Materials Science and Technology of Polymers at the University of Twente

Surface Modification by Plasma Polymerization: Film Deposition, Tailoring of Surface Properties and Biocompatibility

The work described in this thesis concerns the surface modification of materials by thin film deposition in a plasma reactor. In particular, thin polymeric films bearing amine functionalities were synthesized by plasma polymerization of amino group containing monomers. In addition to the synthesis, attention was directed towards the characterization of these films, and the tailoring of their surface properties on a molecular level. Finally, the amino groups introduced by plasma polymerization were used for the subsequent immobilization of graft polymers and biomolecules. Chapter 2 presents a number of topics that are relevant to this thesis. An overview of the most important surface treatment and surface modification techniques is given. Some of the fundamental aspects of plasma and plasma polymerization are also covered. Plasma films are in general insoluble, and therefore not all the characterization methods commonly used for conventional polymers are suitable to analyze plasma polymers. Chapter 3 serves as a brief introduction to the most important techniques that were used to characterize the plasma polymers discussed in this thesis. The synthesis and characterization of amine functionalized plasma polymers is reported in Chapter 4. The influence of plasma parameters such as monomer flow, peak power, and rf duty cycle on the film composition was investigated using different monomers. By careful selection of monomer and process conditions, it was possible to produce thin films with a large variety in chemical structure and physical properties. In general it was found that the amino group functionality of the monomer was increasingly retained in the plasma polymer with decreasing power input. Waveguide mode spectroscopy (WaMS) measurements suggested the presence of an index gradient within the plasma polymerized allylamine films. The oxidation behavior of plasma polymerized allylamine was studied in Chapter 5. The loss of material after extraction in ethanol and the swelling behavior of these films in this solvent were also considered. Surface plasmon resonance (SPR) thickness measurements on plasma polymers showed that the films contained low molecular weight material that could be removed by extraction in ethanol. The film thickness decrease upon this solvent treatment was found to be related to the duty cycle employed during polymerization. WaMS proved to be a powerful tool for the characterization of plasma polymerized films. By allowing a simultaneous study of film thickness and refractive index against different cover media, this method provided new insights in the aging and swelling behavior of plasma polymerized coatings. The duty cycle employed during plasma polymerization of allylamine strongly affected the aging mechanism during storage in air. It was found that films polymerized under low power conditions were more susceptible to surface aging than films deposited at high power input. This behavior was explained by the preferential oxidation of carbon atoms adjacent to amines and by the reorientation of the polar amino groups at the surface. The characterization of plasma polymerized allylamine films using atomic force microscopy (AFM) with chemically functionalized tips is described in Chapter 6. Pull-off force measurements with carboxylic acid functionalized tips in ethanol were found to correlate with the amino group content of these films. The ionization of the amino groups was studied separately by pH-dependent pull-off force measurements with hydroxyl-terminated tips. Adhesive interactions were found to decrease significantly between pH 6.2 and 5.2 upon lowering the pH, due to protonation of the amino groups. The detected ‘force pKa’ of plasma polymerized allylamine films was independent of the power input during deposition. Laterally inhomogeneous pull-off forces were related to the inhomogeneous distribution of the polar groups on the surface. In Chapter 7, a novel method for the attachment of polymer monolayers to plasma modified surfaces is presented. Plasma modified surfaces containing amino groups were successfully immobilized with the radical initiator 4,4’-azobis-(4-cyano-pentanoic acid chloride). The azo groups were then utilized to start the thermally induced polymerization of methyl methacrylate or styrene, leading to surface-attached poly(methyl methacrylate) (PMMA) or polystyrene (PS) chains. It was found in this study that the peroxides or free radicals present in the plasma deposited film contributed to the grafting reaction as well. The dependency of the graft density on the plasma polymerization conditions is described qualitatively as well as quantitatively. High molar mass values were determined for both PMMA (Mn = 2.4*106 g*mol-1) and for PS (Mn = 5.5*105 g*mol-1). PMMA layer thicknesses as high as 130 nm were found on the plasma surfaces after polymerization following the proposed modification concept. The adsorption of proteins to plasma polymer surfaces was investigated in detail in Chapter 8. Si and Au substrates were functionalized with amino or ether groups by plasma polymerization, or with a self-assembled monolayer (SAM) of octadecanethiol. The functional group density at the surface was controlled by using different monomers, or by variation of the average input power during plasma deposition. The adsorption of the proteins fibrinogen, bovine serum albumin and immunoglobulin G to these test surfaces could be measured in situ by SPR spectroscopy. The tenacity of the protein adsorption on the different substrates was also measured, after removing elutable protein with 1 % sodium dodecyl sulfate (SDS) solution. After drying in air, the protein layers were studied by tapping mode atomic force microscopy (TM-AFM). The results obtained showed that both the initial protein adsorption to and the retention on the surfaces were affected greatly by the surface functionalities. All the amine-functionalized surfaces exhibited a high affinity toward the proteins, and thin dense layers of adsorbed protein remained on these surfaces, even after rinsing with SDS solution. A large contrast in protein affinity was observed between the ether group containing plasma films polymerized at different power input conditions. Dramatic reduction in both initial adsorption and retention of all proteins was observed on these films upon decreasing the power input. The low degree of cross-linking, as well as the high retention of ether content during the polymerization under mild power input conditions was thought to result in the production of biologically non-fouling surfaces.