Hydrocarbons with Halogen Containing Functional Groups

Low pressure plasma polymerisation of fluorocarbon thin films has been extensively studied in the last decades due to their remarkable properties such as low dielectric constant and friction coefficients, excellent chemical stability, high tolerance to mechanical stress, thermal stability and high hydrophobic/oleophobic character. More recently, plasma polymerisation of fluorocarbon thin films has also been studied in atmospheric pressure discharges. One of the first investigations of atmospheric pressure deposition of fluorocarbons was reported by Yokoyama et al.[119]. The authors used an APGD with tetrafluoroethylene (TFE) (C2F4) diluted in helium. The deposition rate increased with increasing discharge current and increasing monomer flow rate. The latter behaviour is in contrast to the observation by the same authors that the deposition rate does not depend on the flow rate in the case of ethylene polymerisation, as described in section 3.1. The influence of discharge current and monomer flow rate on the chemical structure of the plasma polymerised TFE films was analysed by XPS. The F/C ratio tended to increase with discharge currents and to decrease with TFE flow rates.

Sawada and Kogoma[92] used an APGD to plasma polymerise TFE films on porous granulated silica particles (average diameter: 152 μm). XPS measurements revealed that the films deposited on the particles were composed of highly branched and cross-linked fluorocarbon segments. SEM pictures showed very smooth and uniform films on the silica particles. Thyen et al.[101] also deposited TFE coatings in a filamentary DBD at atmospheric pressure. Soft and smooth fluorocarbon coatings could be deposited with a fairly high deposition rate of about 100-200 nm/min.

Multiple articles by Vinogradov et al.[110-113] deal with the deposition of different fluorocarbons (CF4, C2F6, C3HF7, c-C4F8, C2H2F4, C3F8) using DBDs in argon. The surface tension of the deposited films depended on the nature of the fluorocarbon molecule used, deposition time and discharge gap. The effect of hydrogen or oxygen addition to the gas feed on deposition rate and film composition was evaluated in detail[112]. Admixture of hydrogen strongly influenced the deposition rate, the coating morphology and structure of the film.

(a)

(b)

Figure 4. SEM picture of polymer film deposited on Si in (a) an argon/c-C4F8 mixture and (b) an argon/C3HF7 mixture[126].

Results also showed that an admixture of oxygen drastically reduced the deposition rate. Also the effect of hydrogen content in the precursor was evaluated[110]. In precursors with low hydrogen content as c-C4F8, relatively smooth films could be obtained for an argon/c-C4F8

plasma as shown in Figure 4a. In mixtures with higher hydrogen contents as in argon/C3HF7

plasmas, the deposited polymers had a tendency to form rough surfaces with holes as demonstrated in Figure 4b.

APGDs fed with helium/C3F6 and helium/C3F8/H2 were also used to deposit fluorocarbon thin films by Fanelli et al.[16]. Helium/C3F6 mixtures generated fluorocarbon films with an F/C ratio of 1.5 at deposition rates up to 34 nm/min. With helium/C3F8/H2 fed APGDs it is possible to tune the F/C ratio of the coating from 1.5 to 0.6 and to change its cross-linking degree by varying the hydrogen concentration in the gas feed. H2 addition promoted an increase in deposition rate which is maximum for fluorocarbon-to-hydrogen ratio close to 1.

Deposition of hydrophobic coatings on various substrates has many important applications such as protective garments, corrosion prevention, micro-device lubrication, water repellent textiles, barrier coatings,… Several other authors used the above mentioned fluorocarbons to plasma polymerise thin hydrophobic layers on different substrates[43,45,52,58].

Prat et al.[85] proposed the utilisation of an APGD to modify the inner surface of commercial polyvinyl chloride (PVC) tubes to enhance biocompatibility for a blood circulating tube. A fluoro-polymer layer on the inner surface of the PVC tube increases its blood compatibility and suppresses the bleeding of plasticisers present in commercially available PVC. It is difficult to coat the inner surface of soft tubular structures by low pressure plasmas due to pressure differences between the tube inside and outside. Using helium/C2F4 gas mixtures, uniform and thick PTFE-like coatings consisting of mainly CF2

groups were obtained, while using helium/C3F6 mixtures coatings with lower F/C ratios were deposited.

The concentration of glucose in the blood of a diabetic patient is controlled by insulin.

However, this insulin disappears fast from the blood and diabetics are obliged to inject insulin multiple times a day. This drastically decreases their living comfort. Therefore, it would be beneficial if the medicine could be little by little supplied in the blood for some days. A possible approach is the encapsulation of insulin inside poly-DL-lactic-co-glycolic acid (PLGA) microcapsules[98]. However, standard PLGA capsules cannot be used immediately for this purpose, because they tend to unexpectedly burst just after administration due to water penetration. Suppression of this phenomenon was obtained by Tanaka et al.[98] by depositing a thin hydrophobic fluorocarbon layer on the PLGA capsules by means of a C3F6/helium APGD treatment. The most important parameter for an effective hydrophobic coating was the treatment time. At excessive treatment times, the PLGA capsules were destroyed still leading to unexpected burst releases of the medicine. On the contrary, lower treatment times could make PLGA capsules enough hydrophobic to avoid burst release and to obtain controlled and delayed medicine release.

Besides fluorine containing monomers, also chlorine containing monomers can be applied for plasma polymerisation at atmospheric pressure as described by Borcia et al.[10] to achieve hydrophobic coatings.