Sabour Un Nisa¹, Eva Bittrich¹, Chris Sturm², Mirko Nitschke¹, Carsten Werner^1,3
1Leibniz-Institute for Polymer Research Dresden, Dresden, Germany
²Universität Leipzig, Leipzig, Germany
³Technical University Dresden, Dresden, Germany
Low pressure plasma treatment is a versatile tool for surface selective modification of polymers. The modification range extends from nanometers up to a few microns along the z-scale depending on particular process parameters inducing sub-surface effects e.g., gradient of optical and mechanical properties.
While most applications rely on improved properties of the uppermost molecular layers only, others take advantage of sub-surface effects for instance wrinkled surfaces for antifouling coatings, sensors, and electro-optical devices. An example of a sub-surface effect is plasma-induced artificial wrinkling of poly (dimethyl siloxane) (PDMS), formed via a graded increase of elastic modulus in the near-surface region due to the conversion of the silicone to silicon oxide (SiO_x).
Considering the magnitude of the modification and the corresponding depth profiles, type of wrinkles, and quality of the created wrinkled microstructures can be tailored by plasma process parameters. Towards this goal is a range of complementary diagnostic techniques, capable of depth profiling on a sub micron scale is employed.
In this work, results from vacuum ultraviolet spectroscopic ellipsometry reveals the depth dependent conversion of PDMS into more rigid SiO_x-like structures in near sub-surface region. In addition, complementary techniques ion beam and X-ray photoelectron spectroscopy analyses provide a comprehensive view of the 3D structure of low-pressure plasma-exposed PDMS. This can be understood as a case study for the diagnostic challenge arising from shallow gradients on plasma-treated polymer surfaces.