Coupling Analysis of Probe Diffusion in High Molecular Weight Hydroxypropylcellulose

The coupling/scaling model of Ngai and Phillies [Ngai, K. L., Phillies, G. D. J. J. Chem. Phys. 1996, 105, 8385] is applied to light scattering spectra of mesoscopic probes in 1 MDa hydroxypropylcellulose (HPC): water. Spectra are bimodal. The coupling/scaling model works for one mode but not the other. Probes smaller than the hydrodynamic radius of the HPC show a two-stretched-exponential relaxation: The slow mode follows coupling/scaling predictions; the fast mode does not. Probes whose diameters are comparable to or larger than the radius of gyration have spectra composed of a fast, stretched-exponential mode that follows coupling/scaling, and a slow, pure-exponential mode not described by coupling/scaling. Interpretations of the success or failure of the coupling/scaling model for various modes and probes are advanced, based on relationships between the observed and assumed spectral line shapes, and on the absolute time scale of each relaxation