Monolayers of some ABA block-copolymers at the airwater interface

Abstract

Monolayers of non-ionic ABA block-copolymers at the airwater interface have been investigated by means of the Langmuir surface balance technique. The A- and B-blocks of the polymer are biodegradable polyester and poly(ethylene oxide) (PEO) chains, respectively. The interfacial properties such as surface pressure and area per molecule have been measured for polymers with variable A-block and a constant B-block. The results showed that there is a dependence between the critical and extrapolated areas per molecule and the length of the A-blocks. The results were interpreted by a modified equation of state for gases in two dimensions which suggested that the final polymer chain conformation was adopted relatively early during the compression and that the B-block was displaced away from the interface only with the two B-block monomers that are bound to the A-blocks anchored at the interface. The parameter, i, characterising the interaction between the molecules at the interface, was an approximately linear function of the molecular weight and the size of the A-blocks, and when extrapolated to the molecular weight corresponding to a pure B homopolymer a value close to the value for a B homopolymer monolayer was obtained. Interpretation of the results by an equation of state developed by Gaines indicated different monolayer compositions in the high and low compressibility regions, showing that the B-block is displaced away from the interface above a surface pressure of approximately 9 mN m −1 which is the collapse surface pressure of a pure PEO monolayer. A comparison between this model and a modified Semenov model showed only a small contribution to the surface pressure from the polymer chain conformational changes, in line with the suggestion that the polymer chain adopts the final conformation at relatively high areas.