MACROSCOPIC FEATURES. b) CholestericsPOLYPEPTIDE LIQUID CRYSTALS : DIAMAGNETIC ANISOTROPY, TWIST ELASTIC CONSTANT AND ROTATIONAL VISCOSITYCOEFFICIENT

The observed anisotropy of the diamagnetic susceptibility, AX, for oriented nematic liquid crystals of poly-y-benzyl-L-glutamate (PBLG) in dichloromethane is 6.8 &0.7 x 10-9 emu/cm3. For a PBLG axial ratio of 150, a value of the twist elastic constant K22 = 582 x 10-8 dynes is calculated by measuring the critical magnetic field for untwisting the cholesteric supramolecular structure in the PBLG liquid crystal. The rate of reorientation of the nematic director of an aligned nematic PBLG structure yields a value for the rotational viscosity coefficient, y 1=4.4 x 104 poise. K22 and y l appear to be sensitive to the polypeptide axial ratio and solvent composition. 1 . Introduction. Synthetic polypeptides form cholesteric lyotropic liquid crystals in various helicogenic solvents. Specific solvent-polymer interactions are not responsible for this behavior. Liquid crystal formation is spontaneous when the rodlike, a-helical polypeptides exceed a critical concentration in solution [I]. Experimental observations in reasonable agreement with theory [2], show that the critical polypeptide concentration depends primarily on the polymer axial ratio (i. e. polypeptide molecular weight). The asymmetry of the intermolecular forces between helices causes a small angular displacement of one rod relative to another. This generates a macroscopic helicoidal supramolecular structure analogous to the cholesteric structure exhibited by thermotropic liquid crystals consisting of asymmetric molecules. A sufficiently strong magnetic [3a-c] (electric [ 3 d ] ) fields exerts a torque on the anisotropic diamagnetic (dielectric) rods, and untwists the cholesteric structure (*) This investigation was supported in part by a NIH research grant (PHs Grant AM, 17497-01) from the National Institute of Arthritis, Metabolism, and Digestive Diseases and The University of Connecticut Research Foundation. (**) Present Address : Guest Worker, NASA Ames Research Center, Moffet Field, California. (***) Address correspondence to : E. T. Samulski, Institute of Materials Science, U-136, University of Connecticut, Storrs, Conn. 06268. to form an aligned nematic structure. In this arrangement, the rodlike molecules are more or less parallel to the nematic director which in turn, is parallel to the applied field. Herein we show that the magnetic field-induced cholesteric-nematic transition can be described satisfactorily by de Gennes' theory [4]. The twist elastic constant of the liquid crystal, Kz2, can be determined using measurements of the critical field strength, H,, for untwisting the cholesteric structure and the diamagnetic anisotropy, AX, of the aligned nematic. The twist or rotational viscosity coefficient, y,, can be calculated from the rate of reorientation of the nematic director after changing the direction of the applied field. 2. Experimental. All of the magnetic measurements were made at room temperature in magnetic fields up to 20 kOe with a P. A. R. model 155 Vibrating Sample Magnetometer. (See ref. [ 5 ] for experimental details.) The spacing between the retardation lines was on the order of tens of microns. Therefore, for various magnetic field strengths, the pitch of the cholesteric structure (twice the retardation line spacing) could be measured by direct optical observation [5] . The synthetic polypeptides were supplied by Pilot Chemical Company (New England Nuclear). Polypeptide solutions were matured for one or more weeks to insure complete solubilization of the polymer and Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1975145 C1-270 C. GUHA SRIDHAR, W. A. HINES AND E. T. SAMULSKI homogeneity of the liquid crystal. In most cases, the have shown that de Gennes' theory applies equally development of the pattern of retardation lines characwell to the lyotropic cholesteric polypeptide liquid teristicof the cholestericstructure occurred afterseveral crystal [7]. In figure 1, we have plotted PIPo versus days. HIH, for the PBLG-dichloromethane liquid crystal, 3. Results and discussion. 3.1 DIAMAGNETIC ANISOTROPY. Unless otherwise indicated, all of the experimental magnetic and optical results described in the remaining sections were obtained from a sample of liquid crystalline polypeptide solution consisting of poly-y-benzyl-L-glutamate (PBLG) and dichloromethane, 25.5 % (wt./vol. ; PBLG M. W. = 550,000). After allowing the sample to mature for two weeks to insure homogeneity, the retardation lines develop and the liquid crystal supramolecular structure is cholesteric with the twist axis randomly oriented throughout the macroscopic sample. In a magnetic field, the cholesteric structure is untwisted and a macroscopically aligned nematic supramolecular structure is formed. During the field-induced cholesteric-nematic transition, a time dependent anisotropy in the observed magnetic moment, Ap(t), develops in the liquid crystal (see ref. [5]). By observing Ap(t) as a function of time, we can obtain a value for the diamagnetic anisotropy per mole of the aligned nematic polypeptide liquid crystal using Ap = Ap(@)/nH, where Ap(co) is the asymptotic value of Ap(t) for large t, H is the applied magnetic field and n is the number of moles of polypeptide residues. For the sample described above, we determined a value of AX = 6.8 & 0.7 x emu/cm3. Similar measurements were made on a PBLG-dioxane (+ 2 % trifluoroacetic acid) liquid crystal sample, 21.4 % (wt./vol. ; PBLG M. W. = 310,000). For this liquid crystal AX = 6.5 f 0.7 x emu/cm3. 3.2 TWIST ELASTIC CONSTANT. In 1968, de Gennes [4] derived a theoretical relationship between the pitch of a cholesteric structure, P, and the applied magnetic field, H. He predicts an increase in the pitch as. the applied field is increased with a logarithmic divergence of P when the field strength approaches a critical value, H,. This behavior has been verified for thermotropic liquid crystals [6]. Duke and DuPr6