We study the friction behaviors of poly(vinyl alcohol) (PVA) gel sliding against glass surface in dilute poly(ethylene oxide) (PEO) aqueous solution with various molecular weights, Mw, and concentrations. At low sliding velocity (10-5, 10-4 m/s), distinct PEO polymer effects are observed: The frictional stress in PEO 2E4 (Mw = 2 × 104 g/mol) solutions is lower than that in pure water, decreasing with the increase in PEO concentration and reaching a minimum at the crossover concentration, c*. However, in higher molecular weight solution, PEO 4E6 (Mw = 4 × 106 g/mol), this friction reduction effect is only observed for very dilute concentration (0.01c* solution), and the friction stress in higher concentration (0.1c*, 0.3c*, and c* solution) is higher than that in pure water, accompanied by the appearance of "plateau". At fast sliding velocity (10-2, 10-1 m/s), all the friction curves in dilute PEO solution superpose with the curve in pure water, independent of Mw and concentration of PEO. These results indicate that in the low sliding velocity region, where adsorption of PVA gel on glass plays the dominant role in friction, PEO chain screens the adsorption of PVA chains to glass surface. In the fast sliding velocity region, PEO chain is either extensively stretched or forms a deplete layer on the glass surface by the high shear rate, so the liquid lubrication with a viscosity of η ≈ ηwater prevails. The results also support the theoretical prediction that the effective concentration of PEO in the confined 2-dimentional space between gel and glass interface is enhanced for high molecular weight PEO.
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