Salt Addition Effects on Mesophase Structure and Optical Properties of Aqueous Hydroxypropyl Cellulose Solutions

Abstract

The effects of addition of neutral salts on the cholesteric mesophase structure and optical properties of aqueous solutions of hydroxypropyl cellulose (HPC) were investigated by spectrophotometry and an NMR technique with D 2 O as solvent. In the measurements of selective light-reflection spectra, the wavelength of maximal reflectance and therefore the cholesteric helical pitch (P), too, varied systematically with a change in strength of a socalled "chaotropic" effect of the additive salts; i.e., their effectiveness in increasing P was pronounced in the order of Cl − <Br − <NO 3 − <I − <SCN − and Cs + <K + <Na + <Ca 2+ <Li + ≤Mg 2+ <Al 3+ , when compared in a series of salts with a common sort of cation or anion. An LCST-type of phase separation behavior of the HPC/water system was also affected by the coexistence of salt ions; the cloud point shifted either down or up in almost the same manner as that found in the salt-sort dependence of the cholesteric periodicity. In the deuteron NMR study, a frequency width of the quadrupolar splitting, detected as a measure of the molecular orientation development within the mesophase, was found to be less affected by the salt addition. It is suggested that the hydrophobic bond-disrupting or promoting activity of the salts added could regulate subtly a conformational state of the cellulosic polymer, to induce a marked change in cholesteric twisting power without disturbing the orientational ordering in each of the pseudonematic layers. KEY WORDS Hydroxypropyl Cellulose / Liquid Crystal / Salt Addition / Chaotropic Effect / Cholesteric Pitch / Cloud Point /

Addition effects of imidazolium salts on mesophase structure and optical properties of concentrated hydroxypropyl cellulose aqueous solutions

“…As shown in our previous studies, [13][14][15] the cholesteric structure and LCST-type phase-separation behavior, and the ensuing optical properties of the lyotropic system of HPC in water, are significantly affected by the addition of a small amount of inorganic salts as the third component. For instance, when LiCl is dissolved in the cholesteric system, the wavelength (l M ) of maximal light reflectance and the cloud point (T c ) shift, respectively, to the blue side and to the lowertemperature side.…”

“…(In this study, the DS value of the HPC sample was determined on the basis of 1 H NMR data according to a method proposed by Ho et al 29 Some of the previous articles 13,17,18 reported DS¼1.8 for the same grade (Lot#5) of HPC from the same commercial origin; the value was determined by quantitative-mode solid-state 13 C NMR measurements.) 30 …”

“…This phase-separation behavior of aqueous HPC solutions would also be affected by addition of imidazolium salts and the changing manner should depend on the ion species and concentration of the used one, as in the case of the previous HPC/water system containing conventional inorganic salts. 13 T c -shifting effects of [CnMim][Br] and of [C4Mim][X] actually observed for the present HPC/imidazolium salt/water system are illustrated in Figures 4a and b, respectively, in which all T c data were obtained at a 62.5 wt% HPC concentration eliciting a clear cholesteric ordering in the respective solutions and are plotted against the added salt concentration; however, T c ¼29 1C for a salt-free reference sample is marked by a dotted line. As can readily be inferred from the examples, the effectiveness of a given imidazolium salt in altering T c , involving the directional sense and extent, would be determined by the arithmetic sum of the respective effects of its constituent ions.…”

“…As for bromine anion, it works as a weak depressant agent to the LCST of the HPC/water system, as has been established formerly, together with several other inorganic ions. 13,14,16 From the plot of T c for the 62.5 wt% HPC samples containing [C6Mim] [Br] in Figure 4a, it can be declared that [C6Mim] + acts as a T c enhancer, at least at this HPC concentration; the elevating effect would have offset, or rather, surpassed the T c -depressing potency of Br -, resulting in a net increase in T c , which should be more pronounced with increasing salt concentration. The T c vs salt concentration plot for the [C4Mim] [Br]-containing samples in the same figure resembles that for the previously investigated LiBr-containing samples of the corresponding HPC content, 13 in the decreasing rate with salt concentration.…”

Biocompatible and Sustainable Optical Strain Sensors for Large‐Area Applications

“…[13,14] Cholesteric liquid crystals are a promising class of responsive optical materials known for their capability to respond to various external stimuli, such as mechan ical stress, [15,16] electric field, [17,18] temperature, [18] and light. [15,19] Likewise, the reflected color of HPC can be tuned simply by sol vent nature, [14] concentration of the polymer [20] or salt, [21] tem perature, [10,14,21] and by applying shear deformation. [22] In this work, we demonstrate a low cost method for the fabri cation of biocompatible strain responsive photonic sensors based on an aqueous HPC mesophase.…”