Formation of silicone elastomer networks films with gradients in modulus

We describe the formation of soft material substrates with position-dependent modulus. Two strategies have been employed in the preparation of such materials. In the first method, we create modulus gradient structures by inter-diffusing two (or more) formulations of silicone elastomers (SE), comprising mixtures of poly(dimethylsiloxane) (PDMS) and poly(vinylmethylsiloxane) (PVMS) with varying molecular weight and content of PDMS, and cross-link them thermally in order to form SE networks. With this method, the resultant substrates exhibit shallow modulus gradients that extend over a few centimeters in length. The second technique is based on employing ultraviolet (UV)-based cross-linking of mercapto-terminated PVMS chains across transparency lithographic masks. We demonstrate that the shape and “sharpness” of the modulus on the substrate depends solely on the position-dependent transparency of the mask to the UV light. As a part of the study we provide detailed material characterization of networks formed by the PDMS–PVMS and PVMS-SH chains.