(198c) 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.