Quantitative Depth-Resolved Analysis of Plasma-Induced Chemical and Structural Reconstruction on Siloxane-Containing Polymer Surfaces

Abstract

Although siloxane-containing polymers are widely utilized in fields ranging from electronics to the life sciences, there is a persistent demand for enhancing their adhesion and antifouling properties. Plasma-induced surface modification is a common approach to achieving these improvements; however, the depth-dependent chemical and structural changes occurring in the treated surfaces remain poorly understood. In this study, we quantitatively elucidated the plasma-induced, time-dependent surface reconstruction of poly(3-[tris(trimethylsiloxy)silyl]propyl methacrylate) (PTMSM) thin films by complementarily employing X-ray reflectivity (XRR), neutron reflectivity (NR), and soft/hard X-ray photoelectron spectroscopies (SoXPES and HAXPES). Reflectivity analysis revealed the formation of a multilayered modified region consisting of an ultrathin, highly oxidized surface layer and an underlying partially oxidized layer that thickens with increasing treatment time. By integrating the compositional information from XPS with the scattering length density (SLD) derived from reflectivity measurements, we determined the depth-resolved mass density, elemental composition, and hydrogen distribution for each layer. Plasma treatment induced a stepwise reduction in hydrogen content and an increase in mass density, resulting in an enhanced surface SLD. The outermost layer exhibited a composition and density comparable to those of amorphous silica (SiO2), suggesting the transformation of the polymer surface into a silica-like structure. The quantitative agreement between the structural profiles and the chemical-state distributions derived from HAXPES validates the proposed multilayer model. This integrated analytical approach provides a robust framework for characterizing chemically heterogeneous polymer surfaces and buried interfaces.