AN NMR SELF-DIFFUSION INVESTIGATION OF AGGREGATION PHENOMENA IN SOLUTIONS OF ETHYL(HYDROXYETHYL)CELLULOSE

Water solutions of ethyl(hydroxyethyl)cellulose have been investigated with the PFG NMR technique and by means of H-1 NMR relaxation measure ments. The echo decays in PFG experiments have been recorded for EHEC with concentrations ranging from the dilute (c < c) to the semidilute (c > c). In addition, the temperature dependence of the echo decay w as investigated. It is shown that the echo decays deviate substantiall y from Gaussian diffusion behavior. We discuss this in terms of polydi spersity, aggregation, and scaling effects. It is argued that the obse rved deviations are too large to be caused by polydispersity effects a lone. We base this conclusion on the observation that the molar mass s caling of the polymer diffusion coefficient goes roughly as M-4. We fo cus on using a log-normal distribution of diffusion coefficients in th e evaluation procedure of the PFG data. It is found that by comparing the diffusion coefficients extracted from this type of procedure with ''D-mean'' as extracted from a ''stretched exponential'' approach, the re is a difference in the evaluated diffusion constant by up to 1 orde r of magnitude. The difference depends on the degree of deviation from linear behavior in the echo decay when represented in a logarithmic p lot vs the relevant experimental parameter.