Effects of charge fraction on the swelling behavior of weak polybasic brushes

Weakly charged polymer brushes attached to planar surfaces have found applications as highly tunable pH responsive coatings in separations and biological industries. The effects of pH on the conformation of brushes have been widely studied, however the chemically modified charge fraction (f) is left mostly unexplored. Here we investigate the influence of charge fraction on the swelling behavior of weak polybasic brushes by changing the pH in the 3 – 10 range using a combination of in-situ ellipsometry and streaming zeta potential measurements. Brushes are synthesized on silicon using surface-initiated copper(0) controlled radical polymerization. The brushes consist of random copolymers of DMAEA – a weak base – and neutrally charged HEA. The various charge fractions enable a thorough understanding of the effects of charged monomer over the whole f range. We couple ellipsometry results with titrations curves obtained from zeta potential measurements to better understand the purely electrostatic effects of f on swelling behavior. We observe swollen brush thicknesses decrease as f decreases for the entire pH range. We see hysteresis in the intermediate pH range, between the swelling and de-swelling cycles, where the extent of hysteresis decreases by decreasing f. The hysteresis behavior is important as it could possibly hinder the effects of brushes in tuning adsorption of chemicals and biologics due to the difference between the expected conformation and the actual conformation of the brush.