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Many cellulose derivatives form lyotropic and thermotropic cholesteric liquid crystalline phases.1s2 The formation of these ordered phases is generally attributed to the relative stiffness of the cellulose chain's3 and by this criterion cellulose itself should also form liquid crystalline solutions in those few solvents in which it dissolves. (A thermotropic liquid crystalline phase is unlikely because, on heating pure cellulose, it decomposes before melting.) However, the evidence for a lyotropic liquid crystalline phase of pure cellulose is somewhat equivocal; observations in N-methyl morpholine N-~xide/water,~ in trifluoroacetic acid/chlorinated alkanes,5 and in lithium chloride/ N,N-dimethyl a~e t a m i d e~.~ do indicate formation of anisotropic solutions, but because of the strong tendency of cellulose to aggregate or crystallize, the fluid liquid crystalline state is often metastable?Under some conditions, solid films prepared from cholesteric liquid crystalline phases may retain the helicoidal supramolecular structure that is characteristic of cholesteric fluids? (Hydroxypropyl)cellulose, when cast from aqueous solution, forms cholesteric films" that, on heating, preferentially reflect circularly polarized light at visible and UV wavelengths. Recently, cellulose films prepared by two different techniques have been shown to display a cholesteric structure." These films were prepared (a) by slow precipitation from lithium chloride/dimethyl acetamide solution and (b) by deacetylation of cellulose acetate or triacetate films cast from liquid crystalline solutions in trifluoroacetic acid. The helicoidal structure was inferred from the induced CD peaks recorded for cellulose films dyed with Congo red." However, induced CD cannot yet indicate the pitch or handedness of the helicoidal cholesteric structure.In this communication, we present direct electron-microscopic evidence for the helicoidal structure of films of cellulose acetate and of regenerated cellulose. Although not directly applicable to fluid liquid crystalline phases, transmission electron microscopy is a useful technique for examining solid films and fibers for "frozen-in'' liquid crystalline order (see, for example, Ref. 12). EXPERIMENTALThree different cellulose films were prepared as described previously." First, a "precipitated film," was prepared by the slow addition of water vapor to a solution (0.75% by weight) of dissolving grade wood pulp (average degree of polymerization 890) in lithium chloride/dimethyl acetamide. After exposure to water vapor for four days, a swollen gel of cellulose precipitated. The precipitated gel film was washed repeatedly with water and dried.

It is now well known that cellulose derivatives form cholesteric liquid crystalline phases in the bulk and in solution. The supramolecular arrangement of the cholesteric materials gives them unique optical properties. Light entering the cholesteric domain is partially reflected. Circularly-polarized light with the same handedness as the helicoidal structure and a wavelength corresponding to the cholesteric pitch will be reflected by the mesophase. Free-standing films displaying a cholesteric order are thus strongly desirable for many different applications.Even though transmission electron microscopy (TEM) seems to be the perfect technique to extend results obtained by optical microscopy, this approach has been used infrequently in the study of liquid crystals. Liquid crystals don't lend themselves to electron microscopy because of their fluidity. Lyotropic systems are destroyed by evaporation of the solvent under high vacuum. However, under specific conditions, solid cellulosic films can retain a helicoidal organization. Classical embedding and ultra-sectioning can thus be applied on such materials as long as they do not dissolve in water.