PDMS elastic properties: influence of fabrication protocol and test method

Silicone rubbers have been integral to the rapid development of microfluidic devices and flexible electronics in recent decades. Polydimethylsiloxane, commonly abbreviated as PDMS and produced using the brand name Dow SYLGARD 184, is a ubiquitous such rubber that is renowned for its ease of manufacturing and biocompatibility. SYLGARD 184 is a soft polymer frequently used in the field of mechanics, microfluidics, and electronics given the straightforward ability to manipulate its material properties via tuning base to curing agent mix ratios and curing schedules. However, this tuning process largely occurs on a case by case basis via trial and error, without a comprehensive understanding of how composition, temperature, or mechanical test method my alter the measured elasticity. Here we compare the reported elastic response of PDMS SYLGARD 184 obtained using a range of mechanical testing methods and compositions to investigate the variability. We also include previously unreported experimental results obtained in our lab using Volume Controlled Cavitation Expansion (VCCE). Both testing method and cure schedule (time and temperature of curing) are found to strongly influence the measured elastic response resulting in shear moduli that range over 3 orders of magnitude for chemical compositions that are reported to be the same. While tension testing is a common test method in mechanical analysis, the wide ranging moduli for stiff PDMS and complete inability to measure softer PDMS compositions reveal the limits of this standard method. As researchers in the field of mechanics strive to quantify the properties of increasingly complex biological and composite materials, converging on a standardized measurement of PDMS is necessary, and is a necessary first step for the community.