Mesostructured Polymer-Surfactant Hydrogel Films: Formation Processes and Properties

Solutions of polyelectrolytes with surfactants are known to show a variety of adsorption behaviours at interfaces. We have recently been exploiting this phenomenon to create novel solid films of polyelectrolyte and surfactant at the air-solution interface, from solutions where the polyelectrolyte is essentially uncharged. In these systems, a self-supporting hydrogel film several hundred nanometres thick forms spontaneously at the solution surface.1 The films contain highly ordered 2D or 3D micellar mesostructures and grow in a few minutes covering the entire interface in an open container. Our initial investigations focused on films made from solutions of polyethylenimine (PEI) and cetyltrimethylammonium bromide (CTAB) and have been extended to a number of other cationic surfactants with a range of molecular structures, influencing film mesostructure and thickness.2

Fig. 1- GIXD pattern for a PEI/CTAB film at the air-solution interface showing a highly ordered 2D hexagonal structure.

We have investigated formation of these membranes and the mesostructures within these films at the solution interface, and in their subphase synthesis solutions, using a range of techniques to understand the interactions driving film formation. Grazing incidence X-ray diffraction (GIXD) has been used to study the nanostructure in the films at the solution surface (Fig 1). Ellipsometry and neutron reflectivity have been used to investigate the evolution of film thickness and mesostructure during growth.3 Small angle neutron scattering has been used to probe polymer-surfactant interactions in the solutions from which the films form and Brewster angle microscopy has provided information on the micron scale surface structure of the films. We have studied the influence on film growth of factors such as ambient humidity, polymer molecular weight, polymer:surfactant ratio,4 charge on the polymer and charge on the micelle.5 This presentation will report our latest investigations on film formation mechanisms and studies of the hydrogel membrane properties after recovery from the solution surface. Initial steps toward applications of the films, including as a system for encapsulation/release,6 and as a template for production of porous inorganic or insoluble polymer structures will also be discussed.

References
(1) Edler, K.J.; Goldar, A.; Brennan, T.; Roser, S.J., Chem. Commun., 2003, 1724-1725.
(2) Wasbrough, M.J.; Edler, K.J.; Hawley, A.M.; Holdaway, J.A.; Price, G.J., Soft Matter, 2012, 8, 3357 - 3362.
(3) Campbell, R.A.; Edler, K.J., Soft Matter, 2011, 7, 11125 - 11132.
(4) O'Driscoll, B.M.D.; Milsom, E.; Fernandez-Martin, C.; White, L.; Roser, S.J.; Edler, K.J., Macromol., 2005, 38, 8785-8794.
(5) O'Driscoll, B.M.D.; Fernandez-Martin, C.; Wilson, R.D.; Roser, S.J.; Edler, K.J., J. Phys. Chem. B, 2006, 110, 5330-5336.
(6) O'Driscoll, B.M.D.; Hawley, A.M.; Edler, K.J., J. Colloid Interface Sci., 2008, 317, 585-592