Comparison of colloid and polymer dynamics

Having demonstrated that phenomenology gives a strong preference for exponential models over their alternatives, we now turn to other lines of evidence that speak to the nature of polymer motion in nondilute solutions. 

Dreval has shown that c[q is a good reducing variable for y(c) of polymer solutions, for all concentrations from dilute solution up to the melt(5). The t c) of hard-sphere solutions has precisely the same feature, namely 17(c) of suspended spheres is, at all accessible concentrations, a universal function of the sphere volume fraction 4 . The quantities c[i ] and (j) are equivalent up to a constant, as is seen by comparison of 17(c) = 170(1 -I- c[i7] -I-...) for polymers with the Einstein relation 17(c) = 7o(l -I-2.5 / -1-...) for spheres. 

Viscoelasticity Colloid and polymer solutions both generally show shear thinning. The frequency dependences for the dynamic moduli of colloid and polymer solutions, written as G (a )fco and G co)/co, are qualitatively the same, namely at smaller co the moduli follow an exponential or stretched exponential in co and at larger co the moduli follow a power law in . At very large frequencies, the dynamic moduli of polymer solutions sometimes show additional features, an additive secondary relaxation or additive baseline or a second power-law decay, that are not seen with colloidal spheres. However, the secondary relaxations apparent for the moduli of soft sphere melts, as studied by Antonietti, ct a/. (11), are only apparent at frequencies considerably larger than those that have been reached experimentally 

Videomicroscopy of colloid suspensions finds that colloid particles in nondilute solutions form fast- and slow-moving clusters studies of Sedlak on the polymer slow mode indicate that random-coil polyelectrolytes also form slow and fast regions(12). Colloidal probes in colloid or polymer solutions both sometimes show re-entrance, in which the concentration dependences of D and rj differ, but only over a limited range of c. At large q and elevated c, the polymer slow mode sometimes becomes -independent, especially at low temperatures. A similar large- behavior does not appear to have been reported for spheres. 

Single-particle motions The single particle structure factor of col- 

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