Adsorption kinetics of polymers in membranes: change of layer thickness with solvency

Abstract

In order to monitor changes in the adsorbed layer thickness of polymers adsorbed within the pores of membranes, an apparatus has been developed that monitors the differential pressure generated by fluid flowing through the membrane. The membranes used were commercially available “Whatman Anodisc” alumina filters and had uniformly sized pores of approximately 0.2 μm diameter. The apparatus consisted of an HPLC pump, a pressure transducer and a system of PTFE pipework, which was connected to a pair of membrane holders. Identical membranes were placed in each holder, one of which acted as an inlet filter for the other. This ensured that no contaminants affected the experimental results. The mean pore radius of each membrane was derived by flowing pure solvent through and applying Poiseuille's equation, which relates pore radius to the pressure difference across the membrane. The kinetics of adsorption of polystyrene from toluene were followed by monitoring the pressure increase that occurred as the adsorbed layer restricted the pores in the membrane. Once adsorbed, the polymer was bound irreversibly at the pore wall and the equilibrium thickness of the polymer layer was determined. By purging the apparatus with a poor solvent for the polymer, the degree of layer collapse was assessed. Toluene was then brought back into contact with the polystyrene layer, and the rate and extent of reswelling determined.