Grafting of Amino and Nitrogen Groups on Polymers by Means of Plasma Functionalisation Inauguraldissertation zur Erlangung des akademischen Grades doctor rerum naturalium (Dr.rer.nat) an der Mathematisch-Naturwissenschaftlichen Fakult¨at der Ernst-Moritz-Arndt-Universit¨at Greifswald vorgelegt von Asmus Andres Meyer-Plath geb. Meyer-Sievers geboren am 7. Mai 1968 in Hamburg Greifswald, den 6. Februar 2002 Dekan: Prof. Dr. K. Fesser 1. Gutachter: Prof. Dr. H. Conrads 2. Gutachter: Prof. Dr. R. d’Agostino Tag der Promotion: 24. Juni 2002 Abstract The main objective of this work is to contribute to the understanding of the grafting of nitrogen and amino surface functional groups on polymers by means of plasmas containing nitrogen and hydrogen. For this purpose, many aspects of plasma surface modification were studied. In the frame of this work, a new, UHV-sealed plasma reactor system was put into operation. The system is special for its clean reaction environment and the possibility to perform quasi in situ XPS measurements. A comparison of the UHV system to a fine vacuum reactor showed that a clean reaction environment is mandatory for reproducible plasma processing and efficient nitrogen and amino functionalisation. A key motivation for the present work was the observation that the non-coating plasma processes reported in literature fail to graft primary amino groups on polymer surfaces with densities that significantly exceed 3 4% NH2/C. In order to investigate this phenomenon in detail, this work followed two experimental tracks: On the one hand, a broad systematic study of plasma processing parameters was performed. On the other, the surface diagnostics methods used for the quantification of amino groups were critically reviewed. For this, a numerical algorithm was developed to reconstruct the element depth profile from angle-resolved XPS data. In the scope of the process parameter study, cw and pulsed microwave (MW) plasma excitation was com- pared to radio-frequency (RF) excitation. The home-built MW source was studied and optimised with respect to ignition behaviour and power efficiency. The performance of the MW and RF plasmas in polymer surface modifications was studied in various gas mixtures containing NH3 and H2,orN2 and H2. Also the differences of glow and afterglow processing of polymers were investigated. Large variations of the nitrogen and primary amino grafting efficiencies were obtained. They triggered a number of new ideas for the underlying reaction mechanisms. Special attendance was devoted to the selectivity of the functionalisation processes for primary amino groups. Nitrogen-containing discharges that were rich in hydrogen achieved selectivities up to 100%. The upper limit of 3 4% amino groups on the surface, however, was not passed. Angle-resolved XPS measurements revealed a systematic problem for the definition of a surface density, which is capable of explaining the upper limit for amino groups. It is either due to a limited labelling depth of amino groups by the applied TFBA derivatisation reaction, or to a limited functionalisation depth of the plasma process. One very efficient nitrogen-grafting plasma process that was developed on polystyrene was applied to seven other unfluorinated polymers. The similarity of the resulting functionalisation demonstrated a good transfer- ability of plasma surface functionalisation processes. Plasma treatments of polymer surfaces, especially in hydrogen-containing gases, are known to be generally followed by uncontrollable oxidation phenomena. The properties of plasma-functionalised polymer surfaces were therefore studied in conjunction with ageing effects. Quasi in situ XPS analysis allowed to distinguish the influence of oxygen contamination during the plasma process from post-process oxidation due to contact of plasma-treated samples to atmospheric oxygen. The surface modification experiments were accompanied by several gas phase diagnostic techniques. In the scope of this work, the UHV reactor system was equipped with optical emission spectroscopy (OES), two-photon absorption laser-induced fluorescence (TALIF), and tunable diode laser absorption spectroscopy (TDLAS). A se- parate plasma source was setup to perform an absolute quantification of the vacuum-ultra-violet (VUV) emission intensity of hydrogen-containing MW-excited plasmas. The techniques were evaluated with respect to their contribution to an understanding of the plasma processing of polymers. The rich experimental data allowed to suggest new reaction mechanisms for the grafting of nitrogen- and amino functional groups. Surface passivation experiments in H2 plasmas of nitrogen-functionalised surfaces initiated a re-evaluation and an extension of the mechanism of selective etching [1]. Together with two other new reaction mechanisms, a hypothetical reaction scheme was suggested. It was studied by the help of two numerical models for heterogenous reactions of radicals with the surface. In order to avoid the complexity of the fragmentation process of NH3, the models were restricted to discharges in N2 and H2. Despite the sparse information on the composition of the gas phase, the data of two experimental series showed a very particular phenomenology that allowed a first test of the model. The test supports the newly-suggested reaction mechanisms. Especially the role of NH2 attachment to open reaction sites for the grafting of amino groups was emphasised. A more stringent test of the model is left to future experiments with extended gas phase diagnostic means. Contents 1Introduction 3 1.1 GeneralIntroduction.......................................... 3 1.2 MicrowavePlasmaPhysics....................................... 5 1.3 Polymer Chemistry ........................................... 11 1.4 BiomaterialProperties......................................... 13 2 Fundamentals of Surface Functionalisation 15 2.1 Gas Phase Plasma Chemistry ..................................... 16 2.2 PlasmaSurfaceInteraction....................................... 20 2.3 Post-Plasma Surface Processes ..................................... 26 3 Experimental 29 3.1 Plasma Reactor System ......................................... 29 3.2 PlasmaDiagnostics........................................... 35 3.3 PolymerSamplePreparation...................................... 42 3.4 SurfaceDiagnostics........................................... 43 3.5 ExperimentalStrategy......................................... 53 4Results 55 4.1 AvailableInformationontheGasPhase............................... 56 4.2 Polymer Functionalisation Experiments ............................... 59 4.3 ComparisonofVariousPolymers................................... 74 4.4 Durability of Functionalisation .................................... 75 4.5 PolymerPassivationExperiments................................... 78 4.6 Damageability of Polymers ...................................... 79 5 Discussion, Conclusions and Perspectives 84 5.1 Discussion................................................ 84 5.2 Conclusions............................................... 100 5.3 FutureResearch............................................. 105 A Technical Data of the Experimental Equipment 122 B XPS Peak Positions 123 C Reaction Coefficients 124 D Infrared Transitions of NH2 and NH 129 E Optical Transitions 132 Glossary 135 Index 137 1 2 1 Introduction Contents 1.1 GeneralIntroduction................................................. 3 1.2 MicrowavePlasmaPhysics.............................................. 5 1.2.1 Introductiontoplasmaphysics....................................... 5 1.2.2 Plasmaexcitationbymicrowaves..................................... 7 1.2.3 Examplesoflarge-areamicrowaveplasmassources............................ 9 1.2.4 Pulsedplasmaexcitation.......................................... 10 1.3 Polymer Chemistry .................................................. 11 1.3.1 Polymers under investigation ....................................... 11 1.3.2 Functional groups .............................................. 11 1.3.3 Functionalisation of polymers ....................................... 13 1.4 BiomaterialProperties................................................ 13 1.1 General Introduction This work investigates functionalisation of polymer surfaces by means of non-coating plasma processes. In non-coating surface functionalisation no additional molecular layer is deposited, as in plasma polymerisation, but functional groups of the polymer are converted to other chemical groups by substitutive radical reactions, a process also called grafting. Functional groups critically determine the characteristic chemical and physical properties of the polymer surface. If only a thin surface layer is being functionalised the specific nature of the bulk material can be preserved. This way, standard materials may obtain novel qualities. In gas phase processes the depth of surface modification is generally smaller than in wet-chemical reactions, where solvents and capillary forces may transport reagents deep into the substrate. Gas phase processing by plasmas utilises an energised, partially ionised gas. The gas is a source of energetic particles and allows to modify even highly inert surfaces. The particles possess enthalpy in form of kinetic and/or potential energy. The former includes rotational and vibrational excitation, the latter ionisation, electronic excitation, and last, not least chemical reactiveness. While some plasma-based surface processes employ ions, either inert (sputtering), or reactive