Structure of Triblock Copolymers of Ethylene Oxide and Propylene Oxide at the Air/Water Interface Determined by Neutron Reflection

Abstract

Neutron reflection has been used to investigate the structure of adsorbed layers of two triblock copolymers with approximate formulas E23P52E 23 and E9P22E9, where E is ethylene oxide and P is propylene oxide. Measurements were made at the air/water interface at the critical micelle concentration and at two much lower concentrations, and at temperatures of 25 °C and 35 °C. Isotopic labeling was used to improve the resolution of the experiment. In general, the adsorbed layer can be described in terms of a minimum of four uniform sublayers. The outermost layer is always a water-free layer containing only propylene oxide (PO) residues. The ethylene oxide (EO) residues form a tail below this layer, which extends into the solution over a distance slightly shorter than the fully extended length. Depending upon the conditions, some PO is also mixed in with this tail region. For example, at the lower surface concentrations, the segment density of the (PO) decays rapidly and there is a relatively small proportion ...