Zeta Potential of Photochemically Modified Polymer Surfaces

Abstract

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 γ^P of the surface tension γ was evidenced, while the dispersive component γ^D 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.