| Abstract
| - It is of fundamental importance to be able to easily distinguish between the viscoelastic properties of a moleculargel (noncovalent cross-linked three-dimensional polymer structure) and a brush (polymer structure that emanates froma surface in three dimensions without cross-linking). This has relevance in biology and in designing surfaces withdesired chemical and viscoelastic properties for nano and genomic technology applications. Agarose and thiol-taggedpoly(ethylene glycol) were chosen as model systems, as they are known, on adsorption, to behave like a moleculargel and brush, respectively. Here, we focus on their viscoelastic differences using a quartz crystal microbalance withdissipation monitoring (QCM-D). Changes in resonance frequency and dissipation for three overtones using QCM-Dwere fitted with the Voigt viscoelastic model to calculate the shear viscosity and shear modulus for the adsorbedagarose gel and the PEG brush. At a surface coverage of 500 ng/cm2, the shear viscosities and shear moduli were0.0025 ± 0.0002 Pa−s and 2.0 ± 0.17 × 105 Pa and 0.0010 ± 0.0001 Pa−s and 5.0 ± 0.3 × 104 Pa for the gel andbrush, respectively. Thus, the adsorbed agarose gel layer was far more rigid than that of the covalently bound PEGbrush due to its cross-linked network. Also, the diffusivity of agarose and PEG in solution was compared duringadsorption onto a bare gold surface. The estimated value for the effective diffusivity of the PEG (without a thiol tag)and of the agarose gel was on the order of 10-11 and 10-15 m2/s, respectively. This low diffusivity for agarose supportsthe contention that it exists as a molecular gel with a H-bonded cross-linked network in aqueous solution. With themethods used here, it is relatively easy to distinguish the differences in viscoelastic properties between an adsorbedgel and brush.
|
