Nanoscale Surface Compositions and Structures of Plasma-Modified Poly(ethylene terephthalate) Thin Films

Nanoscale compositions and structures of the plasma-treated surfaces of polymers often impart significant consequences on the barrier properties of thin films. Despite their technological importance for packaging and coating applications, a molecular-level understanding of their surface properties has been exceedingly challenging to obtain. This has been due to several factors, including their low external surface areas, nanometer-thin regions of surface modification, subtle differences between their surface versus bulk compositions, and the absence of long-range structural order. Nevertheless, recent advancements in solid-state nuclear magnetic resonance (NMR) spectroscopy, in particular using dynamic nuclear polarization (DNP) enhancement, in combination with X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared (FT-IR) spectroscopy analyses provide detailed insights on the compositions of thin surface layers of plasma-modified poly­(ethylene terephthalate) (PET) thin films. Analyses of 2D 13C­{1H} DNP heteronuclear correlation (HETCOR) NMR spectra of plasma-modified PET films enabled signals from sp 3 carbon species associated with thin (30–80 nm) diamond-like carbon (DLC) surface layers to be detected and identified, along with their interactions at embedded DLC-PET interfaces. Complementary XPS spectra provide insights into different surface and subsurface elemental compositions of the plasma-modified PET films, which are corroborated by FT-IR analyses. Subsurface compositions and structures, in particular carbon:oxygen atomic ratios and intermixing of the DLC surface layers and PET regions, are shown to depend on plasma-enhanced chemical vapor deposition conditions, leading to different gas barrier properties of surface-modified PET films.