Suzanne Giasson

Lubrication by charged polymers

Long-ranged forces between surfaces in a liquid control effects from colloid stability to biolubrication, and can be modified either by steric factors due to flexible polymers, or by surface charge effects. In particular, neutral polymer ‘brushes’ may lead to a massive reduction in sliding friction between the surfaces to which they are attached, whereas hydrated ions can act as extremely efficient lubricants between sliding charged surfaces. Here we show that brushes of charged polymers (polyelectrolytes) attached to surfaces rubbing across an aqueous medium result in superior lubrication compared to other polymeric surfactants. Effective friction coefficients with polyelectrolyte brushes in water are lower than about 0.0006–0.001 even at low sliding velocities and at pressures of up to several atmospheres (typical of those in living systems). We attribute this to the exceptional resistance to mutual interpenetration displayed by the compressed, counterion-swollen brushes, together with the fluidity of the hydration layers surrounding the charged, rubbing polymer segments. Our findings may have implications for biolubrication effects, which are important in the design of lubricated surfaces in artificial implants, and in understanding frictional processes in biological systems.

Effect of mechanical properties of hydrogel nanoparticles on macrophage cell uptake

Uptake and intracellular trafficking of hydrogel nanoparticles (NPs) of N,N-diethyl acrylamide and 2-hydroxyethyl methacrylate crosslinked with N,N′-methylene-bis-acrylamide were studied with a RAW 264.7 murine macrophage cell line. Results show that the uptake rate, the mechanism of internalization and the concentration of internalized NPs are correlated to the NP Young modulus. Soft NPs are found to be internalized preferentially via macropinocytosis while the uptake of stiff NPs is mediated by a clathrin-dependent mechanism. NPs with an intermediate Young modulus exhibit multiple uptake mechanisms. The accumulation rate of the NPs into lysosomal compartments of the cell is also dependent on the NP elasticity. Our results suggest that control over the mechanical properties of hydrogel NPs can be used to tailor the cellular uptake mechanism and kinetics of drug delivery.

Friction and normal interaction forces between irreversibly attached weakly charged polymer brushes.

Polyelectrolyte brushes were built on mica by anchoring polystyrene-poly(acrylic acid) (PS-b-PAA) diblock copolymers at a controlled surface density in a polystyrene monolayer covalently attached to OH-activated mica surfaces. Compared to physisorbed polymer brushes, these irreversibly attached charged brushes allow the polymer grafting density to remain constant upon changes in environmental conditions (e.g., pH, salt concentration, compression, and shear). The normal interaction and friction forces as a function of surface separation distance and at different concentrations of added salt (NaCl) were investigated using a surface forces apparatus. The interaction force profiles were completely reversible both on loading and receding and were purely repulsive. For a constant polymer grafting density, the influence of the polyelectrolyte charges and the Debye screening effect on the overall interaction forces was investigated. The experimental interaction force profiles agree very well with scaling models developed for neutral and charged polymer brushes. The variation of the friction force between two PAA brushes in motion with respect to each other as a function of surface separation distance appeared to be similar to that observed with neutral brushes. This similarity suggests that the increase in friction is associated with an increase in mutual interpenetration upon compression as observed with neutral polymers. The effect of the PAA charges and added ions was more significant on the repulsive normal forces than on the friction forces. The reversible characteristics of the normal force profiles and friction measurements confirmed the strong attachment of the PAA brushes to the mica substrate. High friction coefficients (ca 0.3) were measured at relatively high pressures (40 atm) with no surface damage or polymer removal.

Normal and frictional forces between surfaces bearing polyelectrolyte brushes

Normal and shear forces were measured as a function of surface separation, D, between hydrophobized mica surfaces bearing layers of a hydrophobic-polyelectrolytic diblock copolymer, poly(methyl methacrylate)- block-poly(sodium sulfonated glycidyl methacrylate) copolymer (PMMA- b-PSGMA). The copolymers were attached to each hydrophobized surface by their hydrophobic PMMA moieties with the nonadsorbing polyelectrolytic PSGMA tails extending into the aqueous medium to form a polyelectrolyte brush. Following overnight incubation in 10 (-4) w/v aqueous solution of the copolymer, the strong hydrophobic attraction between the hydrophobized mica surfaces across water was replaced by strongly repulsive normal forces between them. These were attributed to the osmotic repulsion arising from the confined counterions at long-range, together with steric repulsion between the compressed brush layers at shorter range. The corresponding shear forces on sliding the surfaces were extremely low and below our detection limit (+/-20-30 nN), even when compressed down to a volume fraction close to unity. On further compression, very weak shear forces (130 +/- 30 nN) were measured due to the increase in the effective viscous drag experienced by the compressed, sliding layers. At separations corresponding to pressures of a few atmospheres, the shearing motion led to abrupt removal of most of the chains out of the gap, and the surfaces jumped into adhesive contact. The extremely low frictional forces between the charged brushes (prior to their removal) is attributed to the exceptional resistance to mutual interpenetration displayed by the compressed, counterion-swollen brushes, together with the fluidity of the hydration layers surrounding the charged, rubbing polymer segments.

Synthesis of macrostructured MCM-48 molecular sieves

Abstract Dual templating using aqueous cetyltrimethylammonium chloride/hydroxide (CTMA Cl/OH) and latex spheres was performed to prepare powders of bimodal silica molecular sieves with mesopore and macropore systems. The CTMA/Si ratio was chosen to generate a cubic mesopore of MCM-48 structure. According to high Brunauer–Emmett–Teller surface area, narrow pore size distribution, X-ray diffraction powder pattern and scanning electron micrographs, the cubic mesophase is located within the macropore walls, which is the characteristic of a hierarchical porous system.

Micropuncture study of renal phosphorus transport in hypophosphatemic vitamin D resistant rickets mice.

SummaryA micropuncture study of inorganic phosphorus (Pi) transport was performed in 6 mice presenting hypophosphatamic vitamin D resistant rickets (Hyp) and results compared to those obtained in 13 normal (N) mice. The mean plasma (P Pi) and fractional excretion of Pi (FE Pi) in N and Hyp mice were 85.10±2.27 mg/l and 15.81±1.90 vs 48.43±3.29 mg/l and 35.34±5.26%, respectively.In N mice, tubular fluid over plasma Pi ratio (TF/P Pi) progressively decreases along the proximal tubule to reach approximately 0.6 in the late accessible part. The fraction of filtered Pi (% E Pi) remaining in this segment of the nephron is therefore 20–25%. In Hyp mice the TF/P Pi in proximal tubule remains relatively high and does not significantly vary with TF/P inulin (mean TF/P Pi=1.19±0.12 S.E., compared to 0.73±0.04 in N mice). Precise conclusions concerning % E Pi at the end of the proximal tubule of Hyp mice are not available because of the scattering of the data. However, all the values of % E Pi (mean % E Pi: 78.68±7.39 compared to 49.10±4.59 in N mice) are far above the total urinary FE Pi (35.34 ±5.26), suggesting a relatively greater distal fractional Pi reabsorption in Hyp mice than in N mice. Since the P Pi, and therefore the amount of filtered Pi is lower in Hyp mice, it is probable that the absolute amount of Pi reabsorbed distally is comparable in the two series of animals.

Fluidity of water and of hydrated ions confined between solid surfaces to molecularly thin films

In contrast to non-associating liquids such as oils or organic solvents, whose viscosity diverges when they are confined by solid surfaces to films thinner than about ten molecular diameters, recent studies reveal that salt-free water remains fluid, with a viscosity close to its bulk value, even when confined to films down to only one or two monolayers thick. For the case of high concentration aqueous salt solutions compressed down to subnanometre films between confining planar surfaces, the hydration sheaths about the ions (trapped between the oppositely charged surfaces) also remain extremely fluid: this behaviour is attributed to the tenacity of water molecules in the hydration layers together with their rapid relaxationlexchange time. Related experiments on highly compressed, polyelectrolyte brushes in aqueous media reveal a remarkable lubricity which is in large measure attributed to similar hydration layers about the charged segments: this water of hydration strongly resists being squeezed out, but at the same time it may rapidly exchange with adjacent water molecules, thereby remaining quite fluid and acting as a molecular lubricant.

Viscosity of ultra-thin water films confined between hydrophobic or hydrophilic surfaces

A surface force balance has been used to investigate the viscosity of salt-free (conductivity) water confined between hydrophilic and between hydrophobic surfaces. We examine the process of jump-in, across the last few nanometres of thin water films, to adhesive contact between the surfaces. We analyse the flow of water out of the gap under slip and no-slip boundary conditions at the confining surfaces. In both cases we find that the effective viscosity of water remains comparable to its bulk value even when it is confined to sub-nanometre thin films.

Polymer Brush Covalently Attached to OH-Functionalized Mica Surface via Surface-Initiated ATRP: Control of Grafting Density and Polymer Chain Length

The controlled grafting density of poly(tert-butyl acrylate) was studied on OH-activated mica substrates via surface-initiated atom-transfer radical polymerization (ATRP). By properly adjusting parameters such as the immobilization reaction time and the concentration of an ATRP initiator, a wide range of initiator surface coverages and hence polymer densities on mica were possible. The covalently immobilized initiator successfully promoted the polymerization of tert-butyl acrylate on mica surfaces. The resulting polymer layer thickness was measured by AFM using a step-height method. Linear relationships of the polymer thickness with respect to the molecular weight of the free polymer and with respect to the monomer conversion were observed, suggesting that ATRP is well controlled and relatively densely end-grafted layers were obtained. The polymer grafting density controlled by adjusting the initiator surface coverage was confirmed by the polymer layer swelling capacity and film thickness measurements.

Stability of Silanols and Grafted Alkylsilane Monolayers on Plasma-Activated Mica Surfaces

We investigated the effect of physical and chemical modifications of mica surfaces induced by water vapor-based plasma treatments on the stability of silanols and grafted alkylsilane monolayers. The plasma-activated substrates were characterized using XPS, TOF-SIMS, and contact angle measurements. They revealed a large surface coverage of silanol groups (Si-OH) and a loss of aluminum atoms compared to freshly cleaved mica surfaces. The stability of plasma-induced silanol groups was investigated by contact angle measurements using ethylene glycol as a probe liquid. The Si-OH surface coverage decreased rapidly under vacuum or thermal treatment to give rise to hydrophobic dehydrated surfaces. The stability of end-grafted monofunctionalized n-alkylsilanes was investigated in different solvents and at different pH using water contact angle measurements. The degrafting of alkylsilanes from the activated mica was promoted in acidic aqueous solutions. This detachment was associated with the hydrolysis of covalent bonds between the alkylsilanes and the mica surface. The monolayer stability was enhanced by increasing the length of the alkyl chains that probably act as a hydrophobic protective layer against hydrolysis reactions. Stable alkylsilane monolayers in water with pH greater than 5.5 were obtained on mica surfaces activated at low plasma pressure. We attributed this stability to the loss of surface Al atoms induced by the plasma treatment.