Salt concentration dependency of the hydrated swollen structure of cholinephosphate-type polyzwitterion brushes

Abstract

The swollen thicknesses of zwitterionic polymer brushes with phosphorylcholine (PC), cholinephosphate (CP), sulfobetaine, and carboxybetaine groups were measured in aqueous sodium chloride (NaCl) or calcium chloride (CaCl2) solutions via force curve measurements by scanning probe microscopy (SPM) and neutron reflectivity measurements. PC and CP have different charge positions in betaine units consisting of ammonium and phosphate. The PC-type polymer brush did not distinctly reduce the swollen thickness in either the NaCl or CaCl2 solution, even at 1000 mM. However, the swollen thickness of the CP-type polymer brushes was clearly reduced in the aqueous CaCl2 solution. Unlike the phosphate of PC, which is located inside the betaine unit, the phosphate of CP tends to form insoluble calcium phosphate, resulting in the collapse of the brush structure. The effect of calcium ion on the swelling structure of polymer brushes bearing phosphorylcholine (PC) and cholinephosphate (CP) groups in aqueous solution were investigated by scanning probe microscopy and neutron reflectivity measurements. The CP-type polymer brushes formed collapsed structure in the aqueous CaCl2 solution because the phosphate of CP, which is located outside the betaine unit, strongly interacts with calcium ions to form insoluble calcium phosphate. In contrast, no significant change was observed in the swollen thickness of the PC-type polymer brush in the aqueous CaCl2 solution.