An innovative way for structuring plasma polymers at the nanoscale

Over 50 years, plasma polymerization has become a well-established technique for the synthesis of functionalized organic thin film, the so-called plasma polymers films (PPF). Nowadays, despite a highly complex growth mechanism including a multitude of surface and gas phase reactions, it is possible to finely control the chemical composition of the PPF by a clever choice of the process parameters. On the other hand, tailoring their morphology at the micro/nano scale is much more challenging limiting further development in the field. Indeed, structuring functionalized organic based-coatings at the micro/nano scale confers to the material enhanced physico-chemical properties, appealing for a broad range of application including the fabrication of highly sensitive biosensors, super-hydrophobic surfaces, drug-release platforms, etc.
In this context, in this work, a novel method for tailoring the morphology of PPF is established. The approach is based on the formation of a plasma polymer bilayer system in which the two layers differ by their chemical composition and cross-linking degree. As a case study, propanethiol-based plasma polymer films are investigated. As revealed by a much higher S/C ratio than in the propanethiol precursor (i.e. 0.83 vs 0.33), it has been demonstrated that the bottom layer contains a large fraction of trapped sulfur-based molecules (e.g. H2S). When further covered by a denser propanethiol PPF formed at higher energetic conditions, a three-dimensional morphological reorganization takes place giving rise to the micro/nano structuration of the organic material. The shape, the dimensions as well as the density of the generated structures are found to depend on the thickness of both coatings involved in the bilayer structure, offering a great flexibility for surface engineering. Annealing experiments unambiguously confirm the major role played by the sulfur-based trapped molecules for inducing the reshaping mechanism. Furthermore, when replacing the top propanethiol PPF by a propylamine-based one, nanostructured NH2-PPF is generated. The whole set of data clearly paves the way for the development of a novel approach for finely tailoring the morphology of functionalized PPF at the micro/nano scale.