Chapter 1 Polymer Structures PDF

CHAPTER 1 POLYMER STRUCTURES LEARNING OUTCOMES List out and distinguish between various classes 01 of polymeric materials such as thermoplastics, thermosetting or cellulosic 02 Differentiate between amorphous or crystalline polymers INTRODUCTION TO POLYMER...

CHAPTER 1 POLYMER STRUCTURES LEARNING OUTCOMES List out and distinguish between various classes 01 of polymeric materials such as thermoplastics, thermosetting or cellulosic 02 Differentiate between amorphous or crystalline polymers INTRODUCTION TO POLYMER WHAT IS POLYMER? Poly means many; mer means to repeat. WHAT IS A large molecules (macromolecules) that POLYMER? are comprised or built up of smaller units or monomers Polymers are made up of many molecules all strung together to form really long chains (and sometimes more complicated structures) REPEATING UNIT The repeating unit of a linear polymer (which is defined below) is a portion of the macromolecule such that the complete polymer (except for the ends) might be produced by linking a sufficiently large number of these units through bonds between specified atoms. The repeating unit may comprise a single identifiable precursor as in polystyrene (1-1), polyacrylonitrile (1-2), polyethylene (1-3), or poly(vinyl chloride): FORMATION OF POLYMER FORMATION OF POLYMER REPEATING UNIT Polyvinyl alcohol is the example of polymer that cannot be produced by linking the same monomer because it does not exist! Poly(vinyl acetate) Alcoholysis with ethanol or methanol Vinyl acetate REPEATING UNIT Protein fibers such as silk are degradable to mixtures of amino acids, but a direct synthesis of silk has not yet been accomplished Poly(vinyl acetate) Alcoholysis with ethanol or methanol Vinyl acetate  Classification of polymer 1 Source 2 Structure of polymers 3 Mode of polymerization 4 Thermal behaviour 5 Tacticity 6 Arrangement of monomer 5 Crystallinity Source a) Natural Polymers b) Semisynthetic Polymers - They are available in nature - Chemically modified natural - Examples : natural rubber, natural polymers silk, cellulose, starch, proteins - Examples : hydrogenated, halogenated or hydro-halogenated natural rubber, cellulose nitrate, methyl cellulose c) Synthetic Polymers - Man-made polymers prepared synthetically - Examples : Polyethylene, polystyrene, poly (vinyl chloride), polyesters, phenol-formaldehyde resins Structure of polymer  Molecular PS architecture Short branch but still considered as linear polymer Poly(methyl Poly(4-methyl pentene- methacrylate) 1) A linear polymer is one in which each repeating unit is linked LINEAR POLYMER only to two others  Molecular architecture Branched polymers are those in which the repeating units are not linked solely in a linear array, either because at least one of BRANCHED POLYMER the monomers has functionality greater than 2 or because the polymerization process itself produces branching points in a polymer that is made from exclusively bifunctional monomers  Molecular architecture Epoxidized Linseed oil Alkyd Polymer : Glycerol, phthalic anhydride, and linseed oil Branched polymers are those in which the repeating units are not linked solely in a linear array, either because at least one of BRANCHED POLYMER the monomers has functionality greater than 2 or because the polymerization process itself produces branching points in a polymer that is made from exclusively bifunctional monomers  Molecular architecture The major example of the second branched polymer type is the polyethylene that is made by free- radical polymerization at temperatures between about 100!C and 300!C and pressures of 1000-3000 atm (100-300 MPa) Depending on reaction conditions, these polymers will contain some 20 to 30 ethyl and butyl branches per 1000 carbon atoms and one or a few much longer branches per molecule BRANCHED POLYMER  Crosslinked / Network Polymer Network polymers can also be made by chemically linking linear or CROSSLINKED / NETWORK branched polymers. The process whereby such a preformed polymer is converted to a network structure is called cross-linking  Crosslinked / Network Polymer Novolac phenol A network polymer is an interconnected branch polymer. NETWORK Network polymers can also be made by chemically linking linear or branched polymers. The process whereby such a preformed polymer is converted to a network structure is called cross-linking  Crosslinked / Network Polymer A network polymer is an interconnected branch polymer. CROSSLINKED Network polymers can also be made by chemically linking linear or branched polymers. The process whereby such a preformed polymer is converted to a network structure is called cross-linking 1 Crosslinked / Network Polymer 2 A network polymer is an interconnected branch polymer. CROSSLINKED Network polymers can also be made by chemically linking linear or branched polymers. The process whereby such a preformed polymer is converted to a network structure is called cross-linking  NETWORK CROSSLINKED NETWORK Network polymers are A subclass of network polymers Vs. polymers that have covalent called crosslinked polymers CROSSLINKED connections connecting every also have a three-dimensional polymer chain, creating a structure with covalent continuous, three-dimensional connections connecting the network structure. polymer chains. Usually come into existence A linear or branching polymer either by interconnecting can be purposely crosslinked polymer chains during the by adding crosslinking agents polymerization process or during polymerization, or it can through subsequent post- be subjected to outside forces polymerization procedures.. such as heat, radiation, or chemical treatments to induce Example: epoxy resins, crosslinks. phenolic resins, and some types of silicone rubber. Example : vulcanized rubber (crosslinked natural rubber), thermosetting plastics (e.g., Bakelite), and certain dental materials. Mode of polymerization Polymerisation is the process of joining together a large number of small molecules to make a smaller number of very large molecules. The reactants (i.e. the small molecules from which the polymer is constructed) are called monomers and products of the polymerisation process are called polymers. This polymerisation process can occur by two different mechanisms: a) chain growth/addition polymerisation b) step growth/condensation polymerisation Chain growth/addition polymerisation Addition polymerisation is the joining together of two or more simple molecules, called monomers, to form a new compound of the same empirical formula, called a polymer which has a very high molecular weight. The addition polymerisation process can only occur when the monomer molecule is unsaturated (i.e. contain double bonds or triple bonds). Thus, addition polymerisation is characteristic of ethene and the other ethenes. The polymers formed by addition polymerisation are thermoplastic. These include Polythene, Polypropylene and Polystyrene. Step growth/condensation polymerisation In condensation polymerisation, two or more monomers join together with the elimination of a small molecule, such as water The monomers must have different reactive functional groups so that they can react together to form a polymer The polymer formed is called condensation polymer  Addition polymerization is when the monomer molecules bond to each other without the loss of any other atoms. Examples of addition polymers include polyethylene, polypropylene, polystyrene, polyvinylchloride, polytetrafluoroethylene, etc.  Monomers of condensation polymers must contain two functional groups so that each monomer can link up with two other monomers Thermal behaviour Thermal behaviour Thermoplastic Thermoset Polymers which change irreversibly Polymers which Example : Example: bakelite are easily softened Polyethylene & into hard and rigid & melamine materials on upon heating Polystyrene heating and cannot formaldehyde be reshaped  a) Thermoplastic b) Thermoset Can be softened or plasticized Soluble and fusible forms in early or repeatedly on application of intermediate stages of their synthesis, thermal energy but they get set or cured and become insoluble and infusible when further The polymer melt can be formed or shaped when in this softened heated or thermally treated state. The curing or setting process involves chemical reactions leading to further When cooled significantly below growth and crosslinking of the polymer their softening point they again chain molecules and producing giant become rigid and usable molecules Upon heating, thermosetting polymers This type of polymer can be readily will not soften, cannot be shaped or recycled because each time it is formed to any great extent, and will reheated it can again be reshaped definitely not flow. or formed into a new article. Thermoplastic products and properties Thermoset products and properties Tacticity Arrangement of monomers Crystallinity a) Crystalline Polymers Highly crystalline polymers are rigid, high melting, and less affected by solvent penetration. Crystallinity makes a polymers strong, but also lowers their impact resistance b) Amorphous Polymers Polymer chains with branches or irregular pendant groups cannot pack together regularly enough to form crystals. These polymers are said to be amorphous. Amorphous regions of a polymer are made up of a randomly coiled and entangled chains. Amorphous polymers are softer, have lower melting points, and are penetrated more by solvents than their crystalline counterparts. c) Semi-Crystalline Polymers Semi-crystalline polymers have both crystalline and regions. Semi-crystallinity is a desirable property for most plastics because they combine the strength of crystalline polymers with the flexibility of amorphous. Semi-crystalline polymers can be tough with an ability to bend without breaking. Thank you