Nanorheology of aqueous polyethylene glycol (PEG)

Abstract

Static and dynamic surface interactions in deionized water were studied between poly(ethylene glycol) (PEG) and an opposed layer of either the same PEG or a highly charged cationic polymer, quaternized polyvinylpyridine (QPVP). The PEG, molecular weight 5000 g mol(-1), was end-attached to hydrophobized mica by hydrophobic-driven adsorption of the lipid portion (distearoyl-phosphatidylethanolamine) of PEG-lipid diblock copolymers. The QPVP homopolymer, 98% quaternized and molecular weight 39,000 g mol(-1), was allowed to adsorb statistically onto mica. Within the scatter of the data the force-distance curves could be fit equally well by the Alexander-de Gennes expression for brush-brush interactions or by exponential decay (decay length 4 nm). However, at every film thickness, the shear modulus of the PEG-PEG interface was less than that of PEG-QPVP by an order of magnitude. When the layers were compressed strongly, they remained demonstrably fluid, even in the most strongly compressed state, in the sense that both systems display a clear transition from terminal behavior (viscous forces > elastic forces) to plateau behavior (elastic forces > viscous forces) in the accessible frequency range. This contrasts strongly with the cases of strongly adsorbed nonpolar polymers of which we are aware, but the terminal relaxation time was slower for PEG-QPVP than for PEG-PEG by an order of magnitude. The transition from static to kinetic sliding, i.e., from linear to strongly nonlinear shear response with increasing shear displacement, was examined in the two limits where shear forces were either dominantly viscous (low frequency) or dominantly elastic (high frequency). The yield stress of the PEG-PEG interface was larger than that for PEG-QPVP by an order of magnitude; this is attributed to greater interdigitation. It is remarkable that the strong hydrophobic attraction observed between hydrophobized solid surfaces in the absence of polymer was transfigured by a layer of adsorbed polymer to become monotonic repulsion. This comparison of the two systems of adsorbed polymers, symmetric (PEG-PEG) and asymmetric (PEG-QPVP), shows the relative insensitivity of the static-force profiles as compared to dynamic shear responses. The static-force-distance profiles were virtually indistinguishable for the selected experimental conditions but the dynamic shear responses displayed large differences.