DIFFUSION OF DEXTRAN IN AQUEOUS (HYDROXYPROPYL)CELLULOSE

Fluorescence photobleaching recovery (FPR) was used to measure the probe self-diffusion coefficients, D(s), of eight labeled dextrans with different molecular weights in aqueous solutions of the semirigid polymer (hydroxypropyl)cellulose (HPC). With added measurements of free dye and a single dye-labeled polymeric latex, the probe hydrodynamic radius, R(h), spanned 5-551 angstrom. For dextrans, the dependence of D(s) upon M obeys D(s) approximately M(-beta) with beta almost-equal-to 1/2 in water. In the most concentrated HPC solution studied, beta almost-equal-to 1. Small probes show strong deviations from the Stokes-Einstein relation. The effect was less severe as probe size increased. Most of the diffusion data fit the Langevin-Rondelez equation, D(s)/D0 = eta0/eta + exp[-(R(h)/xi)delta], where D0 and eta0 are respectively the diffusion coefficient in and viscosity of pure solvent, xi is the correlation length, and delta a parameter. Exceptions to the Langevin-Rondelez relation were found for small probes in dilute solution (R(h)/xi < 0.1). The data are also interpreted using the hydrodynamic scaling model advanced by Phillies and a cylindrical cell model developed by Johansson and co-workers. The effect of the molecular weight of the HPC matrix upon the diffusion of variously sized probes was also studied; small probes were relatively insensitive to matrix molecular weight, but the diffusion of larger probes did depend on the molecular weight of the HPC. Analytical applications of FPR are considered. HPC selectively retards dextrans according to molecular weight, the effect becoming stronger as HPC concentration is increased. The FPR signal for a mixture of two dextrans with different molecular weights can be markedly biexponential in HPC solution, even if it is not pure water. This expansion of the recovery time distribution by the polymer matrix suggests that FPR in a polymer matrix can serve as a medium-resolution method to detect macromolecular polydispersity, similar in spirit to analytical gel electrophoresis but applicable to uncharged polymers, suitably labeled. A preliminary evaluation is promising.