Phenomenological effects of solvent-casting conditions on pore characteristics of regenerated cellulose membranes

An attempt was made (1) to prepare porous regenerated cellulose membranes by casting cellulose cuprammonium solutions and then immersing them into aqueous acetone solutions as coagulant, and (2) to investigate membrane characteristics such as radius of secondary particles S-2 on the surfaces of the membranes, mean pore diameter measured by the water-flow-rate method 2r(f), membrane porosity by apparent density method Pr(d(3)), and membrane thickness of dry membrane L(d), and (3) to clarify phenomenological effects of solvent-casting conditions on pore characteristics of the membrane formed and to explain the effect in terms of the particle growth theory proposed previously by Kamide-Iijima et al. (KI). Surfaces of membranes prepared by immersing cast solutions in coagulation solution having weight fraction of acetone w(Acetone) below 0.30 consisted of the secondary particles of polymer-rich phase (referred to as ''polymer particle''). As the ammonia concentration in the system was higher, polymer particles of polymer-rich phase grow faster; ammonia was one of the most important factors dominating the size of polymer particles, composing the membranes. L(d) was in proportion to the cellulose weight fraction w(Cell) of cast solutions; on the contrary, Pr(d(3)) and 2r(f) were inversely proportional to w(Cell). These experimental findings suggest strongly that density of dried polymer particles increases in proportion to w(Cell) in the solutions. Pore shape in a whole body of a membrane changed drastically from noncircular pores to circular pores when w(Acetone) in coagulation solutions exceeded 0.30, indicating that w(Acetone) dominates phase separation conditions such as phase volume ratio R(= V-(1)/V-(2); V-(1) and V-(2) are volumes of polymer-lean and -rich phases, respectively), compositions of phase separation points. Changes in Pr(d(3)), 2r(f) and tensile strength TS of the membranes, prepared by using coagulation solutions having different w(Acetone), coincide fairly well with that of pore shape. Membranes constructed by larger cellulose particles have larger pores, and this tendency agreed well with results obtained by KI's lattice theory on pore size distribution proposed before.