Frequency Dependence of Proton Spin Relaxation in Liquid Crystalline PAA

Abstract

We report on measurements of the Larmor frequency dependence of the proton spin relaxation time T1 in the nematic and isotropic phase of p-azoxyanisole (frequency range: 3.8 kHz ≦ ωL/2 π≦75 MHz) . In both cases our results clearly support the Pincus-Cahane mechanism of spin relaxation by order fluctuations ("ωL−½-law") and exclude the alternative translational dif­fusion model (“ωL+½-law”). For the isotropic phase it was possible to evaluate the correlation time τ of the liquid crystalline order fluctuations from the observed T1 dispersion. As a function of the deviation ⊿ν=ν-νc from the critical nematic-isotropic transition temperature, νc= (136± 0.5)°C, we found τ=2.71·10-7-⊿ν-0.25s .

Proton Spin Relaxation of a Liquid Crystal with a Glassy Cholesteric State

“…Qualitatively, the dispersion profiles shown in Figs. 4 and 5 are rather similar to results reported in the literature for numerous untwisted nematic compounds [8][9][10], i.e. they have a low-frequency plateau up to about 10 kHz, followed by a strong T, increase.…”

“…We assign this process to the rotation of whole molecules induced through the self-diffusion along the helix axis, a relaxation mechanism first suggested, expected and calculated by Andreev [5] and by Vilfan et al [6], It is superimposed on the familiar relaxation mechanisms confirmed previously in nematic mesogens [8][9][10], where Tj at low Larmor frequencies is usually governed by director order fluctuations and at high frequencies by self-diffusion. The new relaxation mechanism disappears in the glassy state of the com-pound, though there the compound maintains the cholesteric structure.…”

“…(ii) The SD relaxation rate for BET is comparable with the data known for nematic mesogens [8][9][10], where, however, it has often been noticed that the absolute value of the evaluated self-diffusion constant D s deviates from results of more direct techniques. As a rule, the height and the length of the low-frequency T l (v) plateau are hard to combine with the constraints of (3).…”

“…(i) The OF relaxation rate is considerably faster for BET than for untwisted nematic liquid crystals studied previously [8][9][10] at comparable temperatures. Though this result is subject to a large error due to the presence of the DR term, it is reasonable as a consequence of the differing viscosity rj and the relation A ~ rj 1 ' 2 involved in the Blinc model [18,20,24], Between 50 °C and 100 °C, the parameter A increases by a factor of two.…”

“…On the other hand, it has been pointed out that the self-diffusion of molecules along the helix axis should induce a special rotational relaxation contribution typical for cholesteric systems [4][5][6] (diffusion induced rotation or DR) which, however, has not been observed so far. Remarkably, this kind of reorientation and its time scale are well-known from the related changes of the proton NMR spectrum [7], so that it is surprising indeed not to have seen effects on the relaxation rate.The main purpose of this report is to demonstrate the presence and importance of the DR relaxation contribution in a cholesteric mesogen in addition to the familiar proton relaxation processes dominating in nematic liquid crystals [8][9][10], like e.g. director fluctuation modes, self-diffusion or strongly hindered anisotropic molecular rotations.…”