Indentation of polydimethylsiloxane submerged in organic solvents
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Xin Chen and George M. Whitesides Department of Chemistry and Chemical Biology, Kavli Institute, Harvard University, Cambridge, Massachusetts 02138
Joost J. Vlassak and Zhigang Suoa) School of Engineering and Applied Sciences, Kavli Institute, Harvard University, Cambridge, Massachusetts 02138 (Received 5 August 2010; accepted 20 October 2010)
This work uses a method based on indentation to characterize a polydimethylsiloxane (PDMS) elastomer submerged in an organic solvent (decane, heptane, pentane, or cyclohexane). An indenter is pressed into a disk of a swollen elastomer to a fixed depth, and the force on the indenter is recorded as a function of time. By examining how the relaxation time scales with the radius of contact, one can differentiate the poroelastic behavior from the viscoelastic behavior. By matching the relaxation curve measured experimentally to that derived from the theory of poroelasticity, one can identify elastic constants and permeability. The measured elastic constants are interpreted within the Flory–Huggins theory. The measured permeability indicates that the solvent migrates in PDMS by diffusion, rather than by convection. This work confirms that indentation is a reliable and convenient method to characterize swollen elastomers.
I. INTRODUCTION
This work uses a method based on indentation to characterize an elastomeric gel—a network of covalently cross-linked polymers swollen with a solvent. Gels are used in diverse applications, including drug delivery,1–3 tissue engineering,4,5 microfluidics,6,7 and oilfield management.8 In a gel, the network can change conformation and enable large and reversible deformation, while the solvent can migrate through the network and enable mass transport. The deformation and mass transport are coupled— a behavior known as poroelasticity. Most gels are soft, and some are slippery, so that traditional methods for characterizing materials—e.g., tensile and bending tests—are difficult to perform. Furthermore, the change in the conformation of the network and the migration of the solvent result in time-dependent deformation.9 These considerations, along with rapidly developing applications of gels, call for the development of reliable and convenient methods to characterize gels. Indentation has long been used to characterize elasticity and plasticity of metals,10 and has in recent years been used to characterize elasticity,11 viscoelasticity,12,13 and poroelasticity14–21 of gels. A challenge has been to relate the response of indentation to the properties of the material. When the deformation of a material is timedependent, the response of indentation depends on how an a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2010.35 J. Mater. Res., Vol. 26, No. 6, Mar 28, 2011
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indenter is pressed into the material. Examples include pressing the indenter at a constant velocity, or at a constant force, or with oscillating depths, or to a fixed depth. Each of these methods of
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