NMR Spectroscopic Studies of Cation Dynamics in Symmetrically-Substituted Imidazolium-Based Ionic Liquid Crystals

Ionic liquid crystals (ILCs) present a new class of non-molecular soft materials with a unique combination of high ionic conductivity and anisotropy of physicochemical properties. Symmetrically-substituted long-chain imidazolium-based mesogenic ionic liquids exhibiting a smectic liquid crystalline phase were investigated by solid state NMR spectroscopy and computational methods. The aim of the study was to reveal the correlation between cation size and structure, local dynamics, and orientational order in the layered mesophase. The obtained experimental data are consistent with the model of a rod-shaped cation with the two chains aligned in opposite directions outward from the imidazolium core. The alignment of the core plane to the phase director and the restricted conformations of the chain segments were determined and compared to those in single-chain counterparts. The orientational order parameter S~0.5–0.6 of double-chain ionic liquid crystals is higher than that of corresponding single-chain analogues. This is compatible with the enhanced contribution of van der Waals forces to the stabilization of smectic layers. Increased orientational order for the material with Br− counterions, which exhibit a smaller ionic radius and higher ability to form hydrogen bonds as compared to that of BF4−, also indicated a non-negligible influence of electrostatic and hydrogen bonding interactions. The enhanced rod-shape character and higher orientational order of symmetrically-substituted ILCs can offer additional opportunities in the design of self-assembling non-molecular materials

Orientational order in a liquid crystalline mixture studied by molecular dynamics simulation and NMR

We have studied the orientational order in a liquid crystalline mixture consisting of benzene and 4-n-pentyl-4Ј-cyanobiphenyl ͑5CB͒ employing molecular dynamics simulation and NMR spectroscopy. The temperature-dependent order parameters obtained from the NMR experiments were used to determine the average benzene-5CB and 5CB-5CB interaction parameters. It was found, using mean field theory, that the benzene-5CB interaction is ϳ45% of that between the solvent particles. This analysis is based on a cascade of approximations. The validity of some of these assumptions was tested in the computer simulation. Various pair correlation functions were also calculated

Molecular dynamics simulation of a liquid crystalline mixture

We present results from a molecular dynamics simulation of benzene dissolved in the mesogen 4-n-pentyl-4Ј-cyanobiphenyl ͑5CB͒. The computer simulation is based on a realistic atom-atom potential and is performed in the nematic phase. Singlet orientational distribution functions are reconstructed from order parameters employing several methods, and the estimated distributions are compared with those obtained directly from the trajectory. Transport properties have been studied by calculating translational diffusion coefficients in directions both parallel and perpendicular to the liquid crystalline director. The simulated diffusion coefficients were found to be of the same order of magnitude as those measured in experiments. Second rank orientational time correlation functions are used to investigate molecular reorientations and significant deviations from the small step rotational diffusion model are established. Molecular structure and internal dynamics of 5CB have been examined by correlating the time dependence of dihedral angles with effective torsional potentials

Influence of the Chemical Structure on the Mechanical Relaxation of Dendrimers

The rheological properties of macromolecules represent one of the fundamental features of polymer systems which expand the possibilities of using and developing new materials based on them. In this work, we studied the shear-stress relaxation of the second generation PAMAM and PPI dendrimer melts by atomistic molecular dynamics simulation. The time dependences of relaxation modulus G(t) and the frequency dependences of the storage G′(ω) and loss G″(ω) moduli were obtained. The results were compared with the similar dependences for the polycarbosilane (PCS) dendrimer of the same generation. The chemical structure of the dendrimer segments has been found to strongly influence their mechanical relaxation. In particular, it has been shown that hydrogen bonding in PAMAM dendrimers leads to an entanglement of macromolecules and the region is observed where G′(ω) > G″(ω). This slows down the mechanical relaxation and rotational diffusion of macromolecules. We believe that our comprehensive research contributes to the systematization of knowledge about the rheological properties of dendrimers

Magnetic resonance and its applications

The book provides a basic understanding of the underlying theory, fundamentals and applications of magnetic resonance The book implies a few levels of the consideration (from simple to complex) of phenomena, that can be useful for different groups of readers The introductory chapter provides the necessary underpinning knowledge for newcomers to the methods The exposition of theoretical materials goes from initial to final formulas through detailed intermediate expressions

Magnetic Resonance and Its Applications

XX, 782 p. 234 illus.online resource