Polymer plasma treatment

While polymeric surfaces with relatively high surface energies (e.g. polyimides, ABS, polycarbonate, polyamides) can be adhered to readily without surface treatment, low surface energy polymers such as olefins, silicones, and fluoropolymers require surface treatments to increase the surface energy. Various oxidation techniques (such as flame, corona, plasma treatment, or chromic acid etching) allow strong bonds to be obtained to such polymers. 

Wertheimer, M.R., Martinu, L., Klemberg-Sapieha, J.E., and Czeremuszkin, G., Plasma treatment of polymers to improve adhesion. In Mittal, K.L. and Pizzi, A. (Eds.), Adhesion Promotion Techniques — Technological Applications. Dekker, New York, 1999, pp. 139-174. 

The relatively high volatility of Tg[CH = CH2]8 has enabled it to be used as a CVD precursor for the preparation of thin films that can be converted by either argon or nitrogen plasma into amorphous siloxane polymer films having useful dielectric propertiesThe high volatility also allows deposition of Tg[CH = CH2]g onto surfaces for use as an electron resist and the thin solid films formed by evaporation may also be converted into amorphous siloxane dielectric films via plasma treatment. ... 

Some physical techniques can be classified into flame treatments, corona treatments, cold plasma treatments, ultraviolet (UV) treatment, laser treatments, x-ray treatments, electron-beam treatments, ion-beam treatments, and metallization and sputtering, in which corona, plasma, and laser treatments are the most commonly used methods to modify silicone polymers. In the presence of oxygen, high-energy-photon treatment induces the formation of radical sites at surfaces these sites then react with atmospheric oxygen forming oxygenated functions. 

Ortiz-Magan A.B., Pastor-Bias M.M., Eerrandiz-Gomez T.P., Morant-Zacares C., and Martfn-Martfnez J.M., 2001, Surface modifications produced by N2 and O2 RF-plasma treatment on a synthetic vulcanised rubber. Plasmas Polym., 6(1,2), 81-105. 

This general area has been reviewed in depth by Briggs (1990), who gives an extended bibliography of the application of XPS to a number of aspects, including plasma treatment, photooxidation and weathering, biomedical polymers and polymer-metal interactions. 

Mention has already been made of the effectiveness of corona or plasma treatment in increasing the influence of subsequent or concurrent polymer treatment. As examples of polymers used in this way, mention can be made of reactive cationic polysiloxane [294] and polymerisation on the fibre of tetrafluoroethylene or hexafluoropropylene [299]. Water repellency was also improved by the fluorinated polymers. Tetrafluoroethylene gave superior shrink resistance this polymer covered the scale edges of the wool, whereas this did not occur with poly(hexafluoropropylene). 

Apart from modifications in the bulk, also surface modification of PHAs has been reported. Poly(3HB-co-3HV) film surfaces have been subjected to plasma treatments, using various (mixtures of) gases, water or allyl alcohol [112-114]. Compared to the non-treated polymer samples, the wettability of the surface modified poly(3HB-co-3HV) was increased significantly [112-114]. This yielded a material with improved biocompatibility, which is imperative in the development of biomedical devices. 

Before fluorination, the dielectric constant ofpoly(bisbenzocyclobutene) was 2.8, and this value was reduced to 2.1 after plasma treatment. No data were reported in the paper on characterization of structure or properties, except for the dielectric constant of the modified poly(bisbenzocyclobutene). The authors did report that the thermal stability offluorinatedpoly(vinylidenefluoride) was inferior to the original poly(vinylidenefluoride) when treated in a similar way. One of the probable reasons for the low thermal stability is that the NF3 plasma degraded the polymer. According to their results, the thickness of fluorinated poly(bisbenzo-cyclobutene) was reduced by 30%. The same phenomenon was observed for other hydrocarbon polymers subjected to the NF3 plasma process. A remaining question is whether plasma treatment can modify more than a thin surface layer of the cured polymer Additionally, one of the side products generated was hydrogen fluoride, which is a serious drawback to this approach. 

In a previous section, the effect of plasma on PVA surface for pervaporation processes was also mentioned. In fact, plasma treatment is a surface-modification method to control the hydrophilicity-hydrophobicity balance of polymer materials in order to optimize their properties in various domains, such as adhesion, biocompatibility and membrane-separation techniques. Non-porous PVA membranes were prepared by the cast-evaporating method and covered with an allyl alcohol or acrylic acid plasma-polymerized layer the effect of plasma treatment on the increase of PVA membrane surface hydrophobicity was checked [37].The allyl alcohol plasma layer was weakly crosslinked, in contrast to the acrylic acid layer. The best results for the dehydration of ethanol were obtained using allyl alcohol treatment. The selectivity of treated membrane (H20 wt% in the pervaporate in the range 83-92 and a water selectivity, aH2o, of 250 at 25 °C) is higher than that of the non-treated one (aH2o = 19) as well as that of the acrylic acid treated membrane (aH2o = 22). 

The surface analytical techniques mentioned above provide wealth information on the composition and structure of polymer surfaee and changes resulting from modification by plasma discharge. It should be however stressed that, despite of broad spectrum of analytical techniques available, the information is not sufficient to understand all imderlying proeesses in their eomplexity. Espeeially, it is the case of plasma treatment when the interaetions of many plasma eonstituents with polymer surface may play a role. Existing theoretical models are restricted to some specific cases and they usually deseribe only some part of the proeess. 

The parameters of treatment were chosen since these led to the most pronounced changes of polymer surface in our previous experiments [70-74]. It was observed elsewhere that plasma treatment of polymer macromolecules results in their cleavage, ablation, alterations of chemical structure and thus affects surface properties e g. solubility [75]. The chemical structure of modified polyethylene (PE) was characterized by FTIR and XPS spectroscopy. Exposition to discharge leads to cleavage of polymeric chains and C-H bonds followed by generation of free radicals which easily oxidize [10,76]. By FTIR spectroscopy the presence of new oxidized structures within whole specimen volume can be detected. IR spectra in the 1710-1745 cm" interval [71,77] from PE, exposed to... 

Due to plasma treatment the radicals (R, free spin) are generated on polymer chain. Not only C-H but also C-C bonds are likely to brake, the later leading to fragmentation of polymer chain. The radical concentration determined by EPR method is in Table 3. The concentration of free radicals R decreases during the aging to about quarter of the initial after 80 days. The decrease of R is a result of radical recombination [h]. Detailed comment of EPR observation ( aging of radicals) is described too [79]. 

In most of published studies of polymer modification by plasma treatment, great attention is devoted to the roughness and surface morphology of modified... 

Figure 44. A schematic representation of the plasma developed x-ray resist process. Exposure serves to covalenty bind the monomer (m) into the polymer matrix (p). Heating (fixing) drives out (volatilizes) the monomer except where it is "locked in place" by exposure. Plasma treatment converts the silicon to Si02 which retards the etch rate in the exposed areas through formation of a metallic oxide (MO) layer.

Some nonconductors, such as the polymers polycarbonates and polystyrenes, must be subjected to a surface treatment prior to activation to ensure good adhesion of palladium nuclei. Surface treatment can include the use of chemical etchants for plastics or reactive gas plasma treatments (66). 

Bascom, W.D., Chen, W.J, (1991). Effect of plasma treatment on the adhesion of carbon fibers to thermoplastic polymers. J. Adhesion 34, 99-119. 

Biro, D.A., Pleizeier, G, and Deslandes, Y. (1993b). Application of the microbond technique. IV. Improved fiber matrix adhesion by RF plasma treatment of organic fibers. J. Appl. Polym. Sci. 47. 883-894. 

Gao. S. and Zeng, Y. (1993a). Surface modification of ultrahigh molecular weight polyethylene fibers by plasma treatment. I. Improving surface adhesion. J. Appi. Polym. Sci. 47, 2065-2071. 

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