Plasma polymers can be used to modify the surface chemistries of materials in a controlled fashion (without effecting bulk chemistry). An example used here is the modification of the alumina surface of aluminium, which is modified by the plasma deposition of acrylic acid to modify adhesion. The tutorial focuses on the chemistry of the plasma deposit and how to analyse it. 1
Functional composition is analysed using advanced spectroscopies/spectrometries Adhesion is tested by single lap shear test 2
Understanding bonding two materials can be complex. In this example (alumina & resin) there are two materials and two interfaces and a modifying plasma polymer. Any interface is complex! The adhesive bonding of aluminium oxide involves at least four distinct interfaces This would be the same for a coated material interacting with a fluid, or biological media. 3
Plasma polymers are deposited under a wide range of parameters. The plasma parameters employed determine the final product Power input (frequency) this can range from d.c. to a.c. (from radio frequency 13.56 khz to microwave 2.45 GHz). Broadly, higher frequency results in greater fragmentation with the plasma and a more cross-linked product (Hence the property of the plasma polymer will very substantially from the choice of frequency.) Power the power deposited into the plasma is a further variable. This can very from a few watts to 150 s watts. Higher power generally leads to a more cross-linked product. However in quoting power care must be taken, as the power read from a dial meter is not the power deposited into the plasma. Plasma can be sustained at reduced pressure (as in this example) or at atmosphere. The characteristics of the plasma will be very different according to pressure. At low pressure, the involvement of ions (ion bombardment) is an additional factor. The choice of gas determines whether the surface is modified (etched as in the case of oxygen) or whether a new deposit is formed (in this example of acrylic acid). 4
This is a typical glass reactor based up on the Clark reactor originally used at the University of Durham (UK) in the 1970 s. It is pumped by means of a rotary pump. The power source is 13.56Hz and the power (dial) typically 2-100W. Power is coupled via an external (copper) coil or band. These glass reactors typically produce a polymeric material. 5
In depositing plasma polymers from the compound acrylic acid, the objective is invariably to retain the carboxylic acid group (-CO 2 H) in the deposit. This functional group is utilised, for example to react with epoxy resin in the adhesive bonding of aluminium or glass fibres to resins. 6
The power deposited into the plasma has a significant influence on the nature of the final plasma deposit. At low power (2W) the deposit has chemically similarities to conventional poly (acrylic acid(. The time-of-flight secondary ion mass spectrometry (ToFSIMS) data show a deposit containing up to (at least) 6 monomer repeat units. When the power is increased (20W), as shown by ToFSIMS, the material is much more cross linked and sputtered fragments are of much lower mass 7
For adhesive bonding (or many other applications) the retention of -CO 2 H is paramount. X-ray photoelectron spectroscopy (XPS) can be used to determine the retention of -CO 2 H in the plasma polymer deposit. As shown above at 2W the retention is greater than at 20W. The inverse relationship between functional group retention and power is well known. 8
However, interpretation of XPS data is not unambiguous: for example -CO 2 H (acid) and -CO 2 R (ester) have the same chemical shift of approximately 4eV. These two functional groups can be resolved by chemically labelling with trifluoroethanol, that uniquely tags acid groups. TFE labelling introduces three (new) fluorines for every -CO 2 H. [Above, the upfield chemical shift of CF 3 can be seen at approximately 292 ev.] 9
Using TFE labelling it is shown that at 2W most of the -CO 2 H/R environment is indeed -CO 2 H (i.e. carboxylic acid). However, at 20W, the result is more mixed with both acid and ester. 10
In addition to power, a further approach for controlling the surface concentration of carboxylic acid (-CO 2 H ) is dilution of the acrylic acid (in the monomer feed) with an diluent compound such as hexene. Use of low power (eg. 2 W) ensures good retention of acid (-CO 2 H ). A further advantage of copolymerization (plasma copolymerization) with a diluent monomer is to reduce the solubility of the plasma polymerized acrylic acid. As shown in this example, post washing with water, the -CO 2 H group is much better retained in the plasma polymer on the addition of a small amount of hexane to the monomer flow. 11
Plasma polymers are always potentially soluble materials. Increasing plasma power increases plasma polymer resistance to solubility in water, but has no discernable effect in ethanol which removed most of all ppaac films. 12
This work was undertaken to ascertain whether plasma polymer coatings could be viable alternatives to chromate containing treatments which are environmentally undesirable. 13
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The single lap shear test is a reasonable measure of interfacial strength. Initial adhesion strength is good for all acid containing coatings and it can be seen in the images that the failure path is through the epoxy adhesive resin (coloured green). Non chemically functional coatings, i.e. pp Hexane, observe low initial bond strength correlating with the limited bonding mechanisms between the epoxy resin and the coated aluminium. The real measure of the quality of an adhesion promoter is whether the adhesive strength of the bond is durable. To test this we used an accelerated ageing procedure which involved immersing in water for 2 weeks at 60 C. In all cases this resulted in a reduction of interfacial strength and an increase in the near interface failure as judged by the visual observation of metallic coloured areas in the failured joints. 15
For all PP thickness the functinoalised coatings exhibited a similar failure strength which was not influenced by the acid concentration, except the 100% ppaac coating. This was observed to decrease in performance as its thickness increased-an observation consistent with the coating acting as a point of weakness when hydrated for 2 weeks at 60C. 16
Using imaging XPS allowed us to made some observations on the locus of failure at the interface. 17
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From imaging XPS, it was apparent that the failure of the ppaac pretreatment passed between the aluminium-pp and the PP-adhesive interfaces. This analysis also reveated particulate filler in the resin. 19
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