Organisation and dynamics of a polymeric surfactant at the air-water interface

Abstract

Polyethylene(oxide) (PEO) is an intriguing polymer, it is water-soluble but exhibits surface-active properties. When capped hydrophobically at one end by a fluorocarbon group, a novel polymeric surfactant is generated. This has been synthesised by combining anionic polymerisation and an end-capping reaction using isophorone diisocyanate. Three molecular weight polymers were generated, for each of which a hydrogenous and a deuterated version were required, hi aqueous solution, these polymeric surfactants formed surface excess layers at the air-water interface and their surface organisation and dynamic behaviour has been investigated. Surface tension data was obtained using a digital tensiometer and the surface tension isotherm is dependent on both solution concentration and polymer molecular weight. Using neutron reflectometry, the organisation of such adsorbed polymer films at the air-water interface has been obtained over a range of solution concentrations for each polymer molecular weight. Neutron reflectometry data was analysed by both optical matrix and kinematic approximation methods. Both analyses yield the same description, i.e. a two-layer organisation is observed at the air-water interface for all three molecular weight polymers. The PEO layer thickness and the surface organisation are found to be dependent on both solution concentration and molecular weight. The PEO chains are totally immersed in the subphase and strongly anchored at the surface via the fluorocarbon end group. The dynamic behaviour of each PEO adsorbed surface excess at the air-water interface has been studied using surface quasi-elastic light scattering. A resonance between the capillary and dilational waves is observed and the maximum in damping is independent on surface organisation but is molecular weight and solution concentration dependent. The viscoelastic behaviour of the dilational modulus can be described using a simple Maxwell fluid model, from which a relaxation time has been obtained assuming a single relaxational process.