Measurement of forces in interactions between bimodal layers of diblock copolymer adsorbed on mica

Data have been obtained on force of interaction as a function of distance (F(D)) between two mica sheets bearing selectively adsorbed layers of block copolymer, each layer having an identical bimodal distribution of non-adsorbing polystyrene (PS) blocks anchored to the surface by adsorption of poly(2-vinylpyridine) PVP blocks immersed in toluene. These "bimodal brushes" were assembled via two routes: first, by competitive adsorption of two different PVP-PS diblock copolymers; second, by adsorption of PS-PVP-PS triblock copolymers having two different PS block lengths attached to the same PVP anchor. The competitive adsorption route required establishment of the relationship between the compositions of the adsorbed layer and of the adsorbing solution. The F(D) data on the bimodal layers were compared with those on monodisperse layers by scaling the force and distance axes in a manner that brings all the monodisperse data at different chain lengths and surface densities on to the same universal curve. Normalized in this way, the data on the bimodal layers collapse onto the monodisperse curve at high compression in the osmotic pressure limit but diverge at weak compression, owing to the non-universality of the segment distribution of the longer chains. If the distance axis is rescaled in a different fashion, based on an Alexander-de Gennes analysis of a bimodal brush, then the data for bimodal and monodisperse layers converge in weak compression, supporting the idea that this analysis gives an accurate estimate of the thicknesses of bimodal brushes.