title: Adsorbed gels versus brushes: viscoelastic differences.
abstract: It is of fundamental importance to be able to easily distinguish between the viscoelastic properties of a molecular gel (noncovalent cross-linked three-dimensional polymer structure) and a brush (polymer structure that emanates from a surface in three dimensions without cross-linking). This has relevance in biology and in designing surfaces with desired chemical and viscoelastic properties for nano and genomic technology applications. Agarose and thiol-tagged poly(ethylene glycol) were chosen as model systems, as they are known, on adsorption, to behave like a molecular gel and brush, respectively. Here, we focus on their viscoelastic differences using a quartz crystal microbalance with dissipation monitoring (QCM-D). Changes in resonance frequency and dissipation for three overtones using QCM-D were fitted with the Voigt viscoelastic model to calculate the shear viscosity and shear modulus for the adsorbed agarose gel and the PEG brush. At a surface coverage of 500 ng/cm2, the shear viscosities and shear moduli were 0.0025 +/- 0.0002 Pa-s and 2.0 +/- 0.17 x 105 Pa and 0.0010 +/- 0.0001 Pa-s and 5.0 +/- 0.3 x 104 Pa for the gel and brush, respectively. Thus, the adsorbed agarose gel layer was far more rigid than that of the covalently bound PEG brush due to its cross-linked network. Also, the diffusivity of agarose and PEG in solution was compared during adsorption 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 supports the contention that it exists as a molecular gel with a H-bonded cross-linked network in aqueous solution. With the methods used here, it is relatively easy to distinguish the differences in viscoelastic properties between an adsorbed gel and brush.
authors.affiliation: Howard P. Isermann Department of Chemical and Biological Engineering, Rensselaer Polytechnic Institute, Troy, NY 12180-3590, USA.
authors.forename: Amit K
authors.initials: AK
authors.lastname: Dutta
authors.forename: Georges
authors.initials: G
authors.lastname: Belfort
mesh_terms: D000327:Adsorption; D003432:Cross-Linking Reagents / Q000737:chemistry; D004058:Diffusion; D004548:Elasticity; D004587:Electrophoresis, Agar Gel; D005782:Gels; D018625:Microscopy, Atomic Force; D008956:Models, Chemical; D011092:Polyethylene Glycols / Q000737:chemistry*; D011108:Polymers; D012685:Sepharose / Q000737:chemistry; D013314:Stress, Mechanical; D013438:Sulfhydryl Compounds / Q000737:chemistry; D013499:Surface Properties; D014783:Viscosity
chemical_list: D003432:Cross-Linking Reagents; D005782:Gels; D011108:Polymers; D013438:Sulfhydryl Compounds; D011092:Polyethylene Glycols; D012685:Sepharose