Semiflexible Liquid Crystal Research Articles

Evidence of Chemical Reaction in Binary Blends of Polycarbonate and a Semi-Flexible Liquid Crystal Polymer

In this study, we provide the experimental results of the binary blends of a semi-flexible nematic liquid crystal polymer (LCP1) and polycarbonate (PC) within their phase diagram. The LCP1/PC blends were investigated by Differential Scanning Calorimetry (DSC), Optical Microscopy (OM), Wide-angle X-Ray Diffraction (WXRD), Fourier Transfer Infrared (FTIR), Gel Permeation Chromatography (GPC), and Nuclear Magnetic Resonance (NMR) techniques and observed the evidence of chemical reaction between LCP1 and PC in their heat-treated blends possibly by transesterification mechanism.

Interplay of Particle Morphology and Director Distortions in Nematic Fluids.

The existing microscopic theories for elasticity of nematics are challenged by recent findings on systems, whether bent molecules or semiflexible polymers, which do not comply with the model of rigid rodlike particles. Here, we propose an extension of Onsager-Straley second-virial theory, based on a model for the orientational distribution function that, through explicit account of the director profile along a particle, changes in the presence of deformations. The elastic constants reveal specific effects of particle morphology, which are not captured by the existing theories. This paves the way to microscopic modeling of the elastic properties of semiflexible liquid crystal polymers, which is a longstanding issue.

Eutectic liquid crystal polymer blends for self and thermoplastic reinforcement

We studied the eutectic behaviour in binary miscible blends of a rigid and two semi-flexible Liquid Crystal Polymers (LCP) model systems for self and thermoplastic reinforcement. At the eutectic point of the studied LCP-LCP blends, the systems showed suppressed crystal-nematic temperatures and wider nematic range with lower process temperature than the single LCP components. This approach broadens the application of LCPs as high-performance polymers, as well as in reinforcement of engineering plastics. The eutectic points of LCP blends could be a blueprint for development of varieties of multi-component LCP materials.

Self-Assembly of Flexible–Semiflexible–Flexible Triblock Copolymers

The self-assembly of ABA triblock copolymers comprising flexible amorphous poly(ethyl methacrylate) (PEMA, A block) and semiflexible liquid crystal (LC) polyester (B block) was investigated for amorphous block volume fractions (φ) ranging from 20% to 80%. Two copolymer series with different LC block molecular weights (Mn,LC) were examined. At φ < 55%, block copolymers with Mn,LC = 11 600 formed lamellar microdomains in which LC segments mostly extended along the lamella normal but folded to fit in the lamellae. When φ was augmented, the LC segment fold (Nfold) increased to enhance the interface between the LC and amorphous segments and counterbalanced the increase in amorphous lamella thickness by reducing the LC lamella thickness. When φ > 68%, the LC lamellae were divided into cubes, transforming into spheres in the PEMA matrix. When Mn,LC decreased to 6300, the copolymers showed similar morphology. However, the lamellae adopted zigzag configurations showing a greater tilting angle between the lamella normal to the LC chain axis with increasing φ. Thus, the LC and PEMA segments enhanced their interface by mutually sliding along the chain axis instead of increasing Nfold.

Quadrupolar moment calculations and mesomorphic character of model dimeric liquid crystals

Model dimeric compounds CH 3(CH 2) n COOROOC(CH 2) p COOROOC(CH 2) n CH 3, where (R=C 6H 4C(CH 3)HCC 6H 4), of semiflexible liquid crystal polymers (LCP) have been studied by molecular modeling techniques. First, a conformational analysis of the molecules was made in order to find the lowest energy conformation structure. In a second stage, geometry optimization calculations of these conformers were performed at the quantum mechanics semiempirical PM3 level. Some molecular parameters (electric dipolar and quadrupolar moments) were obtained at the improved ab initio Hartree–Fock HF/6-31G(d)//PM3 level and are related with the mesomorphic character of these systems. It is found that the trace of the quadrupole moment tensor correlates with the spacer ((CH 2) p -linkage) length, and this can be an indication of the trend that this magnitude exhibits in aromatic liquid crystal systems.

Mean-field theory for the phase transitions of nematic polymers in a magnetic field: application to magnetic birefringence

With the help of a theory which is based on functional integrals, we calculate the evolution, with temperature and degree of polymerization, of the orientational order parameter of thermotropic nematic polymers under a magnetic field. We compare our results with those obtained experimentally from magnetic birefringence measurements on polyesters. The validity of the application of the mean-field theory in the case of semiflexible liquid crystal polymers which consist of sequences of mesogenic units and flexible spacers is also discussed.

Monte Carlo study of the thermodynamic stability of the nematic phase of a semiflexible liquid crystal model

This paper reports a Monte Carlo (MC) study of the ordering in a semiflexible liquid crystal model designed to give a crude representation of 4-n-octyl-4′-cyanobiphenyl (PCB). A mechanically stable nematic phase is obtained and the question of its thermodynamic stability is addressed by comparing the free energy to that of the isotropic phase. The free energies are calculated by thermodynamic integration using an efficient configuration biased MC scheme. Pair distribution functions, orientational correlation functions, and conformational properties of the nematic and isotropic phases are contrasted. In the isotropic phase the equation of state is compared with theoretical predictions for systems of convex molecules and chains made up of tangent hard spheres.

Effect of an adsorbing surface on the phase behavior of a confined semiflexible liquid crystal

Results of a study of the nematic to isotropic transition of a liquid crystal near an adsorbing surface in a thin film using Monte Carlo simulation are presented. The order parameter profile, heat capacity and overall order parameter are calculated to characterize the system as it undergoes the transition. We show that the strength of the adsorption energy has a large influence on the phase behavior of the system. At low adsorption energies a boundary layer is present which undergoes a transition from a nematic to a partially ordered phase at the reduced temperature τ1 and then undergoes a transition to an isotropic orientation at the reduced temperature τ2 which is separate from the bulk. The bulk undergoes an order–disorder transition at a temperature τ3 between the two orientation transitions τ1 and τ2 of the boundary layer. For strong adsorption energies, the boundary layer increases to the thickness of the sample. In this regime, the complete system undergoes the nematic–isotropic transition simultaneously and the temperature of this transition increases linearly with the adsorption energy. The case of intermediate adsorption energies corresponds to a crossover between the above two limiting situations. Here, the boundary layer is more disordered than the bulk and, yet, the entire sample undergoes the nematic–isotropic transition simultaneously. The observed phase behavior is explained in terms of the influence of the boundary layer on the order of the system. The present situations suggest the existence of a critical adsorption energy above which there is only one transition but below which several transitions occur due to the intervention of the boundary layer.