Organisation and dynamics of amphiphilic systems at the air - water interface

Abstract

The behaviour of polymeric amphiphiles, specifically systems containing poly( methyl methacrylate), PMMA, and poly(ethylene oxide), PEO, spread at the air water interface is reported. An amphiphilic graft copolymer containing a PMMA backbone and PEO side chains forms the main subject of the study. Films of PMMA spread onto a 0.1 wt.% solution of PEO have also been considered. Due to the presence of a surface excess, both PEO and PMMA populate the surface and the latter system provides a model of the graft copolymer. Monolayer behaviour has been characterised by recording surface pressure isotherms. It has been demonstrated that the behaviour of the graft copolymers is highly composition dependent, exhibiting characteristics associated with the constituent homopolymers. In contrast the surface pressure isotherm for PMMA on PEO solution is quite unlike that for PMMA on water. Details of the structural organisation in these systems have been determined using neutron reflectometry. The surface organisation adopted by the graft copolymers varies as the surface concentration increases. The PEO side chains penetrate the subphase and stretch increasingly whereas the PMMA backbone behaves much like the homopolymer. This is in contrast to the behaviour of PMMA on PEO solution where the PMMA occupies a wider region of the interface. The structure of the adsorbed PEO differs from that of the bare solution but maintains a constant surface excess. Dynamic behaviour has been studied using surface quasi elastic light scattering. For the graft copolymers surface wave evolution is very unusual. The dilational parameters provide evidence, e.g. a negative dilational viscosity, indicating that surface wave propagation is complex and not described fully by current theories. Similar behaviour is exhibited by PMMA on a PEO solution. These features are believed to be associated with subphase penetration and are discussed in this context.