Cross-effects between Mechanical and Electric Fields in Liquid Crystalline Solution of Poly(γ-benzyl L-glutamate)

Abstract

KEY WORDSPoly(-benzyl L-glutamate) / Liquid Crystal / Shear Deformation / Electric Field / Cross-effect / Poly(-benzyl L-glutamate) (PBLG) forms a liquid crystal (LC) in a helicogenic organic solvents above a certain concentration.1,2 The cholesteric liquid crystalline texture formed by PBLG has been shown the norm due to the optical activity of the PBLG molecule, and nematics under some particular conditions are rather exceptional.3-6 Since a PBLG molecule in helical conformation has a large permanent dipole moment along the helical axis, 7 alignments of dipoles in the cholesteric texture and properties in electric field are very interesting. Application of strong electric field exceeding the threshold value is expected to align the rod-like molecules in the direction of electric field, resulting in a transitional change from cholesteric to nematic texture. In fact, it is the case.8 On the other hand, the mechanical field, i.e., the shear deformation can also bring about the alignment of molecular axes.9 Therefore the coupling between electric and mechanical fields through molecular orientation is expected to occur appreciably. To see this coupling, we have investigated effects of electric field on mechanical properties and effects of shear deformation on electric properties, i.e., the cross-effect, for PBLG solution in helicogenic solvent, 1,1,2,2-tetrachlorothane. We have found large cross-effects between electric current and dynamic viscosity with elasticity, which will be reported here. EXPERIMENTAL Materials and SamplesPoly(-benzyl-L-glutamate) (PBLG) was synthesized by the N-carboxyanhydride method in chloroform with triethylamine as initiator, and subsequently precipitated in hexane. The crude PBLG was repeatedly precipitated by cooling a dilute solution of the polymer in mixed solvent of 1,1,2,2-tetrachloroethane (TCE) and hexane. The viscosity-average molecular weight was 6 Â 10 4 . TCE was a product of Wako Pure Chemical Industries, Ltd. and used as received. Concentration of sample solution was fixed at 0.30 g/ mL. The concentration range of isotropic-to-cholesteric transition around room temperature for the TCE solution of the present PBLG sample is located at concentrations lower than 30 wt%, i.e., the concentrations of the A point and the B point are approximately 14 wt% and 19 wt%, respectively, so that the sample solution used here is in a cholesteric liquid crystalline state under no electric field. Electric and rheological MeasurementsThe apparatus was a RheoStress of Haak Co., RS-75, which is a stress-control type rheometer, modified to be able to apply an electric field, and is illustrated in Figure 1. Sample solution was loaded between rotators of parallel plate type with 35 mm diameter. The plates were separated by 0.2 mm, between which electric field can be applied perpendicular to the plates, using a generator, Multifunctional Synthesizer 1920A produced by NF Electronic Instruments Co. The electric current was measured by a home-made electric current-to-field translator, being monitored ...