The Method in Physical Chemistry - Case Study Example

Method in Physical Chemistry Method in Physical Chemistry Q2 A Conducting polymers are organic substance that consist of many same units and are either conductors or semi-conductors. Chemists have categorized conducting polymers into two fundamental groups including electron conducting and proton conducting polymers. One of the formidable conducting polymers is polyaniline (PANI). The building units of this polymer are aniline that can form polymers through various processes. Polyaniline is preferred to other organic conductive polymers due to its affordability, good conductivity, ease of preparation and redox properties. Other conducting polymers include polyacetylene and polypyrrole. Like other conducting polymers, polyaniline has an alternating single and double bonds with electrons having ability to move from one side to the other. Polyaniline has three distinctive colors depending on its oxidation state. It also has different colors depending on degree of polymerization with leucoemeraldine is white, emeraldine is green or blue and pernigraniline is violet. Further, polyaniline has both acid and base doping response that increases its conductivity in both types of media. Chemical structure of polyaniline consists of six sided carbon rings with nitrogen between the rings. The inherent organic structure of polyaniline consisting of various alternating benzene ring appears in structure as below. B Preparation of polymers takes place through two mechanisms that include chemical polymerization and electrochemical polymerization. Both involve the use of acid as part of the media in preparation. Chemical polymerization remains comprehensively understood with changes involved during elongation. Both reactions involve breaking of some previous bonds in building unit to join with other units. Chemical polymerization Chemical polymerization is a method used especially in preparation of redox polymers (Inzelt, 2012). Redox polymers relates to polymeric organic compounds containing functional groups that can undergo reversible reactions through both oxidation and reduction process. Reversible redox reactions occur within the polymer central chain in a similar manner as that of quinine or polyaniline polymers. Consequently, chemical reactions involve the use of building units of the polymer, strong oxidizing agent and a strong inorganic acid. In the case of polyaniline, start by placing appropriate quantity of 0.1M aniline in the polymerization vessel. Addition of inorganic acid such as sulfuric acid follows. Adjusting the volume of the mixture to 100 ml is necessary for standardization. Slow addition strong oxidant takes place after placing the polymerization vessel on magnetic stirrer. This ensures that the polymerization of the aniline subunits is equivalent to rate of oxidant addition. For 100 ml solution, volume of appropriate oxidant used is 20 ml with proper concentrations. Filter the product obtained then wash with dilute sulfuric acid to get green polyaniline after drying in vacuum at 60 oC. The oxidant used assist in the formation of active nitrogen joining site in every aniline subunits that forms polymer. Electrochemical Polymerization In electrochemical polymerization of aniline to form polyaniline is a reaction involving transfer of two electrons. The reaction in this method occurs in three stages. The first leads to formation of oligomers that are soluble. The next step involves deposition of the oligomer through growth and nucleation and the final step leads to chain elongation. The process of generating conducting polymers through electrochemical technique involves both charge generation and transfer as indicated below; Conductivity These polymers conduct electricity through the delocalized electrons in the conjugated system. During doping, a redox reaction takes place to form anion or cation radicals usually termed as polaron or soliton. An extra proton usually accompanies oxidation to the radical making it positively charged. A second electron transfer may take place leading to dianion or dication leading to bipolaron. Sometimes, charge transfer may take place between neutral and charged portions of the polymer. Any incoming proton produces excitons that dissociate to produce holes and electrons. The charges then relocate in their transport materials to produce net charge flow. C The ability of conducting polymer to change electrical properties due to their instability in heat and moisture is important in making of the sensors. This property is also applicable when reacting with redox agents. An example is polypyrrole that changes its resistance with varying ability of reducing gas. The ability to conduct electricity increases when reducing gases are not present. Contrary, the resistance increases and conductivity reduces in presence of reducing gas such as ammonia. Oxidizing gases such as nitrogen dioxide cause reverse action of the reducing gas. The effect reducing and oxidizing gases reacting with absorbed ions of oxygen causing change in the electron holes density. This propagation of charge is unique only to conducting polymers. Biosensors on the other side uses polyacetylene oxidation with triiodide that measure glucose concentration. Oxidation of glucose uses oxygen using enzyme glucose oxidase that produces hydrogen peroxide that further oxidizes iodide ions to triiodide ions. The concentration of peroxide then controls the amount of current flow that is also proportional to glucose concentration. The affordability of these conducting polymers makes them all more applicable in various levels such as batteries and display devices such as phones. D Corrosion is a slow destruction of a substance due to chemical reactions. Some environmental factors and chemicals can accelerate this process. Paintings are composed of polymers that increase their working ability. During application of painting, polymers in paints have solvents and their volume is large. Upon drying or evaporation of the solvent and chemical reaction, the coating shrinks. This makes them to become brittle and increase adherence to coated material. In case of lower temperatures, the coating shrinks further causing internal stress making it to crack. The crack is due to high stresses that overcome the tensile strength of the polymers in the paint. The probability of a paint cracking increases with its thickness. Thickness of polymers inherent relates to the structure and properties of its subunits used in polymerization. Peeling of the coating increases the corrosion of the previously fully coated steel. When the coating cracks, external factors affect steel directly causing increased corrosion. The paint coatings protect the steel from corrosion by provision of barrier to any corrosive agents. The paint may also have conducting polymers that lead to nucleation that assist in corrosion prevention. In significantly preventing corrosion of steel while using polymer paints, chemists may add corrosion inhibitors to further slow down eating away of the metal. For instance, cathodic inhibitors additives including magnesium salts are essential in reduction of corrosion rates thorough cathodic reactions retardation (Sastri, 2011). The additives initiate insoluble compounds precipitation within the cathodic regions resulting into creation of barrier film. E Fibers used in the reinforcement of a polymer are usually carbon, glass or aramid but rarely wood and asbestos. The polymers that are reinforced may be epoxy, vinylester polyester or phenol formaldehyde resins. The reinforced polymers have wide application from aerospace to automotive industries. Formation of the carbon fibers occurs through heating of polypropenotrile-polyacrylonite filaments in conditions of atmosphere air to form oxidized polyacrylanite. Oxidize PAN is passed through pyrolysis that involve no air. The produced from pyrolysis is carbon fibre that contain chain carbon bonded to another carbon. The carbon fibre has a great tensile strength. Carbon fibre being unipolar makes it hard to bond with the resin in the matrix. The inability will decrease mechanical strength of the composite being formed. Activation of carbon fibre has to take place to form a functional group such as carboxyl, carbonyl or hydroxyl group through oxygen treatment. These functional groups are use in carbon fibre-resin bond formation. This is one by passing the fibre through strong oxidizing agents such as nitric acid or potassium permanganate in liquid-phase oxidative treatment. The roughened carbon fibre forms greater mechanical strength and has functional groups including carbonyl, carboxyl and hydroxyls. Resin matrix contains epoxy that has functional groups that can easily share electrons to form covalent bonds. This makes it suitable for formation of carbon composite. The activated carbon fibre is then reacted with the resins to form strong covalent bonds. This provides the strongest mechanical strength due to shared electrons. Other attraction forces such as electrostatic force may also add mechanical to a small extent. The diagram shows stages of formation of carbon fiber from polyacrylanite. The structure shows part of resin polymer that reacts with carbon fiber to form carbon reinforced composite. References Inzelt, G. (2012). Conducting polymers: A new era in electrochemistry. Berlin: Springer. Sastri, V. S.(2011). Green corrosion inhibitors: Theory and practice. Hoboken, N.J: Wiley. Read More