Surface Characterization of Linear Low-Density Polyethylene Films Modified with Fluorinated Additives

Abstract

The modification of a polymer surface is valuable in applications where specific chemical and physical properties must be presented to other contacting materials. The surface concentration and orientation of linear and branched fluorosurfactant additives and the corresponding surface properties were investigated in fluorosurfactant-modified films of linear low-density polyethylene (LLDPE). The results from surface-segregated samples were compared to those from samples that were solution-coated with the same additives to achieve surface coverages similar to those attainable with complete surface segregation. The chemical composition and physical structure of the modified film surfaces were characterized using attenuated total reflectance FTIR (ATR-FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and static water contact angle measurements. Results from ATR-FTIR, XPS, and contact angle measurements indicated that, for the fluorinated additives studied, the linear, lower molecular weight additive (DuPont Zonyl FTS, (F(CF2CF2)nCH2CH2OCOC17H35), n ∼ 3.7) segregated more prevalently to the polymer surface than the branched, higher molecular weight additive (DuPont Zonyl TBC, ((F(CF2CF2)nCH2CH2OCO)3C3H5O), n ∼ 4.1). For both additives studied, ATR-FTIR indicated that the effective equilibrium surface concentrations via segregation in bulk-loaded films were lower than those achieved by solution coating additive in amounts equivalent to fully surface-segregated, or one-half the loading of, bulk-loaded films. Topography and phase imaging from AFM showed distinct differences in the physical form of the additives at the polymer surface. The surface-segregated FTS and TBC exhibited grainy, finer scale surface structures than the solution-coated samples. Solution-coated FTS showed overlaid strands of material in the plane of the surface, and solution-coated TBC showed clumplike structures. It is suspected that the difference in surface structure between the two solution-coated samples is due to micellar aggregation behavior of these fluorosurfactants. Angle-resolved XPS (ARXPS) and contact angle results indicated that the perfluoroalkyl end groups of the fluorosurfactants orient at the polymer surface with the CF3 group toward the polymer−air interface. This orientation behavior was clearly seen for bulk-loaded FTS samples and also in solution-coated FTS and TBC samples. For bulk-loaded films, the FTS fluorosurfactant was able to reduce the surface energy of the polymer film at low bulk loadings while TBC had little to no effect at the same loadings due to ineffective surface segregation of the TBC.