Zeta Potential of Photochemically Modified Polymer Surfaces
The present work is focused on the introduction of sulfonic acid groups (−SO 3 H) onto low-density polyethylene (LDPE) surfaces by photosulfonation. The generation of sulfonic acid groups at the polyethylene surfaces and the degree of photosulfonation were examined by FTIR-spectroscopy, contact angle testing as well as zeta potential measurements. The contact angle θ of water decreased from θ = 99° (pristine LDPE) to about θ = 30° (photosulfonated LDPE) and then remained constant. From contact angle data with different test liquids a significant increase in the polar component γP of the surface tension γ was evidenced, while the dispersive component γD remained almost constant. Zeta potential measurements were performed for the investigation of electrokinetic effects at the solid/liquid interface. The ζ-potential of the modified LDPE surfaces shifted to less negative values with increasing UV irradiation time corresponding to a higher hydrophilicity of the photosulfonated polyethylene surfaces. Concomitantly the isoelectric point was shifted to lower pH values, which indicates an increasing amount of acidic -SO3H groups present at the sample surface. However, strongly photosulfonated LDPE surfaces became partly soluble in aqueous media which limits the amount of -SO3H groups present at the modified LDPE surface. To reduce these effects, LDPE samples were cross-linked by e-beam irradiation and then subjected to the photosulfonation process. Compared to standard LDPE, crosslinked LDPE displayed a higher content of -SO3H groups and higher surface polarity after photosulfonation. This was evidenced both by zeta potential and contact angle measurements. It is thus demonstrated that sample pre-treatment by crosslinking provides more stable surfaces which maintain their polar properties during water contact. This is explained by a lower amount of extractable components as a result of radiation-induced network formation.
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This study was performed at the Polymer Competence Center Leoben GmbH (PCCL, Austria) within the framework of the Kplus-program of the Austrian Ministry of Traffic, Innovation and Technology with contributions of Graz University of Technology (TU Graz), Anton Paar GmbH (Graz) and KEKELIT Kunststoffwerk GmbH (Linz). PCCL is funded by the Austrian Government and the State Governments of Styria and Upper Austria. Further thanks to ARC Seibersdorf research GmbH (Dr. J. Wendrinsky) for performing the e-beam irradiation experiments.
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