Plasma Polymerisation: A Fundamental Study Towards the Realisation of New Materials

EPSRC funded project GR/L28548

Summmary of research carried out on EPSRC funded project GR/L28548

Principal Investigator: R. D Short

The low power (P) continuous wave (CW) plasma polymerisations of selected compounds have been investigated. For each compound, the plasma gas-phase was probed by means of mass spectrometry and the solid-phase plasma polymers by X-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS) and infrared spectroscopy. Based on the findings, a more generic understanding of the relationship between plasma P and plasma polymer chemistry and structure has emerged. Mass spectral measurements of the plasma gas-phase and ion flux measurements (in plasmas of acrylic acid and allyl amine) have afforded a deeper insight into the possible role that ions play in plasma polymer growth.

Many commonalties have been identified in the plasma polymerisations of acrylic acid, allyl alcohol and alkyl methacrylates. The latter compounds were the subject of a previous award, GR/H 61391. At low P, substantial retention of the functional groups of these compounds (carboxyl, alcohol and ester, respectively) can be achieved. For example, in the case of acrylic acid 81% retention of the carboxyl functional group has been achieved. As R increases, the amount of functional group retention decreases. In the plasma-gases of these compounds extensive cationic oligomerisation was observed at low R, and the extent of oligomerisation for each compound investigated was found to be inversely proportional to R. For acrylic acid, allyl amine and methyl methacrylate detailed reactions schemes have been proposed. These describe the principal gas-phase reactions and fragmentations of these compounds.

The plasma polymerisation of allyl amine showed some distinct differences from the Ocontaining compounds. Firstly, high levels of retention of primary amine were not achieved at low P. At low P there was considerable scrambling of the amine, and a wide range of different N-containing functionalities were incorporated into the plasma polymers of allyl amine. Secondly, the chemistry of the allyl amine plasma polymer was not affected by plasma P, over the range of input powers investigated (R=1-15 W). The above discrepancies correlated with the different plasma-phase chemistry of allyl amine. Although cationic oligomerisation was observed, the reactions were not as specific as those seen in the O-containing compounds, nor was the extent of oligmerisation P-dependent.

The general trends seen in the plasma polymerisation of styrene fit well those found for the Ocontaining compounds, if the benzene ring is here considered the functional group. At low P there is high retention of the ring. As P increases ring retention reduces. In the plasma gasphase oligomerisation is seen at low P and chain growth is described by radical cationic addition reactions.

In the plasma polymerisations of acrylic acid and allyl amine an ion flux probe, capable of operation in a depositing environment, was used to measure the ion flux as a function of P. The average ion mass with P was estimated from the mass spectral data and the contribution ions make to the total deposit mass was calculated. A complex, but very similar behaviour was found in both the acrylic acid and allyl amine. At low P the ion mass flux was found to contribute substantially (50% for acrylic acid, nearer 100% for allyl amine). In these environments the ions are large and arrive at surfaces contacting the plasma with very low energies, about 5 eV. Therefore, it is reasonable to assume that the ion structure is directly incorporated into the deposit without fragmentation. As P increased the contribution of ions initially fell, but later rose again. Also as P increased ion size decreased (more notably with acrylic acid) and ion energies rose steeply (to greater than 30 eV). Under high P conditions ions could account for the entire deposit mass (acrylic acid or allyl amine) but this is not thought likely as high ion energies will result in fragmentation and sputtering of the depositing surface.

The findings reported offer not just an insight into low P CW plasmas, but also possibly into low P pulsed plasmas, which are currently attracting considerable attention. The findings are scientifically interesting and are of technical importance, pointing to a possible approach for the on-lining monitoring of low P plasmas.

The support of EPRSC is gratefully acknowledged.

Contact:

Prof RD Short (now University of South Australia),
Department of Engineering Materials,
University of Sheffield,
Mappin Street, Sheffield S1 3JD
Telephone: 0114 222 5477
Fax: 0114 222 5943