Thermoreversible gelation in aqueous dispersions of colloidal particles bearing grafted poly(ethylene oxide) chains

Abstract

Colloidal interactions between particles in a dispersion can be tuned by grafting polymeric chains onto the surface of the particles. The affinity between the polymeric chains and the continuous-phase liquid controls the strength of these interactions. In our system the polymerliquid affinity is strongly influenced by temperature, and as a result, dramatic changes occur in the dispersion microstructure on heating. The system is an aqueous dispersion of polystyrene ͑PS͒ particles bearing grafted poly͑ethylene oxide͒ ͑PEO͒ chains of low molecular weight ͑ ϳ 2000͒. At room temperature, water is a good solvent for PEO chains, and the dispersion is a stable, low-viscosity sol. As temperature is increased, water becomes a progressively worse solvent for PEO. Beyond a temperature T c there is a sharp transition in microstructure from a stable sol to a volume-filling gel. The sol-gel transition is reversible and the transition temperature T c can be pinpointed using tan ␦ versus temperature plots. Remarkably, T c is more than 100°C lower than the temperature for PEO ͑2000͒ in water, i.e., the gelation occurs under significantly better-thanconditions. T c is independent of particle concentration, but is strongly influenced by the graft density of PEO chains on the particles. The higher the graft density, the higher the T c for gelation; conversely, at very low graft density, the samples are gels even at room temperature. Above T c , the elastic modulus (G Ј) of the gels reveals a power law dependence with particle volume fraction ͑͒, i.e., G Ј ϰ n. The power law exponent n is independent of the PEO graft density, implying that the various gels have a similar microstructure. We suggest that gelation is the result of a weak secondary minimum in the interparticle potential that can develop in the case of short stabilizing moieties and moderate solvent conditions.

Modeling the anisotropic self-assembly of spherical polymer-grafted nanoparticles

“…[15][16][17] The presence of an isotropic short ranged interparticle attraction coupled to a longer ranged repulsion can also yield anisotropic structures. For instance, the assembly of spherical nanoparticles decorated with short alkanes into stripes on a substrate 18,19 or into nonequilibrium gels in three dimensions 20,21 can be rationalized on this basis. Similarly, molecular dynamics simulations on aqueous solutions of hydrophobic fullerenes uniformly grafted with hydrophilic polyethyleneoxide chains 22,23 show amphiphilelike selfassembly behavior.…”

Temperature-induced gelation of concentrated silicon carbide suspensions

“…The reversible destabilization of a sterically stabilized suspension could be used as a novel near-net-shape-forming method called temperature-induced gelation (TIG) [14][15][16]. Temperature-induced gelation (TIG) originates from the incipient flocculation effect of sterically stabilized suspensions.…”

Theoretical Predictions of Temperature-Induced Gelation in Aqueous Dispersions Containing PEO-Grafted Particles

“…[1][2][3][4][5] However, solvent quality usually changes with temperature, so the stabilizing effects may be reduced or lost upon heating or cooling the system. [6][7][8][9][10][11][12] For instance, many polymer solutions display a lower critical solution temperature, LCST, and when such polymers are grafted onto particles, one may anticipate a decreased stabilization, as the temperature is increased.…”