Interaction forces between particles containing grafted or adsorbed polymer layers

Abstract

The interaction forces between particles containing grafted or adsorbed polymer layers have been investigated using rheological and surface force measurements. Polystyrene latex dispersions with grafted poly(ethylene oxide) (PEO) chains ( M=2000) were used for the rheological measurements. Results were also obtained for latex dispersions stabilised with adsorbed graft copolymers of poly(methyl methacrylate–methacrylic acid) with methoxy capped PEO chains ( M=750). The relative viscosity η r –volume fraction ϕ curves for the latex dispersions with grafted PEO chains were established for three particle radii of 77.5, 306 and 502 nm. For comparison the η r – ϕ curve was calculated using the Dougherty–Krieger equation. This allows one to obtain the adsorbed layer thickness δ as a function of ϕ. The results showed a decrease of δ with increase of ϕ, which was attributed to the interpenetration and/or compression of the PEO chains on increasing ϕ. Viscoelastic measurements as a function of ϕ showed a change from predominantly viscous to predominantly elastic response at a critical volume fraction, which indicated the onset of the strong steric repulsion when the polymer layers begin to overlap. A similar trend was obtained with the latex particles containing adsorbed graft copolymer layers. A scaling law was used to fit the elastic part of the log G′−log ϕ curve (where G′ is the elastic modulus). This fit could be used to estimate the compressibility of the PEO chains. The correlation of the rheology of concentrated sterically stabilised dispersions with interparticle interactions was investigated by measuring the energy–distance curves for the graft copolymer that was adsorbed on smooth mica sheets. Using de Gennes scaling theory, it was possible to calculate the energy of interaction between the polymer layers. The high frequency modulus of the latex dispersions was obtained as a function of the volume fraction and the results were compared with those calculated from the potential of the mean force. The trends in the variation of the modulus with volume fraction were similar for the experimental rheological results and those calculated using the directly measured interaction forces. The above results demonstrated the powerful use of rheological measurements for studying the interaction between sterically stabilised dispersions in concentrated systems.