Polymers with Heat-Setting Proteins. Consider a solution of a
globular protein at a pH above its isoelectric point that will give a gel upon
heating. Now make solutions also containing a polysaccharide, where the
total concentration of protein þ polysaccharide is the same. If the
polysaccharide is anionic (e.g., carrageenan), a homogeneous solution is
often formed; upon heating, a gel results whose modulus (G^0 ) is higher than
that of the protein alone. If the polysaccharide is neutral (e.g., dextran),
phase separation due to incompatibility occurs. After heating the modulus is
lower than that in the absence of polysaccharide, which is presumably due to
the inhomogeneity of the system. These are rules of thumb and exceptions
may be possible.
Polysaccharides with Surfactant Micelles. Consider a solution
of a fairly hydrophobic polysaccharide, such as a cellulose ether. The
hydrophobic groups cause a weak attractive interaction, leading to a
somewhat increased viscosity at low shear rates. If an anionic small-
molecule surfactant is added, say SDS (sodium dodecyl sulfate), at a
concentration above the CMC (critical micellization concentration), micelles
are formed that interact with the polymer; more specifically, one or a few
polymer chains can pass through a micelle. In this way, polymer chains can
be cross-linked. If now the polymer concentrationcis belowc (the chain
overlap concentration), mainly intramolecular junctions are formed. Ifc>
c, however, a gel results. In this manner, viscoelastic gels can be made with
a modulus of the order of 10 Pa.
Filler Particles. We will consider gels containing particles that are
much larger than the pores in the gel network. The primary gel (i.e., without
particles) may be a polymer gel, a fractal particle gel as obtained with casein,
or a heat-set protein gel. We will consider the effect of‘‘filler particles’’on
gel properties, especially the elastic modulus, which has been studied best.
The following factors are known to affect the properties.
- Particleconcentration, generally expressed in the particle volume
fractionj. The modulus will either increase or decrease with increasingj;
see below. - Particlebonding. The particles can be bonded or not to the gel
matrix, as schematically depicted in Figure 17.20, frames 2 and 1. As shown
in Figure 17.20a, bonded particles increase the value of the modulus, which
is logical since they effectively increase the amount of gel material;
nonbonded particles tend to decrease the modulus, since they effectively
behave like voids at very small deformation.
Bonding can occur if the gel material will adsorb onto the particle
surface. Filler particles can be emulsion droplets, and a gel is often made by