Surface characterization of ethylene-vinyl acetate (EVA) and ethylene-acrylic acid (EAA) co-polymers using XPS and AFM

摘要

The air surfaces of ethylene-vinyl acetate (EVA) co-polymers with 9-70 wt% vinyl acetate (VA) and ethylene-acrylic acid (EAA) co-polymers with 3-20 wt% acrylic acid (AA) were studied using XPS at three take-off angles, representing three depths of penetration from 15-58 Angstrom. The semi-crystalline EVA and EAA co-polymers with high wt% ethylene (9-27.5 wt% VA) had higher percentages of VA at the surface than in the bulk, regardless of the type of sample preparation. Excess VA or AA at the surfaces of the annealed semi-crystalline films was probably the result of the rejection of VA or AA units from the growing crystallites. Spin and solution cast films generally had a greater percentage of VA at their surfaces than annealed films; this additional excess may be caused by the lower solubility of the ethylene units in the toluene used for casting, leaving a layer of the more soluble VA units on the air surfaces. Amorphous EVA co-polymer (50-70 wt% VA) annealed films showed an excess of ethylene at the surface. This occurred even though there are only very short sequences of ethylene in these co-polymers and was probably caused by the lower surface free energy of the ethylene repeat units with respect to the VA repeat units. The EAA co-polymers, all of which were semi-crystalline, always showed an excess of AA at the air surfaces, probably because of the rejection of the AA units by the crystallites. AFM bearing-ratio curves showed a fractional coverage of VA on 9EVA and 18EVA annealed surfaces of 0.885 and 0.927, respectively. When the XPS data were used with a simple model, which assumed a partial VA layer over a polyethylene layer at annealed co-polymer surfaces, a 2.0 Angstrom layer of VA on the 9EVA and 18EVA surfaces, with surface coverages of 0.91 and 0.96, respectively, was calculated, in reasonable agreement with the AFM bearing-ratio data. (C) 1998 Elsevier Science Ltd. All rights reserved. [References: 33]