Microscopical Observations on Partially Acetylated Cottons and Related Fibers

Effects of acetylation on the morphology and submicroscopic structu e of cotton have been studied by light and electron microscopy. The cross-sectional area of partially acetylated (PA) fibers increases with degree of substitution. The average refractive index of PA cotton decreases with increase in acetyl content, as do the indices measured parallel and perpendicular to the fiber axis. Refractive index measurement appears to be a feasible method for estimating the chemical composition of PA cotton. At early stages of esterification, unevenness of reaction along the length of fibers may be demon strated by dyeing and swelling techniques. Acetylation causes the surface of cotton to become smoother, and obliterates the micro- fibrillate pattern characteristic of scoured fibers. PA cotton fibers swell when embedded in methacrylic esters by polymerization. The swelling causes separation of the lamellae of the secondary wall, and permits electron microscopical observations on the interior elements of the fiber to be made on thin cross sections of the specimen. Fragments of PA cotton obtained by wet- beating show progressive loss of the fibrillate character of unacetylated cellulose as the acetyl content increases. At high degrees of substitution, the PA cotton fragments resemble the spongy particles obtained from wet- beaten acetate rayon and Arnel. The results of microscopical examination are considered in terms of fiber structure.