Polymer Molecular Weight Dependence on Lubricating Particle-Particle Interactions

Using ultrathin surface coatings of water-soluble polymers to modify interfacial friction is relatively new, but may offer routes to form beneficial coatings while using significantly lower polymer concentrations. In the current study, silica surfaces were modified by the physisorption of poly(vinylpyrrolidone) (PVP) from water solution. Four polymer samples with different molecular weights, ranging from 8 to 1300 kDa, were examined here. Optical reflectivity measurements showed that the saturated surface excess for each PVP sample was ∼1 mg/m2. The amount of trapped water within the 8 kDa PVP film (∼10 wt %) was found to be much less than the trapped water (40–55 wt %) in films formed from higher molecular weight PVPs (40, 360, and 1300 kDa). In addition, QCM dissipation values for the 8 kDa PVP film was more than four times smaller than those measured for the higher molecular weight PVPs, suggesting that the 8 kDa PVP conforms to a flat film (predominantly train orientation), whereas the high molecular weight PVPs slowly reorganize resulting in more lossy films (increased Sauerbrey film thickness). Colloid-probe AFM lateral force measurements showed that 8 kDa PVP films exhibited similar lateral resistance to that seen for uncoated silica surfaces in water, whereas higher molecular weight PVP films showed significantly reduced lateral forces. This lubrication effect, induced by the adsorbed higher molecular weight PVP samples was explored further by measuring the rheology of concentrated particle suspensions. Suspension yield stress data for PVP-coated particles showed a reduction by a factor of 2 in the yield stress when compared to the uncoated particles for suspension concentrations above 60 vol %, i.e., approaching the close-packed limit of spheres.