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It is accepted that the macroscopic properties of functional plasma polymer films (PPF) are defined by their functional density and their crosslinking degree ( χ ) which are quantities that most of the time behave in opposite trends. If the PPF chemistry is relatively easy to evaluate, it is much more challenging for χ . This paper reviews the recent work developed in our group on the application of principal component analysis (PCA) to time-of-flight secondary ion mass spectrometric (ToF-SIMS) positive spectra data in order to extract the relative cross-linking degree ( χ ) of PPF. NH 2 -, COOR- and SH-containing PPF synthesized in our group by plasma enhanced chemical vapor deposition (PECVD) varying the applied radiofrequency power ( P RF ), have been used as model surfaces. For the three plasma polymer families, the scores of the first computed principal component (PC1) highlighted significant differences in the chemical composition supported by X-Ray photoelectron spectroscopy (XPS) data. The most important fragments contributing to PC1 (loadings > 90%) were used to compute an average C/H ratio index for samples synthesized at low and high P RF . This ratio being an evaluation of χ , these data, accordingly to the literature, indicates an increase of χ with P RF excepted for the SH-PPF. These results have been cross-checked by the evaluation of functional properties of the plasma polymers namely a linear correlation with the stability of NH 2 -PPF in ethanol and a correlation with the mechanical properties of the COOR-PPF. For the SH-PPF family, the peculiar evolution of χ is supported by the understanding of the growth mechanism of the PPF from plasma diagnostic. The whole set of data clearly demonstrates the potential of the PCA method for extracting information on the microstructure of plasma polymers from ToF-SIMS measurements. [ABSTRACT FROM AUTHOR]

Abstract This work aims at presenting an innovative method for tailoring the morphology of functionalized plasma polymer films (PPF). 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 have been 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. H 2 S). When further covered by a denser 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 process. The whole set of data clearly paves the way for the development of an innovative approach for finely tailoring the morphology of functionalized PPF at the micro/nano scale. Highlights • Synthesis of a bilayer system formed by two different propanethiol plasma polymers • Formation of micro/nano structured sulfur-based plasma polymers • Study of the reshaping mechanism [ABSTRACT FROM AUTHOR]