Polymorphism and dynamics of MBBA as studied by NMR

Abstract

We report proton NMR experiments on the liquid crystal material N -( p-methoxybenzylid ene)-p-n-butylaniline (MBBA) at 100 MHz in the temperature range 110± 350 K. The phase diagram was investigated by means of second moment and spin-lattice relaxation measurements in order to establish connections between dynamics and phase transitions. The results show that in a slow cooling experiment, two processes contribute to the relaxation, a slow ethyl group motion together with reorientation of the methyl groups. For the glassy nematic state, as well as for the phases observed after reheating a quenched sample, only methyl rotation is observed. The correlation times of these various mechanisms were determined and the results compared with those obtained by previous NMR and dielectric analysis.

Dynamic 1H NMR Investigation of the Liquid Crystal MBBA Confined in Porous Silica

“…1 [22]. Within experimental errors, the results are consistent with those previously reported in the literature [25][26][27][28]. As seen in Fig.…”

“…The corresponding motional parameters are given in Table 2. In the crystalline phases (C5, C0, C4), the activation energies of the motion-generating T 1 minimum are found in the range 9-10 KJ=mol, as generally observed for methyl group reorientations in similar systems [25][26][27][28][30][31][32]. In C0, the distribution of environments results in a smaller activation energy (E ¼ 6 KJ=mol).…”

“…In previous NMR investigations, the low-temperature minima were attributed to the reorientation of methyl groups [25][26][27][28]. Under such assumption, the constant C i is given by [30]:…”

Phase Transformations And Dynamics Of 4-Cyano-4′-Pentylbiphenyl (5cb) By Nuclear Magnetic Resonance, Analysis Differential Scanning Calorimetry, And Wideangle X-Ray Diffraction Analysis

“…This results show that it is sufficient to take into account only intramethyl proton-proton interactions and that the origin of relaxation is mainly 108 T. Mansaré et al due to the C 3 axis reorientation of the methyl groups. As Ea increases in C1 and C2, the crystallization induced an increase of the steric hindrance of the molecules, which prevents ethyl group reorientations from occuring in these phases as it has been observed in MBBA and p-ethoxybenzilidene-p-n-butylaniline (EBBA) [11,15]. In solid state C2, a decrease of this ratio suggests a more important influence of the intermolecular interactions, probably related to a different steric hindrance of the molecules.…”

Electrospun Polymer Fibers Containing a Liquid Crystal Core: Insights into Semiflexible Confinement

“…MBBA is known to have rich polymorphism depending upon if it is fast-or slow-cooled to temperatures below 0 °C. 42,43 We note that the data presented in Figure 8 are of MBBA subjected to slow cooling and that the features which will be discussed below (at low temperatures) were not present in that scenario. Thus, numerous solid-state studies of fast-cooled MBBA have reported several solid phases at low temperatures using a variety of experimental techniques.…”