Effects of magnetic fields on flexural properties of a longitudinal polymer liquid crystal (original) (raw)
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PHYSICAL REVIEW LETTERS, 2012
Using a magnetic Frederiks transition technique, we measure the temperature and concentration dependences of splay K1, twist K2, and bend K3 elastic constants for the lyotropic chromonic liquid crystal sunset yellow formed through noncovalent reversible aggregation of organic molecules in water. K1 and K3 are comparable to each other and are an order of magnitude higher than K2. At higher concentrations and lower temperatures, K1 and the ratios K1=K3 and K1=K2 increase, which is attributed to elongation of self-assembled lyotropic chromonic liquid crystal aggregates, a feature not found in conventional thermotropic and lyotropic liquid crystals formed by covalently bound units of a fixed length.
European Polymer Journal, 1994
Orientational elastic deformations in a magnetic field and phase transition temperatures of a thermotropically mesogenic "combined" aromatic polyester with mesogenic groups, both in the main and in the side chains, have been investigated in the range of 900 to 10,200 mol. wt. It was shown that in this molecular weight range the birefringence of a completely oriented nematic and correspondingly, the degree of its orientational order S are independent on molecular weight. When the relative temperature AT changes from-2 to-30°C the orientational order parameter increases from 0.35 to 0.55. The bend elasticity constants K 3 coincide in the order of magnitude with those for high and low molecular weight nematics investigated previously and their dependence on molecular weight has not been detected.
Journal of Rheology, 1995
An analysis of the dynamic behavior of polymeric liquid crystals is given in terms of a second-rank order parameter tensor describing the orientational state of the local microstructure. The continuum equations governing the dynamics of the material are analyzed under both steady-state and transient conditions for isotropic and liquid-crystalline fluids subjected to shear and magnetic fields. Analysis reveals that a great variety of dynamic behavior for liquid-crystalline materials can be accommodated within the framework of a continuum theory in terms of a second-rank order parameter tensor, and that this dynamic behavior approximates well the orientational behavior obtained with the more complex distribution function theories. Under some conditions the associated rheological behavior of the continuum theory exhibits discrepancies in the normal stress behavior compared to rheological calculations based on the distribution function theory. For simultaneous application of shear and magnetic fields, both the direction and the strength of the magnetic field playa major role in determining the dynamic system response.
Friction and Scratch Resistance of Polymer Liquid Crystals: Effects of Magnetic Field Orientation
Journal of Materials Research, 2004
We have studied PET/0.6 PHB, an alternating copolymer in which PET is poly(ethylene terephthalate) and PHB is p-hydroxybenzoic acid with the mole fraction of 0.6 PHB. It is a longitudinal polymer liquid crystal (PLC) with the LC sequences in the main chain and oriented along the chain backbone. Material not subjected to the magnetic field, specimens oriented along and perpendicularly to the flux of the magnetic field, were investigated. Static friction, dynamic friction, scratch penetration depth, and healing of the material were determined. Static and dynamic friction parameters for oriented samples have significant higher values for oriented samples. The best scratch resistance is found for the sample aligned along the magnetic field. The results are explained in terms of morphology revealed by scanning electron microscopy and phase structures.
Magnetic and magnetorheological characterization of a polymer liquid crystal ferronematic
Journal of Applied Physics, 2009
Cobalt ferrite ͑CoFe 2 O 4 ͒ nanoparticles ͑ϳ12 nm diameter͒ were suspended in the polymer liquid crystal hydroxypropyl cellulose ͑HPC͒/m-cresol to obtain a new type of ferronematic. Suspension of these particles in 35% wt HPC in m-cresol did not affect the appearance of the liquid crystalline phase as evidenced by small angle x-ray scattering. Magnetic measurements performed on the 35% wt HPC/m-cresol/CoFe 2 O 4 ferronematic showed the appearance of ferromagnetic behavior and magnetic hysteresis. In addition, rheometry of the samples showed magnetorheological effect upon application of a dc magnetic field, with the ferronematic having the largest response.
Effect of Magnetic Particles on Structural Changes and Magneto-Optical Behavior of Liquid Crystal
Acta Physica Polonica A, 2020
Structural changes in the nematic liquid crystal 6CHBT doped with a low concentration (1×10 −4 , 5×10 −4) of magnetic nanorods induced by weak magnetic field in the increasing/decreasing mode, as well as in pulsed regime are investigated using surface acoustic waves and magneto-optical effect. The responses of both surface acoustic waves attenuation and optical transmission of the linearly polarized laser beam (532 nm) passing through the sample were investigated. The role of the concentration of magnetic nanorods on structural changes and corresponding magneto-optical behavior concerning the system stability and switching time under applied field was determined. The obtained results of both methods confirmed an effective orientational coupling between particles' magnetic moments and liquid crystal molecules and showed the effectivity of measurements relying on suspension stability and switching processes.
Morphology and dielectric properties of polymer dispersed liquid crystal with magnetic nanoparticles
It has been shown that introduction of magnetic nanoparticles (MN) of various shapes with the concentration wt.% into polymer dispersed liquid crystal (PDLC) causes two effects: the size of liquid crystal droplets decreases, and the amount of the latter with through holes increases. MN increase the effective value of permittivity by more than one order within the frequency range , as well as the electron and ion components of conductivity. MN reduce the exponent in the frequency dependence of the electron component of conductivity. The changes caused by the presence of the nanoparticles quantitatively depend on their shape.
Orientation kinetics of thermotropic main-chain liquid-crystalline polymers in a magnetic field
Polymer, 1991
This work presents a quantitative analysis of the kinetics of orientation of main-chain thermotropic liquid-crystalline copolyesters in a magnetic field. Preferred chain orientation has been induced in samples with weight-average molecular weight up to 14 400. The kinetics of the orientation process have been explored as a function of the time in the field, sample temperature and field strength (up to 1.12 T), and in particular the influence of molecular weight has also been determined. It has proved possible to achieve a degree of preferred orientation up to (P2) = 0.90. Curve fitting the data leads to the determination of maximum (P2) values and characteristic time constants.