1. Plasma co-polymerization for functionalized surfaces Dr Alison J Beck Research Fellow Research Centre in Surface Engineering Department of Materials Science and Engineering The University of Sheffield UK

2. Tutorial covers 2 What is plasma co-polymerization? Why use plasma co-polymerization? The choice of co-monomers Effect of varying the proportion of co-monomers in the gas feed Examples of plasma co-polymerization: 1.acrylic acid and hexane 2.acrylic acid and 1,7-octadiene 3.acrylic acid and allylamine (strongly interacting monomers) Further reading

3. What is plasma co-polymerization? In its simplest form, plasma polymerization uses the vapour of one monomer to form the feed gas. Plasma co-polymerization uses a mixture of the vapours of more than one monomer, usually two. The proportion of each monomer in the feed gas may be varied to increase the variety of plasma polymers that we can produce. This additional parameter is helpful as it opens up further possibilities to control the chemical and physical properties of the plasma polymer. In plasma co-polymerization, a second monomer (for example a simple hydrocarbon) is co- polymerised alongside the monomer containing the functional group of interest. However, plasma-co-polymerization can be used successfully with a wide range of precursors. This will be discussed further, later in the tutorial. Plasma co-polymerization has the potential to enhance many areas including: fundamental studies and control of adhesion and bonding controlling the physical properties and chemistry of the interphase in composite materials enable convergence of nano/micro technologies with biology (e.g. lab on a chip) producing chemical gradients on surfaces (e.g. has utility in biomaterials and in-vitro studies of cells) Control of protein/biomolecule attachment 3

4. Why use plasma co-polymerization? Plasma co-polymerization provides us with another parameter which helps to provide additional control over the physical and chemical nature of the plasma polymer. For example, its often desirable to work at low W/F to minimise monomer fragmentation and so retain as much of the functional group from the monomer as possible. This often has the drawback of producing a plasma polymer that is partially soluble in organic solvents or water as it has little cross-linking. Such levels of solubility can limit to the use of plasma polymers if they are to be exposed to liquids. The introduction of a co-monomer to increase cross-linking (e.g. one containing two double bonds) can increase the stability of plasma polymers to solvents. For example, this technique has been used to improve surfaces to culture cells in aqueous media for tissue engineering. Plasma co-polymerization also allows coatings with broadly similar physical properties but a range of chemistries to be prepared (at a fixed W/F). This has been applied in studies to optimize the properties of composites and to control the level of adhesion between the reinforcing fibre and matrix of the composite. 4

5. The choice of co-monomers Hydrocarbons A saturated hydrocarbon (e.g. hexane) may be added to the monomer feed mixture to effectively reduce the concentration of the functional group from the functional monomer (e.g. the carboxylic acid group from acrylic acid) An unsaturated hydrocarbon, such as 1,7-octadiene [CH 2 =CH-(CH 2 ) 4 -CH=CH 2 ], acts as a diluent in a similar way to hexane. However, as it has two double bonds, it can also increase the level of cross-linking compared to the plasma polymer prepared from say, acrylic acid alone. Functional co-monomers Monomers with carboxylic acid (e.g. acrylic acid) or amine groups (e.g. allylamine) may be co-polymerized in the plasma. However, this is complicated by the strong interaction between acrylic acid and allylamine in the gas phase. Nevertheless, this example results in the interesting phenomenon of a co-pp with a zwitterionic component (-NH 3 + - 2 OC-). 5

6. Functional monomer (A): e.g. acrylic acid Hydrocarbon monomer (B): e.g. hexane Table 1 below shows representative monomer mixtures ranging from pure acrylic acid to pure hexane. In this case, the total monomer flow rate is 1 cm 3 min -1. If ideal gas behaviour is assumed, the ratio F A /(F A + F B ), where F is the monomer flow rate, is also the mole fraction of monomer A in the gas feed. Effect of varying the proportion of co-monomers in the gas feed 6 FAFA FBFB F A /(F A + F B ) 101.0 0.750.250.75 0.5 0.250.750.25 01.00 Table 1 Example of nominal flow rates (cm 3 min -1 ) of monomer vapour in the gas feed F total = 1.0 cm 3 min -1 ) Flow rate measurement The flow rate of each monomer is measured individually. The final flow rate of the mixture of monomers should also be measured.

7. Figure 1 shows how varying the proportion of acrylic acid and hydrocarbon vapour in the monomer feed has affected the chemistry of the plasma co-polymer. Example 1: plasma co-polymerization of acrylic acid with hydrocarbons 7 Figure 1 Functional group composition of the plasma polymer surfaces from x-ray photoelectron C1s core level spectra. The hydrocarbon co-monomers were (a) hexane and (b) 1, 7-octadiene. Total flow rate of monomer, F = 1 cm 3 min -1 and power, P = 1 W (a)(b) The addition of a hydrocarbon to the acrylic acid feed “dilutes” the carboxylate in the co-pp at a fixed value of W/F. The concentration of carboxylate (at these low powers it is mainly carboxylic acid) decreases as the proportion of hydrocarbon in the monomer feed increases. However, there are also some additional oxygen-containing groups, due to monomer fragmentation in the plasma and interaction of the plasma polymer with oxygen on exposure to the atmosphere. The overall trends in surface chemistry with F acrylic acid /(F acrylic acid + F hydrocarbon ) are similar in these examples. However, the minor differences are likely to be due to a combination of factors including the greater deposition rate and additional plasma phase reactions that occur for octadiene.

8. Example 2: Low solubility plasma co-polymer coatings 8 Plasma co-polymerization for functional coatings with improved stability in an aqueous environment Up to about 50% acrylic acid in the monomer feed gas (i.e. F acrylic acid /(F acrylic acid + F octadiene ) = 0.5) the ratio of oxygen to carbon in the surface of the plasma co-polymers was stable on a short exposure to water (figure 2 (a)). In addition, the carboxylate functional group was stable after a longer exposure (3 hours) to water (figure 2 (b)). Figure 2 Effect of soaking plasma co-polymers in water. (a) O/C ratios (2 minutes exposure) (b) Carboxylate concentration (3 hour exposure). Power 2W and Total flow rate 2 cm 3 min -1 (a)(b) The low solubility is due to increased levels of cross-linking due to the addition of the diene. This technique was used to provide surfaces (containing an optimum concentration of carboxylate groups) that were favourable for the culture of skin cells in aqueous media in the laboratory.

9. Example 3: acrylic acid and allylamine. Complications of strongly interacting monomers 9 When acrylic acid and allylamine are mixed together in the absence of a plasma they are expected to react to form allylammonium acrylate salt, as shown in Scheme I. A total nominal flow rate of 4 cm 3 min -1 and a power of 5 W was used in this example. For the mixture containing 50% of the vapour of each monomer (i.e. the flow rate of each individual monomer was 2 cm 3 min -1 ) the total flow rate was found to be 1.5 cm 3 min -1. This confirms the expected high level of interaction between the two monomers. The plasma co-polymer may actually be formed from a tertiary mixture of acrylic acid, allylamine and allylammonium acrylate salt (or a related, ion-neutral complex formed in the proton rich environment of the plasma). Scheme I

10. 10 Example 3: acrylic acid and allylamine. Complications of strongly interacting monomers (continued) The results of the XPS analysis of the plasma co-polymers (see Figure 3) show that the concentration of nitrogen increased and that of oxygen decreases as the proportion of allylamine in the monomer feed increases. The XPS data (figure 4) illustrate that some of the nitrogen in the plasma polymer is present as protonated amines, reflecting the structure of the allylammonium acrylate salt portion of the monomer mixture. The coatings have the potential to exhibit zwitterionic characteristics on exposure to aqueous solutions. Figure 4 Results of curve fitting N 1s spectra for plasma polymers prepared from (a) 100%, (b) 50%, and (c) 25% allylamine. The functional groups that contribute to the peaks are, A amines, amides, nitriles, and imines B protonated amines. Figure 3 XPS analysis of plasma co-polymers of acrylic acid, allylamine and mixtures of the two

11. References and further reading A. J. Beck, J. D. Whittle, N. A. Bullett, P. Eves, S. Mac Neil, S. L. McArthur, A. G. Shard, “Plasma Co-Polymerisation of Two Strongly Interacting Monomers: Acrylic Acid and Allylamine,” Plasma Processes and Polymers, 2 (8), 641- 649, 2005. N Lopattananon, AP Kettle, D Tripathi, AJ Beck, E Duval, RM France, RD Short, FR Jones, “Interface molecular engineering of carbon-fiber composites”Composites: Part A 30 (1999) 49–57 M. R. Alexander, Tran M. Duc, “A study of the interaction of acrylic acid/1,7-octadiene plasma deposits with water and other solvents”, Polymer 40 (1999) 5479–5488 A. J. Beck, R. D. Short, “Gas-phase esterification during plasma polymerization of propanoic acid and 1-propanol,” Journal of Vacuum Science & Technology A-Vacuum Surfaces and Films, 16(5), 3131-3133, 1998 A. J. Beck, R. D. Short, F. R. Jones, “A mass spectrometric study of the radiofrequency-induced plasma polymerisation of styrene and propenoic acid,” J. Chem. Soc., Faraday Trans., 94(4), 559-565, 1998 R. Daw, S. Candan, A. J. Beck, A. J. Devlin, I. M. Brook, S. MacNeil, R. A. Dawson, R. D. Short, “Plasma copolymer surfaces of acrylic acid and 1,7-octadiene: Surface characterisation and the attachment of ROS 17/2.8 osteoblast-like cells,” Biomaterials, 19, No.19, 1717-1725, 1998 A.P. Kettle, A. J. Beck, L. O'Toole, F. R. Jones, R. D. Short, “Plasma polymerisation for molecular engineering of carbon fibre surfaces for optimised composites,” Composites Science and Technology, 57(8), 1023-1032, 1997 A. J. Beck, F. R. Jones, R. D. Short, “Plasma co-polymerisation as a route to the fabrication of new surfaces with controlled amounts of specific chemical functionality,” Polymer, 37(24), 5537-5539, 1996 11