Evolution of Layer Density and Thickness during Poly(ethylene oxide) Adsorption onto Silica

The conformation of polymer layers adsorbed on solid surfaces is an important concern for applications such as steric stabilization1 and controlled bridging flocculation of colloidal particles.2,3 Usually, the conformations of adsorbed homopolymer chains are described as trains, loops, and tails and can be characterized by different measurables: the bound (train) fraction of segments, the layer thickness, and the layer density or density profile. Experimentalmethods forprobing conformational aspects of adsorbed layers are subject to some restrictions. Neutron reflectivity4 and neutron scattering5 measurements of layer density profiles are static in the sense that data acquisition typically requires hours and cannot usuallyaccommodateevolving layers. NMR,6 IR,7 electron spin resonance (ESR),8 andmicrocalorimetry9 techniques have been used to determine the average bound fraction of adsorbed chains as a function of the adsorbed amount. These methods have yet to be incorporated in a timebased mode; rather, the current state of the art involves interruption of dynamic studies at different points corresponding to different adsorbedamounts. Ellipsometry, in principle, can measure both the adsorbed amount and the layer density on a planar surface in situ. However, thismethodusually requires a highly reflective substrate for sufficient optical contrast. Although the sensitivities of somemodern ellipsometers allow the determination of the adsorbed amount on a low optical contrast surface suchas polystyrene (n≈1.59),10 the resolution of the layer thickness and refractive index at low coverage is often poor, leading to large uncertainties in these two quantities.10,11 Reliable in-situ measurements of the evolving polymer layer density have rarely been achieved with ellipsometry.10-14 We demonstrate here the utility of Brewster angle reflectivity for in-situ measurements of the evolving adsorbed layer density ona relatively low refractive index substrate (silica glass). “Silica glass” refers to a sodalime slide containing an outer layer of silica, due to acid treatment, as described in detail previously.15 This current work was motivated by previous reflectivity findings15 for poly(ethylene oxide) (PEO) adsorption kinetics on silica glass.