Copolyelectrolyte monolayers: organisation and surface wave dynamics

Abstract

The organisation and dynamic behaviour of a copolyelectrolyte monolayer is discussed A linear diblock copolymer of poly(methyl methacrylate) (PMMA) and poly(4-viny ethylpyridinium bromide) (QP4VP) has been the main focus of the study, although films of both the unquaternised copolymer, PMMA-P4VP and a PMMA homopolymei have also been examined for comparative purposes. The polymers were spread or subphases of water and potassium chloride solutions of varying concentrations to determine changes in structure and dynamics with polymer surface concentration and subphase salt concentration. Monolayer behaviour has been characterised from surface pressure isotherms and the use of Brewster angle microscopy. It has been demonstrated that the shape of the isotherm is dependent on the potassium chloride concentration of the subphase. Information on the organisation of the system has been determined by neutron reflectometry. A systematic variation in organisation occurs as both polymer surface concentration and subphase potassium chloride concentration change. The polyelectrolytic QP4VP block stretches more into the subphase with increasing surface concentration or decreasing salt concentration. The results have been compared to scaling laws for polymer brushes. Dynamic behaviour has been studied by the use of surface quasi-elastic light scattering (SQELS) and resonance between the capillary and dilational waves of the system is observed. The phenomenon of mode mixing and the application of viscoelastic models to the system have also been examined. It has been discovered that an accurate description of the surface viscoelastic properties of the system could not be obtained by the use of standard viscoelastic models. Mode mixing was not observed, even in those systems where negative dilational viscosities were found.