Polyacrylic acid probe diffusion

Extended searches of the spectrum on the time scale on which the spectrum would have decayed, if the Stokes-Einstein equation were correct in solutions of 1 MDa polyacrylic acid, found no sign of a spectral relaxation the spectrum had aheady decayed to the baseline. Lin and Phillies proposed that the discrepancy between Dp and the macroscopic q arises from shear tbinning in the probe diffusion process, namely the microviscosity found on the time and distance scales probed by the polystyrene spheres is far less than the macroscopic viscosity q. 

For clarity, the remaining literature in this section is discussed in chronological order. Work by Phillies and Quinlan had been preceded by extensive optical probe diffusion studies of HPC water solutions Brown and Rymden used QELSS to examine 72 nm radius PSL spheres diffusing in solutions of hydroxyethylcellulose (HEC), hydroxypropylcellulose (HPC), carboxymethylcellulose (CMC), and polyacrylic acid(PAA)(56). The focus was polymer-induced cluster formation, indicated by the substantial decreases in Dp and increases in the second spectral cumulant as seen at very low (0.001 g/g) concentrations of HEC and HPC. These changes were substantially reversed by the addition of 0.15% Triton X-100. The Dp of spheres was reduced by the addition of small amounts of fully-charged pH 9 CMC, but addition of TX-lOO had no effect in CMC solutions. Brown and Rymden also examined sphere diffusion in nondilute polymer solutions. Relatively complex dependences of Dp on concentration were suppressed by the addition of TX-IOO. In the presence of TX-lOO, simple stretched-exponential concentration dependences were observed, but the second spectral cumulant still increased with increasing polymer concentration. 

Einsteinian behavior appears in Figure 9.4, based on Lin and Phillies(lO), involving 20-620 nm probes in aqueous IMDa polyacrylic acid. Bu and Russo examine the diffusion of extremely small probes (0.5-55 nm) in HPC water(22). In one of the few published explicit tests of Eq. 2.3 as applied to diffusion, Bu and Russo find that the dependence of D /D o on R matches Langevin and Rondelez s prediction as discussed in Chapter 2. Bu and Russo report that small probes diffuse much more rapidly than expected from jy for larger probes r]p, approaches y. 

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