The effects of functional group mobility on quantitative ESCA of plasma modified polymer surfaces

Abstract

AbstractWetting nitrogen plasma modified polyethylene with a permeable solvent rapidly reduces the surface concentration of amines originally incorporated with plasma exposure. The rate of decrease in surface amine concentration increases with increasing hydrogen bonding capabilities of the solvent. A slower decrease in the surface amine concentration occurs for dry, modified polymers not wet with a solvent. A model consistent with a solvent mediated dilution of surface groups into the interior of the polymer is proposed. Upon protonation with aqueous H3PO4 or H2SO4, subsurface amines readily migrate back to the surface; HCl and HNO3 do not produce this result. The rate of concentration of protonated amines into the ESCA sampling depth is dependent on the permeability of the aqueous acid and the solubility of the corresponding protonated ammonium salt in polyethylene. The mobility of surface functional groups seriously affects the use of chemical derivatization reagents for the ESCA of modified polymer surfaces and raises important questions concerning the significance of a polymer surface analysis. The reaction of surface amines with pentafluorobenzaldehyde is used to illustrate this point.