ABSTRACT
Abstract-The present work is focused on the surface modification of low-density polyethylene (LDPE) by UV irradiation with a Hg lamp in the presence of sulfur dioxide and air. This process, also called photosulfonation, results in introduction of sulfonic acid groups onto the polymer surface, as well as several micrometers into the polymer bulk. To characterize the modified LDPE surfaces, contact angle as well as zeta potential measurements were utilized. The contact angle θ of water (sessile drop) decreased from θ = 99° to approx. θ = 30°, indicating highly polar surfaces. The zeta potential ζ of the modified LDPE surfaces shifted to less negative values with increasing UV irradiation time. This result is explained by an increase of the hydrophilicity of the LDPE surface. Concomitantly, the isoelectric point shifted to lower pH values, which indicates an increasing amount of –SO3H groups present at the sample surface. To confirm the presence of sulfonic acid groups, FTIR spectroscopy was used to characterize the composition of the polyethylene samples before and after sulfoxidation. Also, LDPE samples were cross-linked by e-beam irradiation and then subjected to the photosulfonation process. Cross-linked LDPE offers a higher degree of modification with –SO3H groups. Moreover, the hydrophilic property of the modified polyethylene was studied as a function of ageing time (in air). We observed that the hydrophilicity of the photosulfonated LDPE surfaces was gradually lost during storage under ambient atmosphere. Within 30 h, the contact angle of water increased from θ = 30° to θ = 65°. This hydrophobic recovery is due to reorientation of polar groups from the surface into the subsurface layer. By storing under a polar atmosphere (48 h in the presence of CH3OH vapour), the polar surface properties of sulfonated LDPE could be recovered. The degree of hydrophobic/hydrophilic recovery was found to depend also on cross-linking before photosulfonation.
