BARRIER PROPERTIES OF PLASMA-MODIFIED PALYPROPYLENE AND POLYETHYLENETEREPHTHALATE

Plasma treatment changes the solvent absorption and permeation as well as the swelling properties of polymers. Enchanced solvent absorption and swelling are effects of an improved solvent compatibility. The pla sma introduces a large number of different groups at the polymer surfa ce depending on the nature of the plasma. Fluorine-containing plasmas can replace hydrogen atoms of the polymer molecule with fluorine atoms . Moreover, fluorine-containing plasma polymer layers can be formed. A ll these processes reduce the resulting surface free energy, reduce th e diffusion length of solvent molecules, and produce a barrier layer. We have studied the formation of solvent barriers by plasma fluorinati on and by crosslinking by ultraviolet (UV) radiation. Thin foils of po lypropylene (PP) and polyethyleneterephthalate (PET) were used as subs trates. CF4, SF6, and SOF2 were applied as sources of fluorine atoms. Hexafluoropropene, tetrafluorethylene, and perfluorohexylethylene form plasma polymer layers on the polymer substrates. Test solvents were n -pentane, tetrachloroethylene, dimethylsulfoxide, and mixtures of n-pe ntane and methanol. The permeation rate of solvents through plasma-mod ified polymers was measured gravimetrically. Mass spectrometry was app lied to analyze the permeating components of the solvent mixtures. Flu orination of surface layers by plasma-chemical (CF4, SF6) means consid erably reduces the permeation rate of PP (95% barrier effect) and PET (100%). The preferred permeation of one component of the pentane/metha nol mixture is influenced by the polarity of plasma-introduced groups at the polymer surface.