Cholesteric liquid crystalline character on the surface of chitosan/polyacrylic acid composites

The cholesteric liquid crystalline structure in chitosan/polyacrylic acid c omposite films was studied by surface techniques. A periodical lamellar-lik e structure was observed in the permanganic acid etched film surface by bot h scanning electron microscopy (SEM) and atomic force microscopy (AFM), ins tead of the thumb-print texture which can be detected with polarized optica l microscopy. It is suggested that the periodical lamellar-like structure i s induced by the etching selectivity between cholesteric layers due to diff erent molecular arrangement on the film surface. Four kinds of perpendicula r disclinations, i.e. chi --> tau (-) + lambda (+), chi --> lambda (-) + ta u (+), chi --> tau (-) + tau (+) and chi --> lambda (-) + lambda (+) were f ound in the composite films from SEM observations. The smallest periodicity of lamellar-like structure (equals to half pitch) is 20 similar to 40 nm m easured with AFM.