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Research Papers: Mixed and Boundary Lubrication

Lubricity from Entangled Polymer Networks on Hydrogels

[+] Author and Article Information
Angela A. Pitenis, Juan Manuel Urueña, Ryan M. Nixon, Tapomoy Bhattacharjee

Department of Mechanical and
Aerospace Engineering,
University of Florida,
Gainesville, FL 32611

Brandon A. Krick

Department of Mechanical
Engineering and Mechanics,
Lehigh University,
Bethlehem, PA 18015

Alison C. Dunn

Department of Mechanical
Science and Engineering,
University of Illinois at Urbana-Champaign,
Urbana, IL 61801

Thomas E. Angelini

Department of Mechanical and
Aerospace Engineering;
Institute for Cell Engineering and
Regenerative Medicine;
J. Crayton Pruitt Family
Department of Biomedical Engineering,
University of Florida,
Gainesville, FL 32611

W. Gregory Sawyer

Department of Mechanical and
Aerospace Engineering;
Department of Materials
Science and Engineering,
University of Florida,
Gainesville, FL 32611
e-mail: wgsawyer@ufl.edu

1Corresponding author.

Contributed by the Tribology Division of ASME for publication in the JOURNAL OF TRIBOLOGY. Manuscript received July 10, 2015; final manuscript received November 9, 2015; published online July 26, 2016. Assoc. Editor: George K. Nikas.

J. Tribol 138(4), 042102 (Jul 26, 2016) (4 pages) Paper No: TRIB-15-1253; doi: 10.1115/1.4032889 History: Received July 10, 2015; Revised November 09, 2015

Structural hydrogel materials are being considered and investigated for a wide variety of biotribological applications. Unfortunately, most of the mechanical strength and rigidity of these materials comes from high polymer concentrations and correspondingly low polymer mesh size, which results in high friction coefficients in aqueous environments. Recent measurements have revealed that soft, flexible, and large mesh size hydrogels can provide ultra low friction, but this comes at the expense of mechanical strength. In this paper, we have prepared a low friction structural hydrogel sample of polyhydroxyethylmethacrylate (pHEMA) by polymerizing an entangled polymer network on the surface through a solution polymerization route. The entangled polymer network was made entirely from uncrosslinked polyacrylamide (pAAm) that was polymerized from an aqueous solution and had integral entanglement with the pHEMA surface. Measurements revealed that these entangled polymer networks could extend up to ∼200 μm from the surface, and these entangled polymer networks can provide reductions in friction coefficient of almost two orders of magnitude (μ > 0.7 to μ < 0.01).

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Figures

Grahic Jump Location
Fig. 1

(a) Hydrogel samples were prepared by polymerizing a 67% pHEMA substrate soaking these samples in a 50% TEMED solution for 10 min. Samples were submerged in a bath of 10% acrylamide solution and retracted immediately at a prescribed velocity. Entangled pAAm networks grew from within and extended out of pHEMA substrate as it was slowly drawn out of the solution. (b) Illustration of entangled, high water content pAAm networks grown onto the pHEMA substrate. (c) Increasing polymerization time yields increasing surface layer thickness. Microscopy revealed that the thickness of these entangled polymer networks increased with increasing time in the acrylamide solution. Error bars represent the standard deviation of the measured thickness over five different regions along the width of the sample.

Grahic Jump Location
Fig. 2

Friction coefficient as a function of pHEMA immersion time in the acrylamide bath. Ultra low friction coefficients, μ < 0.01, were observed at the shortest immersion times and correspondingly thinnest layers. Error bars represent the standard deviation of the measured friction coefficient over the last 40 reciprocating cycles.

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