A mass spectrometric and ion energy study of the continuous wave plasma polymerization of acrylic acid

The plasma polymerization of acrylic acid has been probed by means of mass spectrometry (neutrals and positively charged species) and ion energy spect roscopy. The interpretation of mass spectra is facilitated by the compariso n of mass spectra obtained from plasmas of a C-13-labeled acrylic acid (wit h the label at the carboxyl C) and plasmas of an unlabeled acrylic acid. Ma ss spectrometry of the neutrals shows that under the action of plasma, even at low plasma power inputs, P, there is substantial fragmentation of the a crylic acid. Fragmentation is homolytic at the carboxyl functional group, a nd this is attributed to energy transfer from electrons. In the positive io n mass spectrometry, oligomers of the series (nM + H)(+) as high as n = 4 w ere detected. Low ion energies were measured at low P and are consistent wi th minimal fragmentation on arrival at a self-biased surface. On the basis of these measurements, we speculate that at low P, gas-phase cationic oligo mers may be responsible for up to 50% of the deposit's total mass. Grafting of "intact" acrylic acid accounts for the remaining mass, and the two proc esses (gas-phase oligomerization and grafting) give rise to a highly functi onalized. plasma polymer, containing predominately carboxyls. Irrespective of the exact weighting given to these alternative mechanisms, it is the abu ndance of intact acrylic acid which determines the extent of functional gro up retention. It follows that as P increases and the amount of intact acryl ic acid declines steeply, high levels of functional group retention cannot be achieved.