Surface modification of ethylene copolymers molded against different mold surfaces. Part 2. Changes at the outermost surface

Abstract

The influence of the mold surface on the surface composition of thermoplastics has been investigated. Two different random copolymers, poly(ethylene-co-acrylic acid) (EAA) and poly(ethylene-co-vinyl acetate) (EVA), with varying comonomer contents were used. Specimens were prepared in molds coated with films of perfluorinated ethylene-propylene copolymer (FEP) and of poly(ethylene terephthalate) (PET). Samples were also molded against air and vacuum. Changes in the concentration and arrangement of the functional groups at the outermost surface were studied using X-ray photoelectron spectroscopy (XPS or ESCA), Fourier transform infra-red techniques (FTIR), and contact angle measurements. The concentration of functional groups at the outermost copolymer surface depended on the nature of the surface against which the random copolymers were molded. Results are interpreted in terms of differences in surface energy between the mold surface and the copolymer. The polar acrylic acid groups in EAA increased when molded against the polar PET mold surface and decreased when molded against nonpolar mold surfaces. Air exposure affected the EAA copolymer surface so that the nonpolar parts migrated to the outermost polymer surface, which resulted in a decreased content of acrylic acid groups. Acetate groups in EVA were found to be in excess at the surface when molded against both polar PET and nonpolar FEP. The signal of O 1s in the XPS spectra depended on the mold surface and the time in the XPS vacuum environment. This can be explained in terms of preferential arrangement of the acrylic acid and the vinyl acetate groups.