Decoupling from structural relaxation of short- and longtime dynamics of a paramagnetic tracer dissolved in a liquid-crystal polymer

Abstract Dynamic processes effective on the nanometre length scale are of interest in supercooled fluids and polymers owing to their influence on structural and dynamic properties of these materials. In particular, nanometre and nanosecond scales mark the crossover from liquid to solid properties when approaching the glass transition. A possible explanation of the complex dynamic response of these systems for these length and time regions is provided in terms of heterogeneity. Heterogeneity in supercooled materials manifests itself by the presence of different domains, which relax towards equilibrium with different rates. Therefore, experimental techniques, which are sensitive to slow and fast molecules and able to follow their dynamics, are of paramount importance in this field of research. The present work is devoted to investigate heterogeneity in a polymer. We focused our attention on a side-chain liquid-crystal polymer (SCLCP). In this study a nanometre-scale molecular probe is dissolved in the SCLCP and its rotational dynamics are studied by linear and nonlinear electron spin resonance (ESR) spectroscopies, which are sensitive to the long and short time scales respectively of the rotational relaxation. In particular, linear ESR spectroscopy evidenced the presence of heterogeneity while on the short time scale detected by nonlinear ESR the correlation decay turns out to be homogeneous.