The mechanical properties of a surface-modified layer on polydimethylsiloxane

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Xiaoyue Zhu Department of Biomedical Engineering, University of Michigan, Ann Arbor, Michigan 48109

Shuichi Takayama Department of Biomedical Engineering, University of Michigan, Ann Arbor, Michigan 48109; and Macromolecular Science and Engineering Program, University of Michigan, Ann Arbor, Michigan 48109

M.D. Thoulessa) Departments of Mechanical Engineering and Materials Science & Engineering, University of Michigan, Ann Arbor, Michigan 48109 (Received 15 March 2007; accepted 21 May 2007)

Surface modification of the elastomer polydimethylsiloxane (PDMS) by exposure to oxygen plasma for four minutes creates a thin, stiff film. In this study, the thickness and mechanical properties of this surface-modified layer were determined. Using the phase image capabilities of a tapping-mode atomic force microscope (AFM), the surface-modified region was distinguished from the bulk PDMS; specifically, it suggested a graded surface layer to a depth of about 200 nm. Load-displacement data for elastic indentation using a compliant AFM cantilever was analyzed as a plate bending on an elastic foundation to determine the elastic modulus of the surface (37 MPa). An applied uniaxial strain generated a series of parallel nanocracks with spacing on the order of a few microns. Numerical analyses of this cracking phenomenon showed that the depth of these cracks was in the range of 300–600 nm and that the surface layer was extremely brittle, with toughness in the range of 0.1– 0.3 J/m2.

I. INTRODUCTION

Polydimethylsiloxane (PDMS), a clear elastomer, is a very common material that is used in a myriad of applications in bioengineering, electronics, and microelectromechanical systems. Specifically, some of these applications include micromachined mechanical and chemical sensors,1 stamp material for soft lithography,2,3 and microfluidics devices.4–6 PDMS is widely used because it is biologically inert, gas permeable, an insulator, and good for rapid prototyping of devices. However, for applications where laminar flow or wetting of fluids is desired, the inherent hydrophobicity of the PDMS surface is not ideal. Therefore, the surface of PDMS is often made hydrophilic by oxidation techniques that emulate environmental exposure,7,8 only in an expedited manner. In addition to changing the surface chemistry, oxidation creates a stiff, thin, surface-modified layer.9–11 The oxi-

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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0029 J. Mater. Res., Vol. 23, No. 1, Jan 2008

http://journals.cambridge.org

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dized PDMS exhibits mechanical behaviors that are characteristic of elastically mismatched layered materials, but at the nanoscale. Specifically, moderate uniaxial tensile strains (for an elastomer) produce periodic parallel cracks in the stiff surface-modified layer,12 and compressive strains induce surface buckling.9 This behavior has proved to be useful for applications for which patterns on a nanoscale are desired. For example, a recent study12 showed that when the nanoc

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