Fractionation of cellulose acetate for the investigation of molecular weight influences on the morphology of membranes

Cellulose acetate (CA) with an apparent weight average molar mass, Mw∗, of 150 kg/mol was fractionated with respect to M by means of the mixed solvent methyl acetate (MeAc)/2-propanol (2-POH) applying a new method that uses spinning nozzles to promote the rapid attainment of phase equilibria. Two of the fractions obtained in this manner were employed to prepare membranes from solutions in methyl acetate with 2-propanol as coagulating agent. Electron micrographs demonstrate that the molar mass of CA influences the morphology of the membranes markedly under otherwise identical conditions. For Mw∗=128 kg/mol, one obtains considerably denser structures than for Mw∗=263 kg/mol; furthermore, the pores are considerably less interconnected in the former than in the latter case. The efficiency of the fractionation process and the reasons for the differences in membrane structure are discussed.