Civil Engineering Reference
In-Depth Information
Figure 14.6
Optical microscopy of an aqueous suspension of microcrystalline cellulose at a
concentration of 16 wt%, showing a nematic i eld 10 minutes (a), and a chiral nematic phase typical
i ngerprint texture 1 day at er suspension's preparation (b) [116].
In this context, literature [90] states that at room temperature, acetoxypropyl cellu-
lose exhibits both chiral nematic phases—the lyotropic and the termotropic one. When
subjected to specii c conditions of shear l ow, the cellulose derivative cholesteric liquid
crystal suf ers transformations, such as cholesteric helix and cholesteric-to-nematic
transition. h e i lms prepared from anisotropic solutions of termotropic acetoxypropyl
cellulose in an isotropic solvent exhibit anisotropic mechanical properties, generated
by the molecular orientation of the solution under shear stress. h us, liquid crystalline
solutions give rise to i lms with anisotropic mechanical properties; the i lms are brittle
when stretched parallel to the shear direction and ductile when stretched perpendicu-
lar to it.
h e lyotropic behavior of ethyl cellulose in various solvents (methanol, ethanol, diox-
ane, acetic acid, acetic anhydride, m-cresol, phenol, etc.) was studied as a function of
the critical concentration of the liquid crystal phenomenon produced by refractometry,
polarized light microscopy, and optical transmission [118, 119]. It has been observed
that critical concentration increases with decreasing the solubility of the solvent, and
also that the critical concentrations for alcohols were much higher than those of other
solvents with similar solubility parameters.
In cellulose derivatives such as hydroxypropyl cellulose, cellulose acetate butyr-
ate, ethyl cellulose, cellulose triacetate, etc., formation of the liquid crystalline phase
