Chemical and microstructural characterization of propanethiol plasma polymer films by XPS and ToF-SIMS

Plasma polymer films (PPF) are promising organics materials for tailoring surface functional properties presenting a high crosslinking degree which influences its mechanical and thermal properties. In particular, propanethiol plasma polymer films (SH-PPF) could be useful for applications such as supports for gold nanoparticles or deoxyribonucleic acid (DNA) immobilization.
SH-PPF were synthesized by plasma polymerization of propanethiol using continuous wave (CW) or pulsed (P) radio-frequency (RF) plasmas. The films were synthesized for equivalent power (Peq) ranging from 14 W (P) to 100 W (CW), and were subsequently characterized by ToF-SIMS and XPS in order to evaluate chemistry as well as microstructure. SH-PPF were also chemically derivatized by N-ethylmaleimide in order to assess the SH group density at their surface by XPS and ToF-SIMS.
The S/C ratio in the SH-PPF was evaluated by in-situ XPS measurements. It varies from 1.00 for Peq = 14W (P), to 0.55 for Peq =38W (P) and stabilizes at ?0.45 for Peq = 55 W (P), 71 W (P) and 100W (CW), indicating a decrease of %at. S with PRF.
ToF-SIMS measurements and subsequent Principal Component Analysis (PCA) of the data were mainly used to evaluate i) the functionnalization rate of the surface and ii) the dependence of the crosslinking degree on Peq. PCA was carried out by mean-centering and scaling a matrix composed of intensities which are normalized to the total ion intensity recorded in the ToF-SIMS spectrum. PCA analysis reveals that samples synthesized at Peq = 14 and 24 W are located at distinct values of PC1>0, while the ones synthesized at Peq = 55W (P), 71 W (P) and 100 W (CW) are all characterized by the same value of PC1 which is < 0, the 38W (P) sample is located at PC1 around zero. Based on the analysis of the PC1 loadings, samples synthesized at Peq = 14W and 24W are mainly characterized by sulphur containing peaks. Such behaviour is in agreement with XPS observations. The second component is related to the crosslinking degree of the SH-PPF, and demonstrates that increasing Peq from 55W (P) to 100W (CW) increases the C/H ratio from 0.51 to 1.04. These results suggest a lower crosslinking degree for films synthesized at lower Peq.
The surface thiol content of the films was evaluated by reaction of the SH-PPF with N-ethylmaleimide. XPS measurements were used to study the reaction kinetics by following the N1s line. By ToF-SIMS characterization, evidence was found for the presence of a C6H8NO2S- fragment, which is a N-ethylmaleimide group attached to a sulphur atom.
In conclusion, XPS and ToF-SIMS techniques are suitable tools for measuring the sulphur content and microstructure of the SH-PPF. In addition, these two techniques allow to evaluate the thiol content of the SH-PPF, by characterizing the reacted SH-PPF with N-ethylmaleimide.