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plasma polymer in the continuous wave plasma that is comparable to chemically
polymerised polyethylene [2-14]. The different structure of plasma polyethylene
can be illustrated by the absence of the
ρ
(CH
2
)
≥
4 vibration, which is characteris-
tic for (CH
2
)
4 sequences in commercial polyethylene [15]. The butadiene
plasma polymerisation has not been so intensively studied, however, a highly
branched and crosslinked polymer was found to be formed. Styrene polymerisa-
tion needs a low activation energy of < 1 eV to form a regularly structured poly-
mer by an ordinary radical polymerisation. However, in the continuous wave
plasma, especially on exposure to high-energy doses (high wattage), an undesir-
able fragmentation of monomers in the plasma is observed. Moreover, the depos-
ited plasma polymer layer is exposed to high vacuum-uv doses which additionally
crosslink the polymer and form additional radical sites in the polymer.
A pulsed low-energy plasma should help in avoiding the monomer fragmenta-
tion by enhancing the pure chemical radical polymerisation in the gas phase.
Pulsed plasma polymerisation was first introduced by Tiller in 1972 [16], later
continued by Yasuda [17], Shen and Bell [18, 19] and then further developed by
Timmons [20, 21]. Theoretically, one short plasma pulse should be enough to
start the radical chain polymerisation. However, the number of monomers stick-
ing at radical sites of the growing chains is limited because of the low pressure
(1-20 Pa). Therefore, the radical chain reaction is disrupted and disproportiona-
tion (or a radical-radical recombination or a reaction with oxygen molecules from
residual gas) occurs. Therefore, pulsed plasma with long plasma-off periods and
short plasma pulses offers a good compromise to produce polymer structures with
a minimum of irregularities which are produced during the plasma-on periods. In
Fig. 1 the theoretically expected structures of pulsed plasma (homo) polymerised
bifunctional monomers with OH, NH
2
or COOH groups are shown.
The reactivity of monomers is very different. The most active monomers, char-
acterised by small activation energies for starting the radical polymerisation, are
vinyl and acryl compounds, and less active are allyl monomers. Dienes are also of
high activity, and olefinic double bonds are of lower activity. It should be noted
≥
Figure 1. Structures of pulsed plasma polymerised allylalcohol (a), allylamine (b), and acrylic acid (c).
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