Polymer Molecular Weight Dependence on Lubricating Particle–Particle Interactions

Abstract

Using ultra-thin surface coatings of water-soluble polymers to modify interfacial friction is relatively new, but may offer routes to form beneficial coatings whilst 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 kDa to 1300 kDa, were examined here. Optical reflectivity measurements showed that the saturated surface excess for each PVP sample was ~1 mg/m 2. 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 kDa, 360 kDa, 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), while the high molecular weight PVPs slowly reorganize resulting in more lossy films (increased Sauerbrey film thickness). 2 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.

The rheology of polyvinylpyrrolidone-coated silica nanoparticles positioned at an air-aqueous interface

“…[22] Our previous study confirmed the stability of PVP to water rinse-off, with negligible mass removed following adsorption on silica. [49] For the current experimental conditions the adsorbed PVP was shown to be stable to the sample washing procedure outlined in Yu et al [22] and throughout the duration of the study. [36,37] One advantage of combined Langmuir trough + BAM is that the structural changes associated with compression of the particle-laden interface can be measured in-situ without the need for ex-situ analysis of a Langmuir-Blodgett film.…”

“…2), this observed independence likely results from the deposited PVP layer around the nano silica particles. In our recent study we showed that two PVP-coated surfaces (silica) produce a lubrication effect when compressed together, [49] with the lubrication effect maintained, albeit to a reduced effect, when polymers are submerged in solutions of increasing electrolyte concentration. [50,51] As such, the particleparticle contacts become mobile at a critical compressional stress, allowing the film to reorganize and densify to an almost self-similar structure before eventually collapsing (buckling or wrinkling).…”

Synergy between Composite Nanoparticles and Saponin β-Escin to Produce Long-Lasting Foams

“…18 Our previous study confirmed that the adsorbed PVP layer was stable and resistant to water rinse-off throughout the duration of the test. 18,21,40 Quartz Crystal Microbalance with Dissipation (QCM-D) Monitoring. The adsorption of saponin β-escin on silica and PVP-coated silica was studied using an E4 QCM-D from Q-Sense (Gothenburg, Sweden) at 25 ± 0.02 °C.…”

“…The sensor cleaning procedure is outlined in a previous publication. 40 All measurements began by running the background fluid until a stable baseline was attained before pumping the test solution into the sensor cell. The changes in frequency (Δf) and dissipation (ΔD) were measured simultaneously at different overtones.…”

Resonance properties of quartz crystal microbalance immersed in high solid content suspensions

“…The quartz crystal microbalance is a tool frequently used to study soft matter systems providing insight into deposited/adsorbed layer properties, 2 interactions between chemical and/or biological species, and the stability of deposited/adsorbed layers under different physicochemical conditions. [3][4] There is growing interest to study the influence of point contact loads (particles) on the QCM sensor resonance properties, with complexity now extending to probe the resonance properties in the presence of colloidal dispersions.…”