Design And Property Studies Of Tough Hydrogel

In this paper,hierarchical tough hydrogel was synthesized by design of the main chain structure or controlling physical/chemical crosslinking,including long chain branching double network hydrogel(LCB DN Gel),double network hydrogel with chemically and physically dual-crosslinked second network(D-DN Gel)and DP hydrogel crosslinked by hydrogen bond and hydrophobic association(dual physically crosslinked hydrogel,DP Gel).Firstly,2-acrylamido-2-methylpropane sulfonic acid(AMPS)was copolymerized with 2-hydroxy-4-(methacryloyloxy)benzophenone(HMBP)to sythesize the first network.Polyacrylamide was branched to the first network via photo induced initiating on basis of HMBP.P(AMPS-HMBP)/PAAm was prepared.FTIR indicated that PAAm was successfully branched to the first network and mechanical properties were enhanced effectively by long chain branching structure,compared to traditional double network hydrogel.When CAAm was 8 mol·L-1,tensile fracture stress and compressive modulus of LCB-8M were 950 kPa and 783 KPa,which 4~10 times as large as the corresponding ones of LCB-2M.Moreover,the tensile fracture strain was remaining at high level of 3000%.LCB DN Gel possessed ultra-long fracture tensile strain.In tensile or compressive loading-unloading tests,dissipated energy was strengthened dramatically by entanglements between branching and main chains or branching chains.LCB DN Gel exhibited Mullins effect,as a result dissipated energy couldn't recover in successive loading-unloading cycles.SEM results indicated embedded micro-network in large holes were induced by LCB.Secondly,D-DN Gel was designed on basis of chemically crosslinked PAMPS as the first network and AAm copolymerized with AAc as the second network.In detail,D-DN Gel was prepared by immersing PAMPS/PAAm-AAc in 0.06 mol·L-1 FeCl3 solusion for 12 h to form riversible crosslink between –COO-and Fe3+.Owning to induce of ferric coordination interactions,mechanical properties of D-DN Gel was much higher than these of double network hydrogel.When CAAc/CAAm was 20%,tensile fracture stress,compressive stress and dissipated energy of D-DN-20% were 6.1 MPa,37.3 MPa and 13.8 MJ·m-3,which were 2~12 times of correspond ones of PAMPS/PAAm-AAc and PAAm-AAc-Fe3+.Tensile loading-unloading tests showed that fatigue resistance was improved by inducing riversible ferric coordination.Dissipated energy could recover to 69.0% of initial loading-unloading cycle with resting in 60? for 15 min.SEM indicated that the pore size of embedded micro-network in large holes increased as AAc added.At last,P(NVP-AAc-SMA)terpolymer was random-copolymerized of N-vinyl-2-pyrrolidone(NVP),acrylic acid(AAc)and stearyl methacrylate(SMA).Then P(NVP-AAcSMA)was fully swollen in water,and dual physically crosslinked hydrogel(DP Gel)was achieved by hydrogen bond and hydrophobic association.When introducing hydrophobic association into hydrogel,the Young's modulus and tensile stress were increased by 1~3 order and 1~2 order respectively compared to hydrogen bond crosslinked P(NVP-AAc)Gel.When loading-unloading tensile strain at 300%,dissipated energy of DP Gel could recover above 95% of initial one,after dealing with 70? for 3 min.Besides the effect of pH on the mechanical properties was also studied.UV-vis spectra showed the hydrogel experienced from opaque to transparent when the concentration of SMA increased.SEM showed the holes of hydrogel became smaller and density of holes increased as SMA added.