Mechanism behind polypropylene surface modifications by OH radicals: An experimental study

We quantitatively investigated the roles and mechanisms of OH radicals in polypropylene (PP) surface modification utilizing a previously proposed vacuum ultraviolet photodissociation reactive species supply method. Water contact angle (WCA), attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy, and X-ray photoelectron spectroscopy (XPS) were employed to characterize the modifications in surface hydrophilicity and components. The relationship between the WCA decline rate and OH radical density at the PP surface was quantified as (2.1 +/- 0.1)[OHs] + (0.5 +/- 0.1) degrees/s, where [OHs] is in ppm. The ATR-FTIR and XPS results demonstrated that C-OH, C--O, C-O-O, and O-C--O groups were gradually introduced onto the PP surface at different treatment stages. The formation and behavior of low-molecular-weight oxidized materials (LMWOMs) on the PP surface during OH radical treatment were investigated. A modified layer composed of LMWOMs eventually formed on the PP surface. The physicochemical mechanisms underlying these phenomena at the micro-level were also discussed. A possible surface oxidation model was proposed based on WCA results, surface functional groups, and LMWOMs, indicating that the oxidation of the PP chains by OH radicals was initiated at the topmost surface and advanced in the depth direction.