Dear Sir,

Polymer gels are viscoelastic solids which exhibit a wide variety of dynamic rheological

behavior demonstrated here using a series of lightly crosslinked fluorosilicone gels with a

range of equilibrium moduli. At long time scales, the elastic response of the gel network

dominates and the measured equilibrium parameters are highly dependent on the gel

microstructure and the presence of defects. The equilibrium gel response is almost purely

elastic and shows a plateau at low frequencies where the gel response becomes independent

of frequency. That plateau is more apparent for stiffer polymer gels. In non-ideal gels which

have entanglements, network defects and polymer solvent, viscoelastic effects become more

important as the frequency is increased. At short time scales, physical entanglements and

solvent effects become dominant and the measured gel response becomes almost

independent of the gel network quality. The importance of time scales is readily apparent in

all forms of characterization including linear oscillatory rheology, adhesion and hardness

testing.

Linear oscillatory rheology provides a precise tool for determining the critical gel point and

for understanding gel behavior by manipulating both the applied strain and frequency. The

viscoelastic properties are critically important for good adhesive properties. The presence of

dissipation mechanisms (as evidenced by the viscous modulus) are important for creating a

large work of adhesion whereas the presence of an equilibrium modulus is required for

keeping the adhesive in place. The frequency dependence of the work of adhesion closely

mirrors the linear oscillatory rheology. At low frequencies, the work of adhesion is strongly

dependent on the equilibrium modulus of the gel. At high frequencies, the adhesive

response becomes independent of the gel network quality and instead is dominated by

physical entanglements. Hardness testing can also provide useful qualitative information

about gel dynamic response, though because of the non-uniform strain field applied, the

frequency dependence is not as pronounced.

The increase in modulus is seen in high molecular weight entangled polymer melts as well. Typically larger the frequency, shorter the length and time scales probed by the experiment. At very high frequencies in melts one sees the Rouse like response where both G' and G" increase. At this frequency one probes the chain dynamics between entanglements in Gels I would expect a similar mechanism when the dynamics between cross-links is probed at high frequency.