Weak cation exchange membrane adsorbers were prepared by UV grafting ionic monomers from microporous nylon membranes in aqueous media using Type II photoinitiators. Protein binding was influenced by two factors: a) spacer arm length of >= 6 atoms, and b) the presence of hydrogen bonding moieties in the spacer arm. Increasing the spacer arm length reduced the steric effects associated with the grafted polymer backbone, while intra-chain hydrogen bonding interactions between near neighboring repeat units stiffened the side arm, also making the ligand more sterically accessible and hence more efficiently utilized for protein capture. Static protein binding capacities for lysozyme and IgG of > 150 mg/mL were achieved, while protein binding efficiencies, as measured by the molar ratio of ligand to protein captured, are significantly enhanced when compared to those of other ligand monomers having shorter spacer arms.