Consequences of Shape Anisotropy On Colloidal Glasses

Theoretical predictions on glasses formed from hard particles show that a moderate deviation from spherical shape decreases the elastic modulus in the glass state at a specified volume fraction. The particles used in these calculations consist of two fused spheres (homodicolloid shape) with a variable long direction, L, and a fixed short direction, D. The aspect ratio, L/D is varied from 1.0 to 2.0 and a minimum in the elastic modulus is found at L/D=1.4. It is also found that making the modulus nondimensional by multiplying G' by L²D/kT, and rescaling the absolute volume fraction on the glass point (φ/ φ _g-1), forces all data to collapse onto a single curve. Here we present recent experimental work that confirms many of the predictions of the model.

Using seeded emulsion polymerization (with or without divinyl benzene crosslinker), polystyrene particles are synthesized as either spheres or homodicolloids with and aspect ratio of 1.3. The particles are stabilized with the addition of a nonionic surfactant monolayer to the surface. The ionic strength of the suspension is tuned such that the particles experience nearly hard interaction potentials. The particles are concentrated to above the glass transition volume fraction and the elastic modulus is measured. The results show that the dicolloids do, in fact have a lower modulus than the spheres at the same volume fraction. In addition, by adjusting the particle diameter to account for soft Coulomb repulsions and scaling on the theoretical predictions of the glass transition volume fraction, we also see collapse of the two shapes as predicted by the model. The slope of the modulus data, however, is higher than predicted. This is likely due to the fact that the experiments were done at a frequency of 1Hz while the theory predications are for zero frequency. The confirmation of two (G' spheres > G' dicolloids, collapse of shapes onto single curve) out of three predictions of the model by these experiments represents a powerful confirmation of the physical basis of the theory and the inherent assumptions that it embodies.