title: Nanotribological Investigation of Polymer Brushes with Lithographically Defined and Systematically Varying Grafting Densities.
abstract: Following controlled photodeprotection of a 2-nitrophenylpropyloxycarbonyl-protected (aminopropyl)triethoxysilane (NPPOC-APTES) film and subsequent derivatization with a bromoester-based initiator, poly(2-(methacryloyloxy)ethylphosphorylcholine) (PMPC) brushes with various grafting densities were grown from planar silicon substrates using atom transfer radical polymerization (ATRP). The grafting density correlated closely with the extent of deprotection of the NPPOC-APTES. The coefficient of friction for such PMPC brushes was measured by friction force microscopy in water and found to be inversely proportional to the grafting density due to the osmotic pressure that resists deformation. Deprotection of NPPOC-APTES via near-field photolithography using a range of writing rates enabled the fabrication of neighboring nanoscopic polymeric structures with dimensions ranging from 100 to 1000 nm. Slow writing rates enable complete deprotection to occur; hence, polymer brushes are formed with comparable thicknesses to macroscopic brushes grown under the same conditions. However, the extent of deprotection is reduced at higher writing rates, resulting in the concomitant reduction of the brush thickness. The coefficient of friction for such polymer brushes varied smoothly with brush height, with lower coefficients being obtained at slower writing rate (increasing initiator density) because the solvated brush layer confers greater lubricity. However, when ultrasharp probes were used for nanotribological measurements, the coefficient of friction increased with brush thickness. Under such conditions, the radius of curvature of the tip is comparable to the mean spacing between brush chains, allowing the probe to penetrate the brush layer leading to a relatively large contact area.
authors.affiliation: Department of Chemistry, University of Sheffield , Brook Hill, Sheffield S3 7HF, U.K.
authors.forename: Zhenyu J
authors.initials: ZJ
authors.lastname: Zhang
authors.affiliation: Department of Chemistry, University of Sheffield , Brook Hill, Sheffield S3 7HF, U.K.
authors.forename: Mark
authors.initials: M
authors.lastname: Moxey
authors.affiliation: Department of Chemistry, University of Sheffield , Brook Hill, Sheffield S3 7HF, U.K.
authors.forename: Abdullah
authors.initials: A
authors.lastname: Alswieleh
authors.affiliation: Department of Chemistry, University of Sheffield , Brook Hill, Sheffield S3 7HF, U.K.
authors.forename: Steven P
authors.identifier: 0000-0002-8289-6351
authors.initials: SP
authors.lastname: Armes
authors.affiliation: Biocompatibles UK Ltd., Chapman House, Farnham Business Park, Weydon Lane, Farnham, Surrey GU9 8QL, U.K.
authors.forename: Andrew L
authors.initials: AL
authors.lastname: Lewis
authors.affiliation: Department of Physics and Astronomy, University of Sheffield , Sheffield S3 7RH, U.K.
authors.forename: Mark
authors.identifier: 0000-0001-9919-7357
authors.initials: M
authors.lastname: Geoghegan
authors.affiliation: Department of Chemistry, University of Sheffield , Brook Hill, Sheffield S3 7HF, U.K.
authors.forename: Graham J
authors.identifier: 0000-0002-4315-9076
authors.initials: GJ
authors.lastname: Leggett