Distinct Temperature Dependence of Cholesteric Pitch in Lyotropic Cholesteric Solutions of Polypeptide

Abstract

Examination was made for the thermotropic liquid crystalline nature of oc-helical polypeptides. 1 -8 The type of liquid crystal formed was cholesteric because of the chirality of constituent macromolecules and several cholesteric properties were reported. One most interesting property is the strong temperature dependence of cholesteric pitches, and detailed studies were performed using the representative materials, poly[(y-benzyl L-glutamate)-co-(y-dodecyl L-glutamate)s] (PBDoLG) which formed a cholesteric mesophase in a temperature region above 60--100°C. 5 • 7 • 8 For these materials, cholesteric pitch (P) was comparable to the wavelength of visible light in an initial mesophase temperature region, increased substantially with rising temperature, and diverged at TN. As temperature was raised further, the cholesteric helical structure again appeared and the pitch decreased. The cholesteric sense inversion from righthanded helix to left-handed one was clarified to be responsible for this rewinding phenomenon. The temperature dependence of pitches is described by the following empirical equationwhere ljP 0 and Tc are constants characteristic to the rna terial. 7 • 8 Equation 1 is similar to that deduced theoretically by Kimura et a/. 9 • 10 Although the cholesteric twist results directly from the asymmetric carbons, namely the dispersion force as presented by Goossens, 11 they consider that in the specific polymeric system, the twist would be also forced by steric interaction due to the screw-like van der Waals surfaceY Both independently affect not only the magnitude but also the sense of cholesteric helix. In such a case, the temperature dependence of the pitch appears complicated and as one possibility, the thermally-induced sense inversion is expected if twist arises with opposite sense. The polypeptides may be a typical example of this class of materials since side chains surrounding the oc-helical main chain take the helical arrangement.In this study, examination on the temperature dependence of cholesteric pitches was extended to lyotropic cholesteric mesophases of PBDoLG dissolved in dodecyl alcohol. Temperature dependence was similarly observed as in the thermotropic system and found to be well described by the eq 1. Constants included in the eq l were uniquely determined for all cholesteric solutions with polymer concentrations from 100 val% to 40vol% and concentration dependence is shown. EXPERIMENTALThe polymer used here is PBDoLG with dodecyl content of 39%, prepared by ester interchange reaction between poly(y-benzyl L-glutamate) (DP= 250) and dodecyl alcohol described elsewhere. 3 The dodecyl alcohol was used as supplied by Tokyo Kasei Kogyo Co., Ltd. The polymer was dissolved into dodecyl alcohol at temperatures above 60°C. The volume fractions (val%) of polymers were evaluated from weight fractions (wt%) assuming the additivity of densities of the two components; 1.17 g ml-1 for PBDoLG and 0.83 g ml-1 for dodecyl alcohol.Cholesteric pitch was determined by circular dichroism (CD) using a JASCO a...

Mesomorphic properties of chiral nematic star-shaped liquid crystals containing melitose as cores

“…During the last decade, cholesteric liquid crystals (ChLCs) have attracted considerable interest for their unique optical properties, such as thermochromism, selective reflection of light and circular dichroism and advanced applications for instance nonlinear optical devices, full-color thermal imaging, and organic pigment [1][2][3][4][5][6][7][8][9][10][11]. ChLCs usually contain cholesterol derives LCs and chiral nematic (N * ) LCs.…”

Nacre Is a Liquid-Crystal Thermometer of the Oceans

“…This decrease in pitch does not occur in every case; it is possible, but less common, to have a positive pitch dependence with temperature, 17 for example, in cellulose (supramolecular) liquid crystals, 18 or divergent behavior showing the two opposite effects as a sense inversion in different temperature ranges. 19,20 Pitch is chosen as an indicator of cholesteric behavior in molecular liquid crystals, where cholesteric layers are not a mathematical reality. In this supramolecular liquid crystal, however, the director within a layer is disordered, and it is the individual layer spacing that is the more relevant quantity.…”

“…Thus, there is a shorter distance between layers of parallel rods: the pitch of a cholesteric liquid crystal generally becomes shorter. This decrease in pitch does not occur in every case; it is possible, but less common, to have a positive pitch dependence with temperature, for example, in cellulose (supramolecular) liquid crystals, or divergent behavior showing the two opposite effects as a sense inversion in different temperature ranges. , …”