The role of ions in the continuous-wave plasma polymerisation of acrylic acid

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Abstract
Ion flux, mass spectral and mass deposition rate measurements have been made in radiofrequency-induced continuous-wave plasmas of acrylic acid. At 1 W input power, an ion flux of 0.05±0.1×1018 ions m-2 s-1 was measured for acrylic acid. At this power, ions corresponding to (2M+H)+ and (3M+H)+ were prominent in the positive-ion mass spectrum. When this spectrum was corrected for the transmission function of the quadrupole mass spectrometer (conservatively taken as intensity∝m-1), it was evident that the cationic portion of plasma contained many ions of high m/z, as opposed to small fragments of acrylic acid. The m/z of the ‘average’ ion was calculated as 115. The mass of ions arriving at a solid surface in the centre of the plasma was then calculated by multiplying the flux by the average mass to give 9.6 µg m-2 s-1. This value represents a significant fraction of the total mass deposited, determined by means of a quartz crystal mass balance (45.5 µg m-2 s-1). Repeating the calculation for a 5 W plasma yields an ion mass flux of 39.6 µg m-2 s-1 (measured mass deposition of 57.3 µg m-2 s-1). At 15 W, the calculated mass deposited (based on ion flux) exceeds that measured by the quartz mass balance. The ‘average’ ion mass decreased as plasma input power increased. Based on these data, and XPS measurements of the solid-phase deposit we make a first attempt at describing semi-quantitatively the possible role of ions in deposit formation.

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