Solving curing-protocol-dependent shape errors in PDMS replication
Résumé
PolyDiMethylSiloxane (PDMS) is an elastomer increasingly used to produce soft objects by replication, in a variety of fields including soft electronics, microfluidics, tribology, biomechanics and soft robotics. While the replication of nano-to micrometric scales is usually considered faithful, little is known about the replication quality on larger macroscopic scales. Here, we show that the top surface of parallelepipedic PDMS blocks, molded on a rigid plate, deviates from its expected planeity, the amplitude of the deviation being dependent on the crosslinking protocol. As a practical solution, we identify a suitable twosteps protocol which eliminates those replication errors. Using finite element simulations, we show that the effect originates from a thermal contraction when the sample cools from the curing temperature down to the operating temperature. This phenomenon actually applies at any length scale, and finely depends on the sample's aspect ratio and boundary conditions. Our results should help mitigating replication errors in all applications where a well-defined sample geometry is required.
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