Segmental rotational diffusion of spin-labeled polystyrene in dilute toluene solution by 9 and 250 GHz ESR

A copolymer of styrene containing less than 5 mol % of chain units spin-lab eled by attachment of the nitroxide via a short tether has been synthesized . ESR spectra of dilute toluene solution of the copolymer have been obtaine d using 9 and 250 GHz ESR. Parameters characterizing polystyrene segmental rotational diffusion in toluene solution over a broad temperature range hav e been determined from nonlinear least-squares fits of theoretical ESR spec tra to the experimental ESR spectra. The model used was that of relatively fast internal rotation of the nitroxide about its tether, with slower polym er chain segmental motion. Together they lead to effective rotational diffu sion with an anisotropic diffusion tensor. In addition, constraints in the form of an orientational potential restrict the range of angles over which this diffusion occurs relative to the polymer backbone, and the latter is a ssumed to reorient on an ultraslow time scale. This is referred to as a mod el of microscopic order but macroscopic disorder (MOMD). Rates for the slow er polymer chain segmental motion (from the 250 GHz spectra) ranged from 3. 6 x 10(8) s(-1) at 311 K to 0.15 x 10(8) s(-1) at 215 K, with a substantial orientational potential of about 2kT over this temperature range. Although there was reasonable agreement between the results obtained at 9 and 250 G Hz, there were systematic discrepancies such that the orienting potentials obtained from the 250 GHz spectra were about twice (or more) those from the 9 GHz spectra, and the rotational diffusion tensor components from the 250 GHz spectra were at least twice those from the 9 GHz spectra. This implies a breakdown of the MOMD model for the 9 GHz spectra presumably due to thei r sensitivity to the slower overall tumbling motion at this low spectral fr equency. For the faster "time scale" of the 250 GHz spectra, such a slow mo tion is "frozen out", rendering these spectra consistent with the MOMD mode l. Nevertheless, the results at both frequencies yielded a common activatio n Energy, E-exp = 20.7 +/- 1.5 kJ/mol, which, when corrected for the viscou s flow contribution, yielded an E-a = 11.9 +/- 1.5 kJ/mol, which is in good agreement with recent results from fluorescence studies.