Handedness cellulosics

Werbowyj and Gray (79) examined the relationships between the cholesteric pitch and optical properties of HPC in water, CH3COOH and CH3OH. The reciproc pitch varied as the third power of the HPC concentration. Optical rotatory dispersion results show HPC has a right-handed superhelicoidal structure regardless of structure. As will be discussea below, a change in solvent can reverse the handedness of other cellulose derivatives. 

Ritchey et al. (113) showed the introduction of trifluoroacetate groups at the unsubstituted hydroxyls of cellulose acetate causes a reversal in handedness of the cholesteric structure. Likewise the introduction of an aceto group in acetox3rpropyl cellulose changes the twist (116). 

As Guo and Gray (114) point out, the relationship between the handedness of the supramolecular helicoidal structure and the molecular structure 01 cellulose derivatives and solvents is not at all understood" at the present time. 

Further challenges need to be addressed for the functional development of liquid-crystalline cellulosics, coupled with the elucidation of fundamental aspects of their chiral nature. In addition, the potential thermotropicity of cellulose microcrystallites with the surfaces modified adequately and manipulation of the cholesteric sense (handedness) in the helically arranged molecular assembly are embryonic but fascinating subjects in this research field. 

Lyotropic liquid crystalline cellulose derivatives exhibit unique optical properties because of their helicoidal supramolecular structure.The chiro-optical properties of the helicoidal structure can be described by a pitch p (or its inverse, the twist p ) p = 2o/fi, where is the reflection wavelength and h is the mean refractive index of a sheet, and the corresponding handedness of the twist right-handed helicoidal structure being assigned to a positive pitch p > 0) and left-handed helicoidal structures to a negative pitch p < The nematic mesophase can be... 

AEC) in chloroform, with the increase of degree of acetylation, the rigidity of the cellulose chain increases monotonically. However, the pitch of the mesophase first increases to infinity and then decreases with an inversion of the handedness of the chiral nematic phase, from left-handed to right-handed. 

The theory predicts that the handedness of cellulosic liquid crystalline solutions, designated by the sign of the pitch, depends not only on temperature (T) and on steric repulsion of the chain X), but also on an attractive interaction parameter, %, which depends on the nature of the solvent. The chiral forces are balanced when (x XkT) = 0. In this compensated condition, the pitch of the mesophase should become infinite, and the mesophase resembles a normal nematic phase. 

The values of pitch for lyotropic AEC solutions in AA (50% w/w) are given in Table 1, where AEC-2 is a pure AEC with medium DA and AEC-3 is the mixture of EC and fully acetated AEC, which has the same average DA as AEC-2. However, in the liquid crystalline solutions, even at the same concentration, AEC-2 and AEC-3 have different pitch and handedness. This phenomenon was also observed in the lyotropic AEC/chloroform system. The difference in chiro-optical properties may come from the complex interactions of multiple chiral centers present in each repeating unit of the cellulose chain, not from simple racemic mixtures as in the PBG system. 

The threefold helices of cellulose are chiral, l.e., their left-and righthanded conformations are not Identical. It is not yet known whether the structure of Na-cellulose IIB Is characterized by one particular helix handedness, as both conformations are stable and of not very different conformational energy. The x-ray diffraction diagram (cf. Fig. 2B) Is rich In detail and It should be possible to determine the handedness of the Na-cellulose IIB helix from a detailed x-ray refinement. 

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