Spin‐lattice relaxation in the rotating frame in solutions of poly(2,6‐dimethyl‐1,4‐phenylene oxide)

Abstract

AbstractMethyl‐proton spin‐lattice relaxation times in the rotating frame T1ρ are reported for 10‐wt % solutions of poly(2,6‐dimethyl‐1,4‐phenylene oxide) in CDCl3. These measurements were made as a function of molecular weight and at two Larmor frequencies: 30 and 90 MHz. The T1ρs are substantially shorter than the spin‐lattice relaxation times T1, indicating conditions far from the extreme narrowing limit. However, the observed values of T1ρ correspond very well to predictions from an earlier interpretation of T1s. The interpretation was based on four motions likely in this polymer: overall rotatory diffusion, segmental motion, phenyl group rotation, and methyl group rotation. All parameters in the interpretation such as correlation times had been set in the previous interpretation of T1s, leaving none to be adjusted to account for the T1ρs. Nevertheless, agreement between the predicted and observed T1ρs is excellent; this is apparently the first such quantitative interpretation of T1ρs in a dissolved polymer far from the extreme narrowing limit. This agreement at high molecular weight points to the adequacy of the description of local motion from high frequencies to relatively low frequencies and to the improved utility of T1ρ data versus T2 data as a check of low‐frequency contributions to the spectral density. The agreement at lower molecular weights indicates that the description of long‐range motions is sufficient though clearly oversimplified by the omission of the Rouse–Zimm modes.