Abstract
Cellulose triacetate was prepared from cotton cellulose employing a heterogeneous method for acetylation, and using zinc chloride as a catalyst. Viscosity-concentration studies for solutions of cellulose triacetate in the mixed solvents, chloroform-acetic anhydride, methylene chloride-acetic anhydride and tetrachloroethane-acetic anhydride were carried out. Molecular weight determinations were carried out on the prepared samples of cellulose triacetate using a high speed membrane osmometer (Hewlett Packard Mechrolab Model 502). The Mark-Houwink viscosity-molecular weight relationship was investigated for the above systems and the respective K and a values were evaluated. The degradation of cellulose triacetate in the solvents chloroform, methylene chloride and tetrachloroethane in the presence of acetic anhydride was investigated and the separate effects of nature of catalyst, catalyst concentration, nature of solvent and temperature on the rate of degradation were studied. The extent of degradation was followed by the decrease in viscosity with time, and molecular weights were evaluated using a singlepoint viscosity relationship to determine limiting viscosity numbers. The catalysts used were ferric chloride, antimony pentachloride, stannic chloride, tellurium tetrachloride, sulphuric acid and perchloric acid. The degradation was shown to be a first order random process which was explained by a mechanism based on the acetylium ion, (CH[3]CO[+]), being the activating catalytic species. Scission of 1-4 oxygen linkages between repeating units is believed to take place with the simultaneous introduction of acetate groups due to reaction of acetic anhydride with the activated complex. Differences in rate constants and activation energies obtained are discussed on the basis of the proposed mechanism, as well as possible differences in chain configuration of cellulose triacetate in the various solvent mixtures.