Influence of\nInitial Film Properties in UVO-Mediated\nPatterning of an Elastomeric Film Using a TEM Grid

Ultraviolet irradiation of a cross-linked polydimethylsiloxane\n(PDMS) Sylgard 184 film in the presence of atmospheric oxygen (UVO)\nthrough a bare transmission electron microscope (TEM) sample holding\ngrid is a rather simple and widely utilized technique for creating\nmicropatterned surfaces. The surface oxidation of a Sylgard 184 film\ndue to UVO exposure is associated with densification and the formation\nof a silica-like surface layer, which under a TEM grid happens only\nover the exposed areas of the film, resulting in a physicochemical\npattern. It is known that the depth (<b><i>h</i><sub><i>D</i></sub></b>) of the features depends on the duration\nof UVO exposure (<b><i>t</i><sub><i>E</i></sub></b>). In this article, we show for the first time that <b><i>h</i><sub><i>D</i></sub></b> also depends\non the initial film thickness (<b><i>h</i><sub><i>F</i></sub></b>) and the cross-linker percentage (<b><i>C</i><sub><i>L</i></sub></b>, ratio of part A to\npart B) in a Sylgard 184 thin film. We show that for a specific <b><i>t</i><sub><i>E</i></sub></b>, <b><i>h</i><sub><i>D</i></sub></b> progressively decreases\nwith the reduction in <b><i>h</i><sub><i>F</i></sub></b>. On the other hand, <b><i>h</i><sub><i>D</i></sub></b> shows a nonmonotonic dependence with <b><i>C</i><sub><i>L</i></sub></b>, resulting\nin patterns with maximum depth for <b><i>C</i><sub><i>L</i></sub></b> ≈ 10.0%. We attribute this observation\nto the combined effect of resistance against the penetration of the\npropagation front by the rigid substrate as well as stress relaxation\nwithin the exposed parts of the film below the propagating front in\nfilms with higher <b><i>C</i><sub><i>L</i></sub></b> values leading to the variation of <b><i>h</i><sub><i>D</i></sub></b>. The observation reported here\nwould allow the potential fabrication of polymer films with physicochemical\npatterns with feature height on demand by a one-step, facile technique.