Proton Spin Relaxation Dispersion for Some Nematic Homologues of the Liquid Crystal PAA

Abstract

AbstractIn order to elucidate the nature of spin relaxation in nematic liquid crystals in the vicinity of the nematic–isotropic phase transition temperature, we present extensive measurements and a quantitative analysis of the proton spin, T1, relaxation dispersion for some homologues of the PAA series (PAA, PAB, HAB). The studies were made for the nematic state over a broad range of Larmor frequencies. By a detailed inspection of the frequency, temperature and chain length dependences we conclude that in all cases at least three kinds of molecular reorientations with rather different relaxation dispersion contribute to the total T1, i.e., order fluctuations of the director (OFN), self‐diffusion (SD) and an additional mechanism, which we attribute to order parameter fluctuations (OFS). The combined OFN–SD–OFS model extends existing interpretations and allows a quantitative description of the observed phase transition effects. The effects of both OFN and OFS on T1 clearly increase with the molecular length and we deduce from this that not only the ring protons, but also some chain protons undergo these types of fluctuation. Due to the rather high temperature of the mesophases, the diffusional contribution is hard to evaluate exactly and hence there are no significant variations between the homologues within the limits of experimental error. However, a small systematic discrepancy with respect to independent diffusion measurements, available for PAA, indicates that this relaxation model is still incomplete and that a fourth mechanism should be included in the analysis. Possibly, this could be some kind of rotation as previously reported for low‐temperature nematics.