Insights into the Mass Transfer through Vapor-Liquid Interfaces from Molecular Dynamics Simulations

Here, poly(4,4′-oxydiphenylene-co-pyromellitimide) (polyimide, PI) film was treated with low-pressure plasmas gases such as hydrogen (H₂), carbon tetrafluoride (CF₄), carbon dioxide (CO₂), oxygen (O₂), and air to change the surface properties. The contact angle (CA, θ) value of the plain PI film measured as 64.5°±2.1 and increased to 117.2°±1.7 after the CF₄ plasma treatment. Additionally, H₂, CO₂, O_2, and air plasma-treated PI films became more hydrophilic with the CA values of 18.7°±1.1, 11.3°±0.8, 2.7°±0.5 and 11.6°±0.3, respectively. The surface free energy (SFE) of PI films were also changed to 12.7±1.4 mN/m, 75.3± 0.8 mN/m, 74.1±0.3 mN/m, 80.8±0.4 mN/m, and 73.2±0.6 mN/m from 49.7±1.7 mN/m after CF₄, H₂, CO₂, O_2, and air plasma treatments. Furthermore, the surface of the PI film was exposed to successive plasmas of CF₄ and O₂, O₂ and CF₄, CF₄ and CO₂, CO­_2, and CF₄ multiple times for several times. Upon multiple treatments the CA value was increased as high as 120.3°±0.4 and the SFE value was decreased as low as 10.3±1.2 mN/m. The stability tests of the surface-modified PI tested in an ambient environment were found intact and unchanged even after 20 days of aging.