Effect Of Substrate And Medium On The Frictional Behavior Of Poly (Vinyl Alcohol) Hydrogel

We get the physically cross-linked PVA hydro-gel by the freezing-thawing method, and study the effect of factors on the PVA hydro-gel friction,such as sliding velocity(v), normal pressure(P),surface free energy(γ),concentration(c)of PEO aqueous solution and its molecular weight(M_w),by PVA hydro-gel sliding on various substrates in pure water and PEO aqueous solution.We analyze the experimental results by using the attraction-repulsion model and the scaling theory.When PVA hydrogel slides on glass surface in water,which has a little attraction with the gel,the sliding velocity(v)shows great influence on frictional stress(f).We find that at a certain velocity(v_f)the friction mechanism changes and the f-v curve shows the S-shape.In low sliding velocity region,the friction comes from the elastic force of the deformation of the adsorbed polymer chain,and f increases with the enhancement of v.In fast sliding velocity region,the friction mainly comes from the viscous flow of solvent layer.By changing the substrate,we get that the fin low sliding velocity region raise slightly with the increase of surface free energy(γ),in which highγmeans strong interaction between PVA gel and the substrate.With the increasing of v, f raise remarkably with the enhancement ofγ.At the same time,the elastic-hydrodynamic transition velocity(v_f)move toward high sliding velocity region with the increase ofγ.fincreases with P due to its intensification on the adsorption of gel with the substrate.In slow sliding velocity region,the interaction between PVA gel and substrate shows little influence on the sensitivity of f to P.The v_f of the systems will increase with the increase of normal pressure.In PEO aqueous solution,the PVA hydro-gel would lose some water due to the osmotic pressure,which changes the physical property of the gel,such as modulus and volume.Also,the PEO molecules in the solution would prevent the adsorption of gel with the substrate,which makes f decreases comparing with that in pure water. Comparing the friction behavior in pure water with in PEO c aqueous solution,we find that f is almost reduced one order in PEO c^* aqueous solution when v is low,and we can't see the obvious friction mechanism transition.In c＜c^* region,f decreases with the increase of the concentration of PEO aqueous solution in low sliding velocity.However, with the increase of sliding velocity,the effect of PEO on friction diminishes,and all the friction curves superpose with the friction curve in pure water at velocity higher than 10^-2m/s.In this case,the frictional behavior of PVA gel is also related to the viscoelastic response of the PEO coil in the confined space between PVA gel and substrates.In c＞c region,a PEO layer is easy to form even in the slow sliding region, in which the frictional behavior is determined by the viscoelastic properties of the semidilute PEO aqueous solution.