New Method to Probe the Surface Properties of Polymer Thin Films by Two-Dimensional (2D) Inverse Gas Chromatography (iGC).

Polymer-based functional surface coatings are extensively used in advanced technologies, including optics, energy, and environmental applications. Surface thermodynamic properties profoundly impact the molecular interactions that control interfacial behaviors, such as adhesion and wettability, which in turn dictate coating processes and performance. Conventionally, contact angle measurements are used to assess the surface energy of polymer films and coatings, where the wettability of a surface is assessed using probe fluids (liquid drops). However, contact angle measurement oftentimes can be nontrivial due to the roughness or chemical heterogeneity of the solid surface, as well as the potential for the liquid drop to swell or even dissolve the material being measured. Alternatively, inverse gas chromatography (iGC) is a versatile technique to measure surface thermodynamics and Lewis acid-base properties while also providing environmental control such as temperature and humidity. Despite these benefits, the application of iGC has been limited to powders or fibers, while the direct measurement of supported thin films or coatings is still a nascent area of research. This creates a challenge when using iGC as a comprehensive platform for measuring the physicochemical properties of solid surfaces. Here, we demonstrate how to effectively use iGC to characterize the surface energy of supported polymer thin films by using a two-dimensional (2D) film holder and modifying operational controls, such as the concentration range of the injected gas probe molecules. This enables the precise control of surface coverage required for analyzing samples having minimal surface area, such as thin films. Poly(methyl methacrylate) (PMMA) was employed as a benchmark to determine suitable iGC parameters and to validate our approach on polymer thin films. The seminal work presented here expands the capability of state-of-the-art iGC to embrace supported thin films (2D iGC) that could either be smooth or display texture/roughness (patterned films) as well as coatings with heterogeneous chemical/structural composition.