Properties and interactions of physigrafted end-functionalized poly(ethylene glycol) layers

We have studied the properties of layers of end-functionalized poly(ethylene glycol) (PEG) grafted by one end to mica surfaces, and, using a surface force balance, the interaction between two such layers. PEG of molecular weight 3.4 kg/mol, functionalized at both ends with an N-hydroxysuccinimide carbamate (NHS) group, to form NHS-PEG-NHS, does not adsorb on mica. When a trimethylammonium group ((CH3)(3)N+) replaced one of the NHS groups to form ((CH3)(3)N+)-PEG-NHS, a surface layer formed on the mica from its aqueous solution, showing that the functionalized PEG had been attached by its (CH3)(3)N+ end. The properties of the ((CH3)(3)N+)-PEG-NHS layer were examined primarily by surface force measurements as well as by atomic force microscopy, contact angle measurements, and confocal fluorescence microscopy. The layer was found to be very smooth, and the NHS group was found to retain its reactivity also when attached to the surface, demonstrating that the end-functionalized PEG chains can be used as flexible surface-active spacers for binding amino-containing molecules to the surface. Refractive index measurements yield the adsorbance and hence mean interanchor spacings of the PEG chains, while force profiles revealed the role of both electrostatic double layers and steric components of the normal interactions at different surface separations D. Shear forces between the sliding surfaces were first observed at compression ratios (2L(0)/D) similar to 1.2, increasing markedly for (2L(0)/D) > 3, where L-0 is the unperturbed PEG-brush thickness. This may be attributed both to the relatively low (L-0/S) ratio, which allows high interpenetration and therefore high segmental density and viscous shear dissipation between opposite layers, and to bridging effects.