strength of 0.952 MPa and a compressive deformation of 74.1%.With the increase or decrease of CPS concentration,the compressive strength of CPS covalent cross-linking network will decrease,but the compressive deformation will increase.Through the quantitative analysis of covalent cross-linking network with 35 mM CPS,it can be found that both the change of CPS concentration and the change of 4-armed-PEG-NH2 concentration will have an impact on the mechanical properties of covalent cross-linking network,and its mechanical properties will increase with the approach of the critical value,and then decrease.Due to the entanglement or knotting of CPS polymer chains,obvious hysteresis and energy dissipation can be found when the covalent cross-linking network is compressed.However,this hysteresis phenomenon can be solved by giving a certain recovery time.Through the tensile test of the covalent cross-linking network,we found that the compressibility(74.1%)of the covalent cross-linking network was higher than the extensibility(282.8%).This is related to the irreversible breakage of covalent bonds formed by carboxyl groups and amino groups.The second layer of polyacrylamide(PAAm)network was introduced on the basis of CPS covalent cross-linking network to obtain the target product PS/PAAm double-network(DN)hydrogel,which can effectively improve the extensibility of CPS covalent cross-linking network(822.7%).The mechanical properties of PS/PAAm DN hydrogel were characterized.The introduction of PAAm cross-linked network not only increased the extensibility of CPS covalently cross-linked network,but also reduced the energy dissipation from 28.887 kJ m-3 of CPS covalently cross-linked network to 19.136 kJ m-3 of PS/PAAm DN hydrogel,and increased the recovery from 93%of CPS covalently cross-linked network to 94.5%of PS/PAAm DN hydrogel.The change of the concentration of acrylamide(AM)monomer and cross-linking agent N,N’-methylene bis(acrylamide)(MBAA)in PAAm cross-linking network will also affect the mechanical properties of PS/PAAm DN hydrogel.The mechanical properties of PS/PAAm DN hydrogel composed of 5 mol/L(M)AM and 1 mol%(relative to AM)MBAA reached the best(tensile strength of 0.443 MPa,fracture toughness of 2510.891 kJ m-3 and fracture energy of 0.62 kJ m-2).The scanning electron microscope(SEM)shows that PS/PAAm DN hydrogel has a three-dimensional network structure,which can hold more water(81.6%84.4%).In addition,different stretching speeds will also affect the mechanical properties of PS/PAAm DN hydrogel.Finally,the DN hydrogel prepared by chemical cross-linking shows good mechanical properties and the maximum stress recovery of 94.5%after 20 consecutive tensile-relaxation tests.Our experimental ideas and methods can provide reference for the application of other hydrophobic materials in the field of hydrogels,and also provide new insights for the design of hydrogels.