NMR Studies on Orientation of Liquid Crystals of Poly-γ-Benzyl-L-Glutamate in Magnetic Fields

ABSTRACT:The change of pitch for the helical structure of cholesteric liquidcrystalline solutions of poly(r-benzyl-L-glutamate) and of poly(r-ethyl-L-glutamate) in static magnetic fields has been observed by means of diffraction of light from a laser. Both solutions sense the influence of magnetic fields though not so strongly in the latter case as in the former case. Values of the elastic moduli, k22 and kaa, characterizing the structure have been evaluated. The value of k22l(xp-Xt) ranges from 1 to 10, kaa being almost equal to, or slightly smaller in magnitude than k22. The magnetic anisotropy, XP-Xt. would seem to decrease when the polymer concentration is decreased.KEY WORDS Liquid Crystal I Cholesteric Structure I Polypeptide I Magnetic Susceptibility I Electric Field I Diffraction of Light I Polarizing Micrograph I

Section: Effects Of the Field Strength On The Pitchmentioning

confidence: 79%

Section: Methodsmentioning

confidence: 99%

The Effects of Magnetic Fields on the Structure of Cholesteric Liquid Crystals of Polypeptides

Iizuka

1973

“…However, the solvent signals can still be observed in the liquid crystalline state, though they are much broader than those in isotropic solutions and are split into a doublet. Sobajima 1 was the first to observe this dipolar splitting. Orwoll and Vold 8 showed theoretically and experimentally the time dependence of methylene chloride dipolar splitting on the average orientation of the polymer helices with respect to the external field.…”

mentioning

confidence: 94%

NMR Study of the Time Dependent Dipolar Splitting of Methylene Chloride Protons in Poly(γ-benzyl L-glutamate)

Yokoyama

Arai

Nishioka

1977

Poly(r-benzyl L-glutamate) in a concentrated methylene chloride solution is in a liquid crystalline state. The solvent signal of this solution is a doublet due to magnetic dipole-dipole interactions. Separation of the doublet varies with residence time of the sample in the magnetic field after insertion in the NMR probe or after changing the direction of the applied field. This is attributed to the time dependence of the orientation or reorientation of molecules in the magnetic field.The time dependent phenomena were observed by 1 H-FT-NMR and a floppy disk drive system. That is, after changing the direction of the applied field the peak height of the upfield one of the methylene chloride doublets first decreases gradually relative to the downfield one, and still later the peak height of the upfield one increases, and finally both peaks become of equal height. This is interpreted through the anisotropy of the diamagnetic susceptibility Llx for oriented nematic liquid crystals. The theory is given and the potential for obtaining structural information about nematic liquid crystals of polypeptide is discussed.KEY WORDS NMR / PBLG / Liquid Crystal / Methylene Chloride / Orientation / Nematic / Cholesteric / Polypeptide / Poly(r-benzyl L-glutamate) (PBLG) is well known to form lyotropic liquid crystals in various helicogenic solvents, and has an a-helix conformation in these solvents.In the nematic liquid crystalline state, the rod-like molecules are arranged parallel to one another. The packing of rod-like helices appears to be locally hexagonal on a microscopic scale. 4 Specific solvent-polymer interactions are not responsible for this behavior. Liquid crystal formation is spontaneous when rod-like, a-helical polypeptides exceed a critical concentration in solution. Experimental observations are in reasonable agreement with the self-ordering theory, 5 and show that the critical polypeptide concentration depends primarily on the axial ratio of the rod-like polymer molecules. 7 the polymer are not averaged out. Therefore, the NMR signals of PBLG become so broad that they can not be observed by the ordinary high resolution NMR technique. However, the solvent signals can still be observed in the liquid crystalline state, though they are much broader than those in isotropic solutions and are split into a doublet. Sobajima 1 was the first to observe this dipolar splitting. Orwoll and Vold 8 showed theoretically and experimentally the time dependence of methylene chloride dipolar splitting on the average orientation of the polymer helices with respect to the external field.When the PBLG molecules in methylene chloride solution forms a liquid crystal, the polymer segments are so restricted that the direct dipole-dipole interactions of protons in It was expected originally that the doublets would have the same peak height and width, but unequal peak heights of the doublets were reported in a previous study,8 and a detailed discussion was not given. The doublets show equal peak heights at equilibrium, but transient ...

“…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][4][5][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.…”

mentioning

confidence: 99%

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

Imai

Mizunoya

Hiraoka

et al. 2004

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 ...