Adsorption of Peo-b-Ps Copolymers on Surfaces. Infrared and Surface Force Measurements at XRCC

Abstract

Abstract Surface force and IR data on polystyrene-b-poly(ethy1ene oxide) block copolymers adsorbed on mica from toluene will be presented. The copolymers were selected to have a wide range of asymmetry radius radio (β = RF,PE,O)3/5 as defined by Marques and Joanny. The extended length L the block copolymers has been measured with the Surface Force Apparatus and shown to scale as L∼NNPSNPEO 1/3 for moderately asymmetric copolymers (1 ≪ β ≫ NNPEO 1/2). Deviations are observed when the nonadsorbing block is either much larger than the anchor block or approximately the same size. A general method using infrared spectroscopy has been developed which can measure the quantity of adsorbed polymers on mica. A FTIR operated in a dual beam mode has been used to increase the sensitivity to a point where it is possible to easily obtain spectra of adsorbed polymers. Thin mica sheets and a transmission-reflectance device are used to suppress optical interference fringes which otherwise dominate the spectrum and mask the infrared bands due to adsorbed polymers. The Alexander-de Gennes scaling law for the surface density, LNBσ1/3, has been found to be roughly obeyed over a wide range of copolymer molecular weights and radius ratios p. The Marques and Joanny scaling laws for σ, σ 1/Na, describe well the behaviour of the moderately asymmetric materials. However, their model does not appear to predict the proper surface density for material at the cross over point between the 3D and 2D regimes, near β ≈ NA 1/2. Recently we have extended our infrared studies of PEO-b-PS adsorption to high surface area silica particles. The adsorption of the PEO and PS units with the surface of the silica can be measured independently by observing the perturbation in the infrared band due to surface hydroxyl groups. 8pecial infrared cells have been designed to allow for thermal pretreatment of the silica and subsequent polymer addition under vacuum conditions. An overview of our initial findings will be discussed.