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CHAPTER TWO Polymer surface Modification, Plasma Surface Treatment and Plasma Polymerisation 2.1 Polymer Surface Modification 2.1.2 Corona Discharge Corona discharge is widely used in surface modification of polymers for printing and adhesion. A corona discharge is produced when air is ionised by high electric field.4 Surface treatment by corona discharge is a process to make active groups on the surface of materials and to increase their adhesive strength by the collision of electrons against the surface of the material. The electrons are generated by application of high tension and high frequency voltage between two electrodes in air. This process has been used in areas of printing5 and painting as the pre-treatment, particularly for the surface modification of polyolefins (PP, PE, ect.) and fluoroplastics. 2.1.3 Ion Beam Deposition Ion beam deposition uses an energetic, broad beam ion source carefully focused on a grounded metallic or dielectric sputtering target. Material sputtered from the target deposits on a nearby substrate to create a film. Ion beam deposition yields excellent control and reproducibility of film thickness and properties. There are several ion beam-processing techniques, which may be used for surface modification, each having its own advantages and disadvantages. The techniques include ion beam implantation, ion beam 3 assisted deposition, ion beam texturing and ion beam polishing and sharpening technologies. Ion beams have been used to texturize polymer surfaces, especially flouropolymers, to increase adhesion.6,7 Ion beam implantation in particular offers a number of advantages over other techniques; because it is known to facilitate both chemical and structural modification of the near surface volume of a material without the creation of a defined interface between modified and unmodified volume. In so doing, a gradual transition in chemistry, structure and properties is produced, avoiding the possibility of interfacial delamination.8 2. 1.4 Other Surface Treatment Methods Besides the methods mentioned above, there are still many other methods. Flame treatment has been used commonly in the polymer industries to improve the adhesive characteristics of surfaces, or more particularly to enhance ink permanence on polymer surfaces.9 Photon irradiation should be mentioned, which includes modification by ultraviolet (UV) and infrared (IR) lasers to treat very small and localised areas. Ultraviolet irradiation of typically wavelength between 250 and 400nm produces photons that result in activation of polymer surfaces.10 2.2 Plasma Surface Treatment Plasma is an ionised medium consisting of electrons, ions, and neutral particles that are in constant interactions.11 The term plasma was introduced by Irving Langmuir in his studies of electrified gases in vacuum tubes.4 There are equilibrium (thermal) and non-equilibrium (non-thermal) plasmas. Low- 4 pressure plasma as discussed here is non-equilibrium with an electron energy distribution close to a thermal distribution of 10000 Kelvin, while the energy distribution of ions and neutrals corresponds to a thermal distribution of about 300 Kelvin.12 Polymer surfaces can be treated or modified by the plasmas of a gas such as argon, hydrogen, oxygen, nitrogen, ammonia, ect which does not lead to the deposition of material but chemically modifies the surface. 13 The most common application of plasma treatment is surface cleaning or etching to increasing surface wettability, surface energy alteration as well as surface preparation for bonding by removing organic and inorganic materials that prohibits desired bond strengths without affecting the bulk properties and more recently, to improve cell attachment in tissue culture studies. 4 It can also be applied to a wide range of materials including metals, polymers, plastics, and biomaterials. 2.2.1 Plasma Deposition Plasma deposition can be divided into two groups: plasma enhanced chemical vapour deposition and sputter deposition. In plasma enhanced chemical vapour deposition, the plasma is used to help stimulate a reaction on the substrate surface of two or more species from the gas phase. The plasma helps break down the parent molecule and allows the reaction to occur at lower temperatures than chemical vapour deposition. The major advantage of plasma enhanced chemical vapour deposition is its lower pressure capability with respect to other systems as chemical vapour 5 deposition. For example, while deposition temperatures of 700-900oC are required for silicon deposition in chemical vapour deposition, temperatures in the range of 250-350oC are sufficient in plasma enhanced chemical vapour deposition systems. Sputter deposition, also known as physical vapour deposition is usually carried out in diode plasma systems known as magnetrons, in which the cathode is sputtered by ion bombardment and emits atoms or molecules, which are then deposited on a wafer to form thin films. 14 2. 3 Plasma Polymerisation Plasma polymerisation is a vacuum coating process in which monomer gases are guided into a reaction chamber10 either alone or in combination with an activator gas, which itself does not necessarily participate in the polymerisation reaction (e.g. argon, helium, nitrogen, ect.). The plasma is excited and sustained by the application of electromagnetic radiation, under a process pressure of less than 1 mbar. The monomer molecules are activated in the plasma phase and bombard the substrate surface leading to the dissociation of bonds at the interface, surface etching and chemical reaction between the active sites at the surface and the reactive monomer species in the plasma. These processes can be complicated by ablation and polymerisation mechanism taking place simultaneously at the treated surface. This means that, the reactive species in the plasma phase do not originate from the monomer gas, but may be mixed with reactive species from competitive ablation of the already deposited material. 6 15 Plasma polymerisation takes place through several reaction steps. In the initiation stage, free radicals and atoms are produced by collision of electrons and ions with monomer molecules or by the dissociation of monomers adsorbed on the surface of the sample. Actual formation of the polymeric chain, which is the propagation step, can, takes place both in the gas phase and on the substrate film. Termination can also take place in the gas phase or at the polymer surface by similar processes as in the propagation step, but by ending with a closed polymer chain.10 The properties of the layer can be adjusted over a wide range by selecting the appropriate process parameters. Plasma polymerisation processes allows the wettability and corrosion behaviour of inexpensive substrates to be tailored for desired applications. There is also an improvement in the gluing, painting, printing, corrosion resistance and bacterial growth characteristics of the underlying substrate material.16 Apart from the properties just mentioned, plasma polymerised films can have numerous advantages including excellent coating adhesion on almost all substrates, chemical, mechanical and thermal stability and high barrier effect.10 Functional groups such as hydroxyl, ketone and carboxylic acid groups have been introduced to surfaces to adjust the surface free energy and to facilitate chemical bonding between substrates and adhesives.17 7 There are also other advantages of plasma polymerisation including the following: 1. A broad range of functional groups can be introduced at the surface, by variation of the gas that is used. 2. In general, polymerisation is fairly uniform over the whole substrate. 3. The polymerisation is limited to the top layer and does not affect the bulk properties of the polymer. 4. Plasma polymerisation does not involve the use of solvents and may be deposited on all kinds of substrates. (Polymer, metal, glass and ceramic) The main disadvantages of plasma polymerisation include the following: 1. A vacuum system is required which makes the cost of operation higher. 2. Due to the complexity of plasma processes it is difficult to control the chemical composition of the surface after polymerisation. There are also some factors that affect the surface chemistry and structure of the plasma polymers and these include the design or geometry of the reactor, the input power, monomer flow rate as well as the substrate temperature. All these factors should be investigated separately in order to find an optimal process condition. The cross-link densities of films produced by plasma polymerisation have been shown to be higher than that of conventional polymers produced from the same monomers.18 For example conventional polymerisation of 8 polyethylene gives a liner polymer whiles plasma polymerised polyethylene gives a highly cross-linked structured polymer (see fig. 2.1). H Conventional polymerisation H C C H H H H H H C C C C C H H H H H n H Plasma polymerisation CH2 CH CH2 CH CH2 CH CH CH CH2 CH CH CH CH CH3 CH CH CH CH CH2 CH3 CH2 CH2 CH Fig 2.1 Conventional polymerisation Vs Plasma polymerisation of polyethylene. 2. 4 Plasma Surface Modification for Medical Application Although many synthetic biomaterials have physical properties that meet those of natural body tissue, they can often cause adverse physiological reactions such as infection and inflammations. Through surface modification, biocompatibility as well as biofunctionality can be achieved without changing the bulk properties of the material. There are many ways by which to alter the interaction of biomaterials with their physiological environments, of these; 9 plasma surface modification provides device manufacturers with a flexible, safe and environmentally friendly process that is extremely effective. The field of biomedical applications needs polymers that, besides satisfying the physical requirements of their application show biocompatibility during their application in biological environment. Since biocompatibility involves the interface between the device and the biological environment, surface modification technique can be of great help to improve the biocompatibility of surfaces without changing the bulk properties of the materials There are several factors that must be considered when selecting a coating material for medical application and surface modification process for a particular application. For example, a modified surface for medical application must generally exhibit biocompatibility and biostability in the presence of body fluids and tissues. Surface modification must also be achieved without toxic by-products that could be harmful to the patient or degrade the function of the item being coated.19 2.5 Proteins on Surfaces The immobilisation of proteins on surfaces is a first step in the lengthy process of using active biological molecules in man-made nanodevices. The interaction of proteins with solid surfaces is not only a fundamental phenomenon but also a key to several important and novel applications. 10 2.5.1 Proteins Proteins are complex high molecular weight organic compounds that consist of amino acids joined by peptide bonds. There are four distinct protein structures, namely, primary, secondary, tertiary and quaternary structures. The primary structure shows the sequence of amino acids in the polypeptide chain with reference to the locations of any disulfide bonds. The primary structure may be thought of as a complete description of all of the covalent bonding in a polypeptide chain or protein. The Secondary structure is the ordered arrangement or conformation of amino acids in localized regions of a polypeptide or protein molecule. Hydrogen bonding plays an important role in stabilizing these folding patterns. The two main secondary structures are the alpha helix and the anti-parallel beta-pleated sheet. There are other periodic conformations, but the -helix and -pleated sheet are the most stable. A single polypeptide or protein may contain multiple secondary structures. The tertiary structure of a polypeptide or protein is the three-dimensional arrangement of the atoms within a single polypeptide chain. For a polypeptide consisting of a single conformational folding pattern (e.g., an alpha helix only), the secondary and tertiary structure may be one and the same. Also, for a protein composed of a single polypeptide molecule, tertiary structure is the highest level of structure that is attained. Quaternary structure is used to describe proteins composed of multiple subunits (multiple polypeptide molecules, each called a 'monomer'). The arrangement of the monomers in the three-dimensional protein is the quaternary structure.20, 21 11 In addition to their structures, proteins may rearrange themselves in performing their biological functions. In context of these functional rearrangements, these tertiary or quaternary structures are usually referred to as conformations and transitions between them are called conformational changes. Proteins are essential to the structure and functions of all living cells and viruses. Functions of proteins include the immune response and the storage and transport of various ligands. They also serve as a source of amino acids for organisms that do not synthesise those amino acids natively. Proteins are one of the classes of biomolecules, alongside polysaccharides and nucleic acids that make up the primary constituents of living things. They are among the most actively studied molecule in biochemistry. 2.5.2 Protein- surface interactions Protein adsorption and interaction at surfaces are involved in several situations of technical and scientific interest. There are several diagnostic methods based on the interaction between protein molecules at surfaces, such as antibody-antigen binding reactions. Furthermore, an understanding of the behaviour of protein molecules at surfaces is of great importance for the development of new types of biosensors.22 There exist several thermodynamic models which attempt to explain certain features of protein adsorption isotherms including interaction between the protein molecules adsorbed on the surface. Because of the degree of uncertainty related to the time needed for the equilibrium of the interfacial proteins, another approach is often used in protein adsorption studies instead of measuring the adsorption 12 isotherms, one resorts to the adsorption kinetics. There are a lot of processes that takes place during protein adsorption and these include: a. Transport towards the interface b. Attachment at the interface c. Structural rearrangements in the adsorbed state d. Detachment from the interface and e. Transport away from the interface Each of these steps can in principle determine the overall rate of the adsorption. It is, however evident from a variety of experimental studies that protein adsorption is a highly dynamic phenomenon and also evidence of protein- induced exchange reactions on surfaces whereby already adsorbed protein molecules are exchanged with protein molecules from solution. Fig. 2.2 Exchange reaction taking place on a surface- adsorbed protein being exchanged with protein from solution Proteins have specific sites called binding sites to which molecules and ions bind. These sites exhibit chemical specifity and the strength of ligand-protein binding is a property of the binding site known as affinity. There are also solution conditions that affect the adsorption of proteins to surfaces and these 13 are the pH, the ionic strength, ionic components and buffer type, the protein type and the concentration. The nature of the surface is of course important and plays a major role in the adsorption process.23 The present work helps to understand the various solution conditions that affect protein adsorption to surfaces. That is, the effect that different protein concentrations have on the amount of proteins adsorbed, how strongly proteins are bound to plasma polymerised allyamine films after washing with buffers of different pH and the role of film thickness in the protein adsorption. 14