Investigation of the Temperature-Dependent Mechanical Properties of Polydimethylsiloxane (PDMS) Membrane for Thermo-Mechanical Applications

Abstract

Polydimethylsiloxane (PDMS) is a soft polymer that is primarily used for soft lithography (e.g., microfluidics and lab-on-chip devices) and also has wide range of applications, such as for thermomechanical actuators. The unique material properties of PDMS (such as the low values of Young's modulus) renders it to be an attractive material for applications where large range of deformations can be achieved with small variations in the actuating pressure (or actuating forces) thus providing good mechanical advantage. PDMS has been reported in the literature for microfabricating and testing thermally actuated microvalves (for microfluidics applications). These microvalves involve the thermal expansion of a fluid resulting in the deformation of a flexible PDMS membrane. Accurate numerical modeling of such thermo-mechanical actuators made from PDMS necessitates the knowledge of the temperature dependent mechanical properties of PDMS (such as Young's modulus) which is currently lacking in the literature.In this study large deformations were obtained for a thin flexible PDMS membrane (with a square footprint of 7.2 mm and thickness of 200 microns) that was microfabricated on the top of a hermetically sealed cavity (that was 3 mm deep) by subjecting the membrane to thermo-pneumatic pressure arising from the thermal expansion of air trapped in the hermetically sealed cavity and heated from below. This enabled the experimental determination of maximum displacement of the membrane as a function of actuating temperature and therefore the estimation of the temperature-dependent mechanical properties (e.g., Young's Modulus and Poisson's ratio) using parametric simulations using the finite element method (FEM) and based on linear elastic assumption for the deformation of PDMS. Using digital images of the convex shape of the deformed PDMS membrane the maximum deformation was measured as a function of temperature under steady state conditions. Computational Fluid Dynamics (CFD) based commercial solver (Ansys™ 2019R1®) was used to estimate the air pressure inside the hermetically sealed chamber as a function of temperature under steady state conditions (which was verified by analytical calculations). The values of pressure (obtained from CFD simulations) was used as the boundary condition in the FEM model (Ansys™ 2019R1®) for a fixed value of the Young's Modulus and Poisson's ratio to estimate the maximum deformation of the PDMS membrane. By parametric variation of the Young's Modulus and Poisson's ratio (for a particular operating temperature) the actual values were determined based on the computational result that matched the experimental data. The results show that the material properties of PDMS in thermomechanical applications is highly sensitive to the operating temperature. This study enabled the estimation of these material properties of PDMS (i.e., Young's Modulus and Poisson's ratio) as a function of temperature which is currently lacking in the literature.