Salt Addition Effects on the Phase Behavior of Liquid-Crystalline Cellulose Derivatives and Dynamic Control of their Optical Properties by Electrical Stimulation

点击翻译按钮获取中文摘要

Many cellulosic polymers are capable of forming a 'cholesteric' liquid-crystalline phase in a condensed fluid state. Some concentrated solutions and/or melts of cellulose derivatives are colored due to selective reflection of visible light, originating from the cholesteric periodicity. As a typical example, (hydroxypropyl) cellulose (HPC) is known well to exhibit this optical property in aqueous solutions at polymer concentrations of ca. 50-70 wt%. The wavelength (lambdaM) of maximal reflectance can be related to the cholesteric pitch (P) by the de Vries equation, lambdaM=n^^-・P,with an average refractive index n^^- of the liquid-crystalline system. In the present work, we investigated the effects of addition of neutral salts on the phase behavior and optical characteristics of aqueous HPC solutions by spectrophotometry and NMR techniques. Furthermore, we made a novel attempt to control the selective light reflection of the HPC/salt/water liquid crystals by electrical stimulation. The … More following results are worth noting :1. The value of lambdaM and therefore that of P,too, increased with an increase in strength of a so-called chaotropic effect of additive salts ; this effect becomes pronounced, e.g., in order of Cl^-<Br^-<NO_3^-<I^-<SCN^-, when compared in a series of salts with a common cation sort.The addition of various salts into the HPC/water system also affected the LCST-type phase separation behavior. The effectiveness of a given salt in shifting the cloud point (Tc) either down or up was very similar to that of the salt in altering the cholesteric pitch.2. Selected samples of the HPC/salt/water liquid crystals were sandwitched between parallel glass plates separated by a pair of platinum (Pt) plates as a spacer and electrode. Then an electric potential difference was applied between the Pt electrodes. It was found that the reflective coloration and even the optical turbidity of the aqueous cellulosic liquid crystals were changeable by the action of a relatively weak electric field (3.0 V/cm). This is possibly due to electrophoretic migration of the salt-constituent joins as perturbant of the cholesteric ordering. Less