Chain dynamics in poly(amidoamine) dendrimers: a study of proton NMR relaxation parameters

Abstract

^2H NMR relaxation measurements were used in an extensive study of chain mobility in poly-(amido amine) (PAMAM) dendrimers as a function of molecular weight and position along the chain. For ^2H labels in the terminal generation, the spin-lattice relaxation times (T_1) were found to decrease as the number of chain termini increased from 3 (T_1 = 30 ± 3 ms, at 46.0 MHz) to 384 (T_1 = 7.6 ± 0.7 ms, at 46.0 MHz) and were smaller than the T_1 observed when the polymers were labeled in the interior. The values of T_1 determined at lower field strength (4.7 T) were lower than those at higher field strength (7.05 T). By using the spin-spin relaxation time (T_2) as a second variable, consistent correlation times (τ) were obtained at the two field strengths upon application of Schaefer’s log χ^2 distribution function. No dependence of τ on position along the chain was observed when the label was located in the interior repeating units of PAMAM dendrimers of constant molecular weight. No evidence of radial gradients was found. The data are interpreted in terms of alternative theoretical treatments of dendritic macromolecules.