Mass spectrometric study of the radiofrequency-induced plasma polymerisation of styrene and propenoic acid

The specificity of plasma gas-phase reactions, in low-power plasmas, has been investigated. Quadrupole mass spectrometry has been used to detect the gas-phase species in the inductively coupled, radiofrequency-induced 1-10 W plasmas of propenoic acid and styrene. The mass spectra have been compared with those for plasmas of the individual compounds. Both ionic and neutral species, present in the plasma, can be sampled through a 50 mu m aperture which provides an interface between the plasma reactor (10(-2) mbar) and the spectrometer (10(-7) mbar). In styrene plasmas, oligomeric species in the form of radical cations, nM(.+), were observed in the positive ion mass spectrum. Oligomers were also observed in the positive ion spectrum of the propenoic acid plasmas, but in this case they were in a protonated form (nM + H)(+). Oligomers were not observed as neutral species in the plasmas of either compound. The mass spectra of plasmas containing the vapour of both monomers showed no significant new peaks. It is argued that the two monomers do not react with each other in the plasma to produce stable new species. This implies a hitherto unexpected specificity in the plasma gas-phase reactions.