Nuclear magnetic resonance field-cycling proton relaxation study of polymer dispersed liquid crystals

Abstract

The frequency and temperature dependence of the longitudinal proton relaxation time T1 has been studied in liquid crystal droplets embedded in a solid polymer matrix in the nematic and isotropic phase over a broad Larmor frequency range (500 Hz≤νL≤84 MHz) employing the fast-field-cycling technique. The comparison of the droplet data, bulk 5CB data, and the pure polymer data show that T1 is dominated by the cross relaxation at the liquid crystal–polymer interface in the entire frequency range. In the low frequency range (νL≤1 MHz), an additional relaxation process determines T1 in both phases, namely reorientations mediated by translational replacements in the nematic phase and the exchange relaxation in the isotropic phase. The analysis of the cross relaxation rate k reveals that the simplified model of Vilfan is only applicable in the nematic phase and leads to an anchoring time τAS of the molecules at the surface which is ≊1.3×10−4 s. This model, however, cannot be applied in the isotropic phase, but must be extended by the spin diffusion time τP, which in this case determines the cross relaxation rate. The anchoring time τAS, which in the isotropic phase corresponds to the correlation time of the additional process, is much shorter than in the nematic phase, namely τAS≊10−6 s.