emulsifying oil in a surface active polymer solution, say gelatin, after which
the system is allowed to set, e.g., by cooling. Bonding then is ensured, since
the particles will via their gelatin-coated surfaces directly participate in the
gelation. If the gel material does not adsorb, but the gel is quite stiff, a little
surface roughness of the particles causes effective bonding by friction, at
least at the very small strains as applied during measurement of a modulus.
If the gel is made of a dilute nonadsorbing polymer, bonding does not occur;
there will even be a thin layer depleted of polymer around the particles
(frame 1).
- Theparticle modulus. For bonded particles, the van der Poel
theory can be used to predict the relative modulus of the mixed system, i.e.,
Gm/G 0 , as a function ofjand the relative particle modulusGp/G 0 ; hereG 0 is
the modulus of the primary gel. In Figure 17.20a calculated curves are given
for four values ofGp/G 0. ForGp¼0, the result applies also to nonbonded
particles, whatever their modulus. If the particle is an emulsion droplet or
FIGURE17.20 The effect of filler particles on gel properties. (a) Relative modulus
(Gm/G 0 ) as a function of particle volume fraction (j). The broken lines are calculated
for various values of the ratioGp/G 0 , indicated near the curves. The drawn lines are
average experimental values for acid casein gels (C) and polymer gels (polyvinyl
alcohol, P), with emulsion droplets that are either bonded (B) or nonbonded (N) to
the gel matrix. (b) Highly schematic pictures of the gel structure. Shaded area
denotes primary gel. Particles are nonbonded (1); bonded (2); bonded but with
intermediate layer (3); bonded and aggregated (4). (Adapted from T. van Vliet.
Colloid Polymer Sci. 266 (1988) 518.)