Polymer functional group mobility. 2—Partition of ion pairs between hydrophobic and hydrophilic phases of plasma oxidized polyethylene

Plasma modification of polyethylene is modeled most simply as a process generating a two phase polymer system: a thin hydrophilic surface phase and a relatively unperturbed hydrophobic bulk phase. The oxidized surface of polyethylene exposed to a N (PEN) or to an Ar (PEAr) glow discharge plasma contains functional groups of N (i.e., amines) and of O (i.e., alcohols, carbonyls and carboxylic acids) and constitutes an interesting, chemically unique phase of the polymer. Lipophillic protonated alkyl ammonium nitrates, perchlorates and iodides are more soluble in the internal, hydrocarbon phase of the modified polymer and consequently show negligible electron spectroscopy for chemical analysis intensities. On the other hand, the hydrophilic protonated alkyl ammonium phosphates, sulfates and chlorides concentrate in the more polar surface phase and are readily detected by electron spectroscopy for chemical analysis. Electron spectroscopy for chemical analysis of protonated alkyl ammonium chlorides and iodides is complicated by the evolution of HCl and HI, respectively, into the vacuum. Complete protonation of the N functional groups of PEN with H 2 SO 4 is prevented by prior intra polymer reactions of amines with carbonyl and carboxylic acid groups to form less basic imines and amides, respectively. Sodium carboxylate salts are more soluble in the surface hydrophilic phase, whereas tetrabutylammonium carboxylate salts readily partition into the internal hydrophobic phase. The partition of functional groups between the hydrophilic surface phase and the hydrophobic bulk phase will exert a profound influence on their availability for reaction at the polymer surface.