Neutron reflectivity study of the swollen structure of polyzwitterion and polyeletrolyte brushes in aqueous solution

Abstract

The swollen brush structures of polycation and zwitterionic polymer brushes, such as poly(2-methacryloyloxyethyltrimethylammonium chloride) (PMTAC), poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC), and poly[3-(N-2-methacryloyloxyethyl-N,N-dimethyl)ammonatopropanesulfonate] (PMAPS), in aqueous solutions of various ionic strengths were characterized by neutron reflectivity (NR) measurements. A series of the polyelectrolyte brushes were prepared by surface-initiated controlled radical polymerization on silicon substrates. A high-graft-density PMTAC brush in salt-free water (D₂O) adopted a two-region step-like structure consisting of a shrunk region near the Si substrate surface and a diffuse brush region with a relatively stretched chain structure at the solution interface. The diffuse region of PMTAC was reduced with increase in salt (NaCl) concentration. The PMAPS brush in D₂O formed a collapsed structure due to the strong molecular interaction between betaine groups, while significant increase in the swollen thickness was observed in salt aqueous solution. In contrast, no change was observed in the depth profile of the swollen PMPC brush in D₂O with various salt concentrations. The unique solution behaviors of zwitterionic polymer brushes were described.

Keywords:

• polyzwitterions
• polymer brushes
• neutron reflectivity
• ionic strength
• sulfobetaine
• phosphobetaine

Acknowledgements

Neutron reflectivity measurements were performed under the S-type research project (proposal No. 2008S) of High Energy Accelerator Research Organization (KEK), and M. K. appreciates Dr N. L. Yamada for the technical support and improvement in the operating system for the SOFIA reflectometer.