E. Tomchuk

Deuterium NMR study of molecular order and reorientation in the nematic phase of p‐methoxy‐d3‐benzylidene‐d1‐p‐n‐butyl‐d9‐aniline

The deuterium quadrupole doublet splittings and spin–lattice relaxation times (T1) are measured as a function of temperature in the nematic phase of p‐methoxy‐d3‐benzylidene‐d1‐p‐n‐butyl‐d9‐aniline (MBBA‐d13). It is shown that a single order parameter tensor is sufficient to fully describe the orientational order of the molecule even though the molecule is nonrigid. The molecular conformational averages, which are different for various C–D bonds, result in resolved deuterium NMR spectra from which the T1 for each deuteron site can be measured. While the simplest approach in treating order director fluctuations, in which molecular reorientations and collective fluctuation modes are assumed to be uncoupled, fails to explain our T1 data for all deuterons, Freed’s treatment of spin relaxation in liquid crystals does give a qualitative explanation. Using this theory, molecular reorientation times for different deuteron sites are estimated.

Deuteron line shape study of molecular order and conformation in the nematogens PAA and MBBA

The deuteron fast Fourier transform (FFT) spectra of the ring deuterated nematogens para‐azoxyanisole (PAA‐d8) and p‐methoxybenzylidene p‐n‐butylaniline (MBBA‐d8) have been studied as a function of temperature. An interpretation of the spectra is presented. It is based on the assumption of nonequivalent deuterons, a twist angle φ between the planes of the benzene rings, and an asymmetry order parameter δ in addition to the usual nematic order parameter S. The temperature dependences of δ and φ have been obtained.

High resolution carbon‐13 spin–lattice relaxation study of the nematogen p‐methoxybenzylidene–p‐?‐butylaniline

The 13C relaxation measurements were obtained for all resolved carbon resonances in the nematic and isotropic phases of MBBA. Also obtained were the NOE measurements in the isotropic phase. The study indicates that a flexibility gradient occurs along the n‐butyl chain and that the 13C relaxation is governed by the anisotropic rotational diffusion of the entire molecule.

A proton and deuteron spin–lattice relaxation study of the partially deuterated nematogens PAA and MBBA

Proton spin–lattice relaxation times (T1) have been measured in methyl deuterated para‐azoxyanisole (PAA‐d6) and in ring deuterated para‐azoxyanisole (PAA‐d8) samples as functions of frequency and temperature. When compared with recent order fluctuation theory, the agreement between theory and experiment is not as satisfactory for the intermolecular contribution to the spin–lattice relaxation as for the intramolecular contribution. Deuteron spin–lattice relaxation times (T1Q) have also been measured in a ring deuterated p‐methoxybenzylidene p‐n‐butylaniline (MBBA‐d8) sample. The above order fluctuation theory fails to predict the observed temperature dependence and the observed frequency independence of T1Q if the correlation time τc for local motions is taken to be one‐third the dielectric relaxation time τD. However, a fair agreement is obtained if one uses τc= (1/100) τD, a value consistent with Ref. 6.

Proton spin echo study of spin‐spin relaxation in liquid crystals

Proton spin‐spin relaxation times ``T2 were measured as a function of temperature in the nematic phase of p‐methoxybenzylidene‐p‐n‐butylaniline and in the nematic and smectic C phases of p‐n‐heptyloxyazobenzene using the 90°–45° echo. By comparing ``T2 in these two liquid crystals, we infer that short range smectic order exists even 10° above the nematic‐smectic C phase transition in p‐n‐heptyloxyazobenzene. The angular dependence in ``T2 was also determined in the smectic C phase at two different temperatures.

14N NMR Study of Order Fluctuations in the Isotropic Phase of Liquid Crystals

Abstract Nitrogen-14 resonance lineshapes in the isotropic phase of a nematic (p-azoxyanisole) and of a smetctic-A (dietylaxoxybenzoate) liquid crystals have been studied using fast Fourier transform techniques. The temperature variation in linewidth is diferent in each sample, although both depend critically on the short range order fluctuations which exist about 10° to 15° into the isotropic phase. The experimental results for the isotropic phases of p-azoxyanisole and of diethylaxoxybenzoate are explained in terms of short range order fluctuations of the nematic and of the smectic types, respectively.

A Study of the Dipolar Spin System in Selectively Deuterated Nematogens

Abstract The proton spin dipolar relaxation times and dipolar echo responses were studied for the nematic phase of ring-deuterated p-methoxybenzylidene-p-n-butylaniline (MBBA-d) as a function of temperature and frequency. Measurements of the proton dipolar echo responses were also made for methyl-deuterated p-azoxyanisole (PAA-d6). The echo response was used to determine the inter-pair second moment M2 (inter), while the total second moment M2 was extracted from the echo shape. Both of these second moments were determined for the ring protons in PAA-d6 and the end chain protons in MBBA-d8 and are discussed in terms of the averaging caused by the internal motions of the molecules. The dipolar relaxation times measured for the side chain protons in MBBA-d8 indicate a lower “effective” viscosity for this part of the molecule.

Proton Spin Relaxation Study of the Dipolar Spin System in Nematic Liquid Crystals

Abstract The proton spin dipolar relaxation time was measured in the mesophases of p-azoxyanisole, p-n-hexyloxyazoxybenzcne, p-n-heptyloxyazoxybenzene and p-methoxybenzylidene-p-n-butylaniline as a function of temperature and frequency. It was found that the dipolar spin systems in the nematic crystalline phase are predominantly relaxed by the orientational order fluctuations. This is manifested by the magnetic field effects on the order fluctuation modes.

Proton spin relaxation in a smectic liquid crystal

Abstract A proton spin relaxation study in the liquid crystal ethyl-[(methoxybenzylidene)-amino] cinnamate is presented. A “phase change” is observed at ≈ 103°C within the smectic A phase. Some liquid-like mobility exists below this temperature.

Study of proton spin echoes in the nematic liquid crystalline phase

Abstract We studied spin echoes formed in the nematic crystalline phase using 90°- τ-β pulse sequence, where τ is the time between pulses and β the rotation produced by the second pulse. The decay of the echoes as a function of 2τ is nearly exponentail with a time constant T E . Both T E and the echo amplitude at a fixed τ were studied as a function of β in two nematics.