Double replication for characterizing cracks in surface-hardened polydimethylsiloxane

Surface-hardened polydimethylsiloxane (PDMS) with random cracks is studied by means of double replication. The PDMS samples are prepared under different curing conditions, resulting in a different Young's modulus of the bulk. To modify the surface, an excimer lamp at 172 nm is used. The samples are stretched uniaxially until the first cracks appear. As an evaluation under strain is hard or rather impossible, the idea is to replicate the cracked sample in a curable material and to characterize the cracks by inspection of the replica. To protect the sample from mechanical loading, this replication is done by molding in OrmoStamp on glass; these replicas are used for optical inspection to determine the crack spacing. As a characterization of the depth and width of the cracks is highly facilitated when cleaving is enabled, a second replication is performed into a thin layer of SU-8 on Si; these second replica are analyzed by secondary electron microscopy of cross sections. They provide a realistic picture of the crack shape. It is found that the curing conditions affect the crack shape; a U-shape occurs with a low bulk modulus, whereas a V-shape occurs with a high bulk modulus. The parameters width, depth, and spacing are largely unaffected by the curing conditions. This work provides a background to understand the behavior of random cracks, which is, e.g., useful to design a system with controlled cracks that remain stable. An important finding is that as soon as cracking occurs at a certain strain, already a number of cracks develop, yet without any further stretching. The cracks behave independently from neighboring cracks. Upon further stretching, new cracks develop, and the crack width and depth remain similar. Therefore, these random cracks are stable, and sample failure does not occur up to a strain of at least 40%.