Phenyl ring dynamics and chain reorientation in liquid crystal polymers: A deuteron spin relaxation study

Abstract

Reorientation dynamics in a nematic liquid crystal studied by fast field cycling nuclear magnetic resonance A detailed description of phenyl ring dynamics and spin relaxation in a highly ordered main chain/side chain liquid crystal polymer is presented. Models for the different motional processes are discussed with reference to experimental measurements of the deuteron spectral densities, J M (M 0 ), measured in the smectic A and nematic phases between 330 K and 500 K, and measured as a function of orientation at 410 K, close to the minima in the relaxation times for Zeeman and quadrupolar order, T 1Z and T 1Q . Intramolecular motion of phenyl rings about the para-axis is regarded as a diffusive process in a two-fold symmetric potential. Theoretical analysis of this model provides the rate constants and amplitudes for passage across the potential barriers and libration within the minima. These are governed entirely by the height of the potential barrier, the potential shape, and the diffusion constant. It is demonstrated that other models for the intramolecular ring dynamics cannot simultaneously account for the magnitudes of the observed J M (M 0 ) in the vicinity of the T 1Z and T 1Q minima. Large amplitude reorientation of the polymer chain is regarded as rotational diffusion in an orienting potential. In order to reproduce both the temperature and orientation dependences of the J M (M 0 ), an additional low amplitude motion of the polymer about the main chain axis, with correlation times in the 10 Ϫ9 s regime at Tϳ400 K, must be included. This is modeled as a one-dimensional diffusion process modulated by a harmonic potential, representing torsional motion of the extented polymer chains. Analysis of the experimental deuteron spin relaxation data yields values for the correlation times and amplitudes for each of the individual processes, providing estimates for the parameters describing each of the potentials which modulate the intramolecular and whole molecule motion, as well as the temperature dependences of the various diffusion constants.

Probing the Dynamics of the Porous Zr Terephthalate UiO-66 Framework Using 2H NMR and Neutron Scattering

“…A more quantitative analysis is obtained from the 2 H NMR data. The activation energy derived from the temperature dependence of the mean flipping rate constant, 30 kJ mol –1 , is within the range typical for phenyl fragments dynamics in solid polymers, , but the activation barrier is considerably reduced in comparison with other porous metal–organic frameworks. In MOF-5, the activation energy for the π-flip rate constant is 47 kJ mol –1 , in MIL-47 is 45 kJ mol –1 , and in MIL-53 is 41 kJ mol –1 .…”

Order and Dynamics of a Series of Side Chain Liquid Crystal Copolymers through 2H NMR Spectroscopy

“…where k 0 ) 0, k 1 ) D φ , and k 2 ) (3ν + 1)k 1 , being ν ) 0 for (i) 9,17 and ν ) 1 for (ii). 9,10,18 Within model iii 19 it can be written as where b NN′…”

Quantitative analysis of 2H NMR T1Q, T1Z and T2 relaxation times in the SmA phase of a liquid crystal dendrimer

“…The ''decoupled approach'' 17 (see section 3.3) can be used: in fact, as observed in side-chain liquid crystalline polymers, 28,29 the internal motions affecting the mesogenic terminal units can be safely assumed to be uncorrelated with respect to the molecular motions of the whole dendritic molecule, due to the long flexible chains linking the LC moiety to the rest of the dendrimer (see Scheme 1). Moreover, based on previous observations on high molecular weight LC systems, 28,29,67 the intramolecular processes are expected to be the main contributions to the longitudinal relaxation. The liquid crystalline terminal units can be treated as common low weight LCs forming a uniaxial smectic A phase.…”

“…The internal motion is the faster one, D R B 2.1 Â 10 9 s À1 (T = 325 K), as expected. 28,29,67 The overall molecular motions are almost equivalent, D > B 1.4 Â 10 7 s À1 (T = 325 K) and D // B 1.0 Â 10 7 s À1 (T = 325 K), and they are sensibly lower than in common calamitic LC systems. 26,27 In case 'ii' the strong collision model was used to describe the internal motion.…”