ABSTRACT
Some evidence for the change of surface configuration due to the change of the contacting medium could be seen also with other less hydrophilic and hydrophobic polymers. A copolymer of ethylene-vinyl alcohol (EVA) contains a certain number of OH groups according to the copolymer composition. However, the OH groups are not uniformly distributed in the top surface region. On the surface equilibrated in air, the more O atoms exist away from the top surface, which can be observed by the angular dependence of XPS O 1s peak. When the film is immersed in water, the OH groups buried in the bulk phase migrate to the surface, which is now in contact with liquid water. The term ‘‘migrate’’ is used to describe the motion in a certain direction; however, it does not mean the long-range migration such as diffusive migration. This migration can be followed by XPS as a function of water immersion time by adopting the freeze-drying technique to eliminate water without allowing remigration or reorientation of OH groups near the surface. Figure 23.7 depicts a plot of the O 1s to C 1s ratio against sin a, where a is the take-off angle of XPS measurement, representing 90 degrees to 15 degrees as a function of water immersion time. The figure shows the change of the concentration of O atoms near the surface. The concentration profile completely changes its shape as a function of water immersion time. After 60min immersion in water, the highest concentration is observed at the top surface [16].
