Hydrophobic Recovery of Vacuum Ultraviolet Irradiated Polyolefin Surfaces

Film samples of low‐density polyethylene (LDPE) and biaxially oriented poly(propylene) (BOPP) were surface modified by vacuum ultraviolet (VUV) irradiation using a Kr resonant lamp at λ = 123.6 nm in low‐pressure ammonia gas, and were then stored in air. The time‐dependence of the surface properties was monitored using several complementary surface‐sensitive techniques such as contact angle goniometry (CAG), X‐ray photoelectron spectroscopy (XPS), and time‐of‐flight secondary ion mass spectroscopy (ToF‐SIMS), which allows one to determine the surface energy, and chemical composition at different depths. The relative importance of four possible mechanisms involved in surface hydrophobic recovery is discussed, and we show that in our particular case the main mechanism is rotational and/or translational motion of polymer chains and chain segments. This restructuring determines the observed “loss” of functional groups, which occurs within the first few monolayers of the surface (∼1 nm), as shown by the ToF‐SIMS results, and which leads to the observed decrease in the surface energy. In the deeper surface regions (∼10 nm) long‐lived radicals react with oxygen and water vapor upon exposure to the atmosphere, leading to an increase in the concentration of bound oxygen, as observed by XPS. Finally, CAG measurements show that the hydrophobic recovery is reversible and can be significantly reduced by cross‐linking near the surface, as illustrated by depth sensing nano‐indentation measurements on BOPP surfaces. Surface energy, γs, values as a function of storage time for VUV/NH3‐treated a) LDPE (Ed = 14.1 J · cm−2), and b) BOPP samples (Ed = 28.2 J · cm−2). Open symbols pertain to rinsed samples.