Rheological Behavior And Boundary Lubricating Properties Of Hydrophobic Modified Polyacrylamide And Sodium Polyacrylate Solution

Excellent lubrication performance in vivo has aroused extensive attention that many researches have been focused on explaining biolubrication mechanism and designing ultralow friction surfaces. Biolubricants mostly are highly associative and demonstrate complicated rheological behaviors. To illuminate the relationship between hydrophobic association, rheological behavior and lubricating properties, hydrophobically modified polyacrylamide (HMPAM) and sodium polyacrylate solutions were studied.HMPAM was synthesized by free-radical micellar copolymerization using butyl methacrylate as hydrophobe. The basic properties, such as molecular weight, hydrophobe content, hydrophobic association and adsorption investigated via nuclear magnetic resonance spectroscopy (NMR), static light scattering (SLS), fluorescence spectroscopy, quartz crystal microbalance (QCM), etc. The rheological behaviors of HMPAM and sodium polyacrylate (PAA-Na) solutions were determined by both steady state flow tests and oscillation frequency sweep tests. At the same time, the lubricating properties of the polymer solutions were studied between various substrates. The main results are as follows:(1) Intermolecular hydrophobic association prevailed in HMPAM aqueous solution at high concentrations. Addition of surfactant hexadecyl trimethyl ammonium bromide (CTAB) evidently disrupted the interchain association that steady state viscosity and dynamic modulus decreased significantly.(2) In compliant PDMS-PDMS contact, associative HMPAM solutions behaved as boundary lubricant at high concentration, but the unmodified polyacrylamide solution showed no boundary lubricating properties.(3) Adsorption of HMPAM solution on hydrophobic gold surface could be well fitted by a Langmuir model; adsorption mass gradually reached equilibrium as concentration increase, while friction coefficient (μ) in boundary lubrication region decreased as μ～c-1.2. The boundary lubricating properties of HMPAM solutions improved as hydrophobic association increase, and barely remain when hydrophobic association was destroyed by CTAB. Results revealed that interchain association, rather than the robust adsorption layer, plays a significant role in boundary lubrication.(4) Because of polyelectrolyte effect, PAA-Na exhibited more extent conformation in aqueous solution and high storage modulus. PAA-Na in 1M salt solution showed no boundary lubricating properties, but PAA-Na aqueous solution exhibited boundary lubrication in various compliant contact.