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Nanotribological investigation of polymer brushes with lithographically defined and systematically varying grafting densities

Lookup NU author(s): Professor Mark Geoghegan

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This work is licensed under a Creative Commons Attribution 4.0 International License (CC BY 4.0).


Abstract

© 2017 American Chemical Society.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.


Publication metadata

Author(s): Zhang ZJ, Moxey M, Alswieleh A, Armes SP, Lewis AL, Geoghegan M, Leggett GJ

Publication type: Article

Publication status: Published

Journal: Langmuir

Year: 2017

Volume: 33

Issue: 3

Pages: 706-713

Print publication date: 24/01/2017

Online publication date: 02/01/2017

Acceptance date: 21/12/2016

Date deposited: 19/12/2019

ISSN (print): 0743-7463

ISSN (electronic): 1520-5827

Publisher: American Chemical Society

URL: https://doi.org/10.1021/acs.langmuir.6b04022

DOI: 10.1021/acs.langmuir.6b04022

PubMed id: 28042924


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