Vacuum ultraviolet photo-physical chemistry of hydrocarbon polymers.

摘要

The purpose of this study has been to investigate fundamental processes involved in the vacuum ultraviolet (VUV, lambda 200 nm)-induced modification of polymer surfaces and their physico-chemical properties. It is well known that VUV photons provide an important photochemical contribution during plasma treatments of polymers, for example: ablation of material; crosslinking and chemical modification of the near-surface region can also be performed by VUV irradiation. During the last 30 years, VUV treatments have received increasing attention, due to a few key advantages over their plasma counterparts. These include the possibility of treating commercial polymer films at atmospheric pressure, thereby alleviating the need for expensive vacuum pumps and other auxiliary equipment necessary for continuous low-pressure plasma roll-to-roll treatment of flexible substrates. Another important advantage of VUV photochemistry over plasma is that more specific surface chemistries can be achieved with monochromatic VUV radiation, due to selective (photo-) chemistries both on the solid surface and in the gas phase.; The hydrocarbon polymers used for this study were well-characterized low-density polyethylene, LDPE; biaxially-oriented polypropylene, BOPP; polystyrene, PS; and poly(methylmethacrylate), PMMA. Due to the complexity of interactions between VUV photons and polymers, especially when the latter are in a reactive gas, VUV-wavelength-dependent effects on the physico-chemical properties of irradiated polymer surfaces have been investigated under two different set of conditions, namely: VUV exposure in vacuum, and in a reactive atmosphere of low-pressure ammonia, VUV/NH3. In the former case, we investigated wavelength (lambda)-dependent material ablation ("etching") by in-situ quartz crystal microbalance (QCM) measurements, as a function of the irradiation dose, D. Near-surface structural changes (the creation of unsaturation, cross-linking, etc.) and radical-creation reactions resulting from VUV-initiated bond scissions were analysed by attenuated total reflectance infrared spectroscopy (ATR-FTIR) and by X-ray photoelectron spectroscopy (XPS) following irradiation. For all polymers studied the etch rates, R(lambda), were found to correlate well with the corresponding absorption coefficients, alpha(lambda), and with the accumulation rates, K, of various (C=C)-containing groups, determined from quantitative FTIR measurements. PMMA was found to have the highest R values, and the rate of mass loss of BOPP was higher than that of LDPE, regardless of lambda. All polymers were found to form double bonds, with the exception of PS, which is rather stable, probably due to energy dissipation by fluorescence. (Abstract shortened by UMI.)