Probing the behaviour of poly(ethylene oxide)- poly(propylene oxide)-poly(ethylene oxide) surfactants in the formation of hydrophilic polyurethane foam

Polyurethane (PU) foams are widely used in the medical industry in the treatment of chronic wounds but the role of surfactants in the reaction process, which strongly influences foam structure and properties, is not well understood. In this research, a homologous series of non-ionic poly(ethylene oxide)-poly(propylene oxide)- poly(ethylene oxide) (Pluronic) block copolymer surfactants were studied, in order to elucidate the influence of surfactant structure on PU foam performance. The behaviour of aqueous surfactant stabilised foams was investigated using a combination of surface science techniques (foaming ability / stability, surface tension) and small-angle neutron scattering (SANS). SANS has been successfully implemented to probe the adsorbed Pluronic surfactant layer of dynamic foams in-situ in the neutron beam. We propose the air-water interface comprises a paracrystalline stack consisting of a minimum of 5 adsorbed surfactant layers, with thickness ranging from 80-200Å interspersed with somewhat thicker (200Å) films of water. Total adsorbed layer thickness correlates directly with aqueous foam stability. Correlations of aqueous behaviour to the cell structure and performance of PU foams manufactured on an industrial line were made in an attempt to determine the features of surfactant necessary to produce ‘ideal’ PU foam wound dressings. Analysis of foam cell size and fluid absorption properties demonstrated that greatest absorption was observed for small, fine cell size. This was typically produced by the smallest molecular weight,most hydrophobic surfactants of the series implying that the surface activity of the surfactant (i.e. its ability to reduce the surface tension of the system) is more important than its foaming behaviour. This study should provide a more rational approach when designing surfactant formulations for polyurethane foam systems