Abstract
Cellulose, in the form of purified Texas cotton, was acetylated using a mixture of acetic acid, acetic anhydride, carbon tetrachloride (non-solvent) and perchloric acid as a catalyst over a period of six hours. The rates of acetylation and degradation of cellulose using different catalyst concentrations were studied. By acetylating cellulose, using various acetylation mixtures including zinc chloride as catalyst and ether as a non-solvent, the acetylation and degradetion mechanism for cellulose was postulated. From preliminary experiments, petroleum ether as a precipitant and chloroform-acetone as a mixed solvent were selected for the fractionation of cellulose triacetate. Six fractions were obtained by a fractional precipitation methods from the osmotic pressure measurements carried out on the various fractions in chloroform using a rapid dynamic method, the molecular weight of each fraction was determined. It was shown that virtual non-fractionability of cellulose triacetate with regard to molecular weight occurred in this system. Non-fractionability is explained on the hypothesis of a polymer-solvent interaction involving hydrogen bond formation. Non-fractionability was also observed in attempts to fraction ate it from acetic acid and tetrachloroethane solutions. From osmotic pressure data, the Flory-Huggins' interaction parameter, chi, for the cellulose triacetate/chloroform system was evaluated. Solubilities of cellulose triacetate in different solvents, as well as their solvent power numbers, were determined and the results were successfully interpreted within the scope of the proposed hypothesis. The latter was also shown to be applicable in the case of other polymer-solvent systems. The Mark-Houwink viscosity-molecular weight relationship and Huggins' viscosity-concentration relationship for cellulose triacetate solutions in chloroform, tetrachloroethane and acetic acid were studied and their respective K, alpha and k' constants were evaluated. Salient higher values of alpha for the three solvents are attributed to the inherent stiffness of the cellulose triacetate molecules.