Effects of thermal annealing on the heterogeneities in the dynamics of a paramagnetic probe dissolved in azobenzene polymethacrylates

The rotational dynamics of the cholestane spin probe dissolved in nematic azobenzene polymethacrylates (homopolymer and copolymer) was studied by electron spin resonance spectroscopy. Upon isothermal annealing in the isotropic phase, dynamic nanoscale heterogeneities were induced in both polymers, with slow and fast sites being available for reorientation of the spin probe. The stability with temperature of the different sites was found to be very different in the homopolymer and, copolymer. In any case, the rotational correlation time in both the isotropic and nematic phases was well represented by the Vogel-Fulcher law. By comparing the probe dynamics with the polymer structural relaxation, it was possible to locate the reorientation sites at diffierent positions of the polymer structure. Finally, the annealing of the copolymer in the isotropic phase resulted in a dynamically very homogeneous sample in the nematic phase.