Modification of poly[(3-hydroxybutyric acid)-co-(3-hydroxyvaleric acid)] film surfaces in an oxygen low pressure plasma

Poly[(3-hydroxybutyric acid)-co-(3-hydroxyvaleric acid)] (91:9 mass-%) (P(HB-co-9% HV)) film surfaces were modified by an oxygen plasma treatment (75 W, 50 Pa, 5 min). The number of oxygen atoms increases about 12-13% and the polar component of the surface energy increases from 6.3 to 21 mN . m(-1). However, the wettability is stable only after 60 days. Untreated and treated him surfaces are studied by means of X-ray photoelectron spectroscopy with two different depth analyses. With storage time, the oxygen contents of the modified film is constant at a take-off angle of 20 degrees but decreases to the initial value at a take-off angle of 70 degrees. The superficial layer of the polymer film between the top of the film to a depth of 10 nm is divided into three superimposing zones. The interpretation is supported by motions of modified macromolecular chains and buried polar groups, chain cleavage and the formation of low-molecular-weight molecules which leads to a decrease of the glass transition temperature (plasticizer effect) and migration of short chains from the lowest modified zone toward the surface.