Polymer Adsorption at the Solid-Liquid Interface: The Interfacial Concentration Profile
Evanescent wave induced fluorescence (EWIF) and small angle neutron scattering (SANS) experiments have been performed at the polymer solution-solid interface, in the case of adsorption. The interfacial layer is characterized by its total surface excess Γ and by the monomer concentration profile Φ(z) in the direction normal to the interface.
EWIF has been used to study the adsorption of poly(methylmethacrylate) (PMMA) onto sapphire. Polymer molecular weights are 120,000 and 600,000 and some chains have been labelled with anthracene chromophores to allow fluorescence excitation by the optical evanescent wave. The surface excess is measured to be 1.47 mg/m2. Moreover, the first moment of the monomer distribution, <z>, can be evaluated. The observed value <z> = 73Å, is comparable to the chain radius of gyration, R G , in good agreement with the theoretical models. The determination of higher moments of the distribution is not accessible with the present system. It would require the use of chains with molecular weights in the range 106–107.
SANS is not prone to this limitation. The full concentration profile has been measured for two different systems, namely poly(methylmethacrylate) (M w = 265,000) adsorbed onto γ-alumina and poly(dimethylsiloxane) adsorbed onto porous silica. In both cases the results are in support of the scaling law behavior Φ(z) α(a/z)4/3. The profile is self-similar, as predicted by de Gennes, in the central region Z min < z < R G . Distances smaller than Z min correspond to the proximal region. The measured width Z min depends on the particular system. It is close to a monomer length ≃ 2.5 Å in the case of the highly flexible poly(dimethylsiloxane) chains. It is significantly higher, ≃ 11.5 Å, for poly(methylmethacrylate). This latter value is compatible with the PMMA chain persistence length. Finally, the surface excess measured by SANS, Γ = 1.7 mg/m2 is in fair agreement with the value measured by EWIF on the PMMA-sapphire system.
In a separate experiment, the possibility of monitoring fast adsorption kinetics by EWIF is demonstrated.
KeywordsEvanescent Wave Persistence Length Small Angle Neutron Scattering Surface Excess Scatter Length Density
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