Materials Science for Engineers PDF

Materials Science for Engineers Subject Code: 1OE1763 Dr. V. Thiruvengadam Department of Science Email: [email protected] Allianc...

Materials Science for Engineers Subject Code: 1OE1763 Dr. V. Thiruvengadam Department of Science Email: [email protected] Alliance University  Materials Science for Engineers Module-5 Polymers - Structure, property, application and processing Hydrocarbon Molecules, polymer Molecules, chemistry of polymer, molecules, molecular weight, molecular Shape, molecular Structure, molecular configurations of polymers, thermoplastic and thermosetting polymers, copolymers, polymer crystallinity, polymer crystals, defects in polymers, mechanical behaviour of polymers, crystallization, melting, and glass transition phenomena in polymers, Dr. V. Thiruvengadam polymer types - plastics, Elastomers, fibers, advanced polymeric Materials Assistant Professor Department of Science  Introduction to Polymers Naturally occurring polymers - those derived from plants and animals - have been used for many centuries; these materials include wood, rubber, cotton, wool, leather, and silk Other natural polymers, such as proteins, enzymes, starches, and cellulose, are important in biological and physiological processes in plants and animals Modern scientific research tools have made possible the determination of the molecular structures of the above mentioned natural polymeric group of materials and then developed numerous polymers that are synthesized from small organic molecules Synthetic polymers : plastics, rubbers, and fiber materials Dr. V. Thiruvengadam Assistant Professor Department of Science  Hydrocarbon Molecules Most of the polymers are organic in origin and many organic materials are hydrocarbons — that is, they are composed of hydrogen and carbon Furthermore, the intra-molecular bonds between hydrogen and carbon are covalent Each carbon atom has four valence electrons that may participate in covalent bonding, whereas every hydrogen atom has only one valence electron that participate in bonding A single covalent bond exists when each of the two bonding atoms contributes one electron Double and triple bonds between two carbon atoms involve the sharing of two and three pairs of electrons, respectively CH4 C2H4 C3H8 C2H4 C2H2 Methane Ethane Propane Ethylene Acetylene  Hydrocarbon Molecules A saturated hydrocarbon is a molecule in which there is only a single bond between Carbon atoms in the hydrocarbon chain. Example: Paraffin compounds like Methane, ethane, etc. An unsaturated hydrocarbon is a molecule in which there is a double or triple bond between any adjacent Carbon atoms Saturated hydrocarbon Unsaturated hydrocarbon CH4 C2H4 C3H8 C2H4 C2H2 Methane Ethane Propane Ethylene Acetylene Paraffin Hydrocarbon Any of the saturated hydrocarbons with the general formula CnH2n+2, where C is a carbon atom, H is a hydrogen atom, and n is an integer, is referred to as a paraffin hydrocarbon  Isomerism in Hydrocarbon Hydrocarbon compounds with the same composition having different atomic arrangements is called Isomerism Example: There are two isomers for butane, Normal butane and Isobutane Normal butane Iso-butane  Isomerism in Hydrocarbon Isomers of pentane Normal Pentane Iso-Pentane Neo-Pentane  Polymer Molecules The molecules in polymers are gigantic in comparison to the hydrocarbon molecules already discussed; because of their size they are often referred to as macromolecules Within each molecule, the atoms are bound together by covalent interatomic bonds For carbon-chain polymers, the backbone of each chain is a string of carbon atoms Many times each carbon atom singly bonds to two adjacent carbon atoms on either side, represented schematically in two dimensions as follows: Each of the two remaining valence electrons for every carbon atom may be involved in side bonding with atoms or radicals that are positioned adjacent to the chain These long molecules are composed of structural entities called repeat units, which are successively repeated along the chain The term monomer refers to the small molecule from which a polymer is synthesized Hence, monomer and repeat units are different terminologies  Chemistry of Polymer Molecules Consider the hydrocarbon ethylene (C2H4), which is a gas at ambient temperature and pressure and has the following molecular structure: Monomer of polyethylene If the ethylene gas is reacted under appropriate conditions, it will transform to polyethylene (PE), which is a solid polymeric material This process begins when an active center is formed by the reaction between an initiator or catalyst species (R.) and the ethylene monomer, as follows:  Chemistry of Polymer Molecules The polymer chain then forms by the sequential addition of monomer units to this actively growing chain molecule The active site, or unpaired electron (denoted by ·) is transferred to each successive end monomer as it is linked to the chain. Shown in the schematic below The final result, after the addition of many ethylene monomer units, is the polyethylene molecul This polyethylene chain structure can also be represented as (or)  Chemistry of Polymer Molecules Like Polyethylene, there are also other polymers which are as follows; Tetrafluoroethylene monomer, CF2 ═ CF2, can polymerize to form polytetrafluoroethylene (PTFE) Vinyl chloride monomer (CH2 ═ CHCl) can polymerize to form poly-vinyl chloride (PVC)  Chemistry of Polymer Molecules Some polymers can be represented using the following generalized form: Where the R depicts  An atom [i.e., H or Cl, for polyethylene or poly(vinyl chloride),respectively]  An organic group such as CH3, C2H5, and C6H5 (methyl, ethyl, and phenyl) For example, when R represents a CH3 group, the polymer is polypropylene (PP) Chain structure of few of Polymer Molecules Polytetrafluoroethylene Repeat unit Poly(vinyl chloride) Repeat unit Polypropylene Repeat unit of few of Polymer Molecules  Molecular Weight of Polymer Molecules Extremely large molecular weights5 are observed in polymers with very long chains During the polymerization process, not all polymer chains will grow to the same length; this results in a distribution of chain lengths or molecular weights. Ordinarily, an average molecular weight is specified, which may be determined by the measurement of various physical properties such as viscosity and osmotic pressure. 1. Number-average molecular weight 2. Weight-average molecular weight The Number-average molecular weight Mn is obtained by dividing the chains into a series of size ranges and then determining the number fraction of chains within each size range. The number-average molecular weight is expressed as where Mi represents the mean (middle) molecular weight of size range i, and xi is the fraction of the total number of chains within the corresponding size range  Molecular Weight of Polymer Molecules A weight-average molecular weight Mw is based on the weight fraction of molecules within the various size ranges. It is calculated according to where, again, Mi is the mean molecular weight within a size range, whereas wi denotes the weight fraction of molecules within the same size interval  Molecular Shape of Polymer Molecules Schematic representation of repeat unit A real structure of the Polyethylene molecule, indicating and chain structure Polyethylene the zigzag backbone structure. Consider the chain atoms in the figure a third carbon atom lie at any point on the cone of revolution and still subtend about a 109° angle with the bond between the other two atoms. 109°  Molecular Shape of Polymer Molecules A straight chain segment results when successive chain atoms are positioned as in Figure below Schematic representation of repeat unit and chain structure Polyethylene 109°  Molecular Shape of Polymer Molecules Chain bending and twisting are possible when there is a rotation of the chain atoms into other positionsThus, a single chain molecule composed of many chain atoms might assume a shape similar to that represented schematically in (right side) figure, having a multitude of bends, twists, and kinks. Also indicated in this figure is the end-to-end distance of the polymer chain r; this distance is much smaller than the total chain length.  Molecular Shape of Polymer Molecules Chain bending and twisting are possible when there is a rotation of the chain atoms into other positionsThus, a single chain molecule composed of many chain atoms might assume a shape similar to that represented schematically in (right side) figure, having a multitude of bends, twists, and kinks. Also indicated in this figure is the end-to-end distance of the polymer chain r; this distance is much smaller than the total chain length. The bends, twists, and kinks in the long chain polymer molecules leads to extensive intertwining and entanglement of neighboring chain molecules, a situation similar to what is seen in a heavily tangled fishing line These random coils and molecular entanglements are responsible for a number of important characteristics of polymers, like the large elastic extensions displayed by the rubber materials.  Molecular Structure of Polymer Molecules The physical characteristics of a polymer depend not only on its molecular weight and molecular shape, but also on differences in the structure of the molecular chains Modern polymer synthesis techniques permit considerable control over various structural possibilities Types of polymer molecular structure: linear, branched, crosslinked and network in addition to various isomeric configurations Linear polymers Linear polymers are those in which the repeat units are joined together end to end in single chains These long chains are flexible and may be thought of as a mass of “spaghetti,” as represented schematically in figure above, where each circle represents a repeat unit For linear polymers, there may be extensive van der Waals and hydrogen bonding between the chains Some of the common polymers that form with linearstructures are polyethylene, poly(vinyl chloride), polystyrene, poly(methyl methacrylate), nylon, and the fluorocarbons  Molecular Structure of Polymer Molecules Branched polymers Polymers may be synthesized in which side-branch chains are connected to the main ones, as indicated schematically in figure above, these are called branched polymers The branches, considered to be part of the main-chain molecule, may result from side reactions that occur during the synthesis of the polymer The chain packing efficiency is reduced with the formation of side branches, which results in a lowering of the polymer density Polymers that form linear structures can also be branched. For example, high-density polyethylene (HDPE) is primarily a linear polymer, whereas low-density polyethylene (LDPE) contains short-chain branches  Molecular Structure of Polymer Molecules Crosslinked polymers In crosslinked polymers, adjacent linear chains are joined one to another at various positions by covalent bonds, as represented in the figure The process of crosslinking is achieved either during synthesis or by a nonreversible chemical reaction Often, this crosslinking is accomplished by additive atoms or molecules that are covalently bonded to the chains Many of the rubber elastic materials are crosslinked; in rubbers the cross linking process is called vulcanization  Molecular Structure of Polymer Molecules Network polymers Multifunctional monomers forming three or more active covalent bonds make threedimensional networks (shown in Figure) and are termed network polymers Actually, a polymer that is highly crosslinked may also be classified as a network polymer These materials have distinctive mechanical and thermal properties; the epoxies, polyurethanes, and phenol-formaldehyde belong to this group Molecular Configurations of Polymer Molecules We have already seen Isomerism in hydrocarbons: Same composition having different atomic arrangements Isomerism can also be seen in polymer molecules: Same composition having different atomic configurations Isomerism in polymer molecule are classified in to two types 1. Stereoisomerism 2. Geometric isomerism  Molecular Configurations of Polymer Molecules Stereoisomerism denotes the situation in which atoms are linked together in the same order but differ in their spatial arrangement. For one stereoisomer, all of the R groups are situated on the same side of the chain. It is called an Isotactic configuration In a Syndiotactic configuration, the R groups on alternate sides of the chain Isotactic In a Atactic configuration, the R groups on random sides of the chain Conversion from one stereoisomer to another (e.g., isotactic to syndiotactic) is not possible by a simple rotation about single- Syndiotactic chain bonds These bonds must first be broken; then, after the appropriate rotation, they are re- formed. Atactic Molecular Configurations of Polymer Molecules The second type of isomerism that happens in polymers is Geometric isomerism Geometric isomerism are possible within polymer repeat units that having a double bond between chain carbon atoms It is based on, on which side the carbon atom (that participating in double bond) the side groups are attached with In one geometric isomer, the side groups are in same side with respect to the double bond. It is called Cis geometric isomer In another geometric isomer, the side groups are in opposite side with respect to the double bond. It is called Trans geometric isomer Molecular Configurations of Polymer Molecules Consider the isoprene repeat unit CH3 group and the H atom are positioned on the same side of the double bond. This is termed a cis structure, and the resulting polymer, cis- polyisoprene, is natural rubber CH3 group and the H atom are positioned on the opposite side of the double bond. This is termed a Trans structure, and the resulting polymer, Trans- polyisoprene Conversion of trans to cis, or vice versa, is not possible by a simple chain bond rotation because the chain double bond is extremely rigid  Thermoplastic and Thermosetting polymers Classification scheme for the characteristics of polymer molecules  Thermoplastic and Thermosetting polymers The response of a polymer to mechanical forces at elevated temperatures is related to its dominant molecular structure Polymeric materials can be classified in to two categories based on its behavior with rising temperature. They are 1. Thermoplastics polymers and 2. Thermosetting polymers  Thermoplastic polymers Thermoplastics soften when heated (and eventually liquefy) and harden when cooled — processes that are totally reversible and may be repeated On a molecular level, as the temperature is raised, secondary bonding forces are diminished (by increased molecular motion) so that the relative movement of adjacent chains is facilitated when a stress is applied Irreversible degradation results when a molten thermoplastic polymer is raised to very high temperature Thermoplastics are relatively soft Most linear polymers and those having some branched structures with flexible chains are thermoplastic Examples of common thermoplastic polymers include polyethylene, polystyrene, poly(ethylene terephthalate), and poly(vinyl chloride)  Thermosetting polymers Thermosetting polymers are network polymers They become permanently hard during their formation and do not soften upon heating Network polymers have covalent crosslinks between adjacent molecular chains During heat treatments, these bonds anchor the chains together to resist the vibrational and rotational chain motions at high temperatures. Thus, the materials do not soften when heated Crosslinking is usually extensive, in that 10% to 50% of the chain repeat units are crosslinked Only heating to excessive temperatures will cause severance of these crosslink bonds and polymer degradation Thermoset polymers are generally harder and stronger than thermoplastics and have better dimensional stability Most of the crosslinked and network polymers, which include vulcanized rubbers, epoxies, phenolics, and some polyester resins, are thermosetting polymers  Homopolymer and Copolymers In a polymer molecule, when all of the repeating units along a chain are of the same type, the resulting polymer is called a homopolymer If the polymer chains composed of two or more different repeat units, then it is called copolymers Consider a copolymer that is composed of two repeat units Depending on the polymerization process and the relative fractions of these repeat unit types, different sequencing arrangements along the polymer chains are possible Based on the sequencing arrangements, copolymers can be classified in to four types 1. Random copolymer 2. Alternating copolymer 3. Block copolymer 4. Graft copolymer  Homopolymer and Copolymers 1. Random copolymer Example Two different repeat units are randomly dispersed along the polymer chain Examples of commercially relevant random copolymers include rubbers made from styrene-butadiene copolymers and resins from styrene-acrylic or methacrylic acid derivatives 2. Alternating copolymer Two different repeat arranged in alternate position in the polymer chain Nylon 6,6  Homopolymer and Copolymers Example 3. Block copolymer Identical repeat units are clustered in blocks along the chain Acrylonitrile butadiene styrene (ABS) 4. Graft copolymer Homopolymer side branches of one type may be grafted to homopolymer main chains that are composed of a different repeat unit Polystyrene chains grafted onto polybutadiene  Polymer Crystallinity The crystalline state may exist in polymeric materials. However, because it involves molecules (instead of just atoms or ions, as with metals and ceramics), the atomic arrangements will be more complex for polymers Polymer crystallinity can be explained as the packing of molecular chains to produce an ordered atomic array For example, figure shows the unit cell for polyethylene and its relationship to the molecular chain structure; this unit cell has orthorhombic geometry Of course, the chain molecules also extend beyond the unit cell shown in the figure  Polymer Crystallinity Molecular substances having small molecules (e.g., water and methane) are normally either totally crystalline (as solids) or totally amorphous (as liquids) In the case of polymers, as a consequence of their size and often complexity, the are often only partially crystalline (or semicrystalline), having crystalline regions dispersed within the remaining amorphous material Any chain disorder or misalignment will result in an amorphous region, a condition that is fairly common, because twisting, kinking, and coiling of the chains prevent the strict ordering of every segment of every chain.  Polymer Crystals It has been proposed that a semicrystalline polymer consists of small crystalline regions (crystallites), each having a precise alignment, which are separated by amorphous regions composed of randomly oriented molecules These polymer crystals are regularly shaped, thin platelets (or lamellae) approximately 10 to 20 nm thick, and on the order of 10 μm long The molecular chains within each platelet fold back and forth on themselves, with folds occurring at the faces; this structure, aptly termed the chain-folded model, is illustrated schematically in figure Each platelet consists of a number of molecules; however, the average chain length is much greater than the thickness of the platelet  Polymer Crystals Many bulk polymers that are crystallized from a melt are semicrystalline and form a spherulite structure As implied by the name, each spherulite may grow to be roughly spherical in shape The spherulite consists of an aggregate of ribbon like chain-folded crystallites (lamellae) approximately 10 nm thick that radiate outward from a single nucleation site in the center The detailed structure of a spherulite is illustrated schematically in the figure Shown here are the individual chain-folded lamellar crystals that are separated by amorphous material Tie-chain molecules that act as connecting links between adjacent lamellae pass through these amorphous regions  Polymer Crystals Many bulk polymers that are crystallized from a melt are semicrystalline and form a spherulite structure As implied by the name, each spherulite may grow to be roughly spherical in shape  Crystallization, melting, and glass transition phenomena in polymers Three phenomena that are important with respect to the design and processing of polymeric materials are crystallization, melting, and the glass transition 1. Crystallization 2. Melting and 3. Glass transition Crystallization is the process by which, upon cooling, an ordered (i.e., crystalline) solid phase is produced from a liquid melt having a highly random molecular structure The melting transformation is the reverse process that occurs when a polymer is heated The glass-transition phenomenon occurs with amorphous or noncrystallizable polymers that, when cooled from a liquid melt, become rigid solids yet retain the disordered molecular structure that is characteristic of the liquid state For a semicrystalline polymers, crystalline regions will experience melting (and crystallization), whereas noncrystalline areas pass through the glass transition  Crystallization An understanding of the mechanism and kinetics of polymer crystallization is important because the degree of crystallinity influences the mechanical and thermal properties of these materials An understanding of the mechanism and kinetics of polymer crystallization is important because the degree of crystallinity influences the mechanical and thermal properties of these materials The crystallization of a molten polymer occurs by nucleation and growth processes For polymers, upon cooling through the melting temperature, nuclei form, in which small regions of the tangled and random molecules become ordered and aligned in the manner of chain-folded layers (as shown in figure) Subsequent to nucleation and during the crystallization growth stage, nuclei grow by the continued ordering and aligning of additional molecular chain segments; that is, the chain-folded layers remain the same thickness, but increase in lateral dimensions, or for spherulitic structures there is an increase in spherulite radius  Melting The melting of a polymer crystal corresponds to the transformation of a solid material, having an ordered structure of aligned molecular chains, into a viscous liquid in which the structure is highly random. This phenomenon occurs, upon heating, at the melting temperature, Tm The thickness of chain-folded lamellae depends on crystallization temperature; the thicker the lamellae, the higher the melting temperature Impurities in the polymer and imperfections in the crystals also decrease the melting temperature Finally, the apparent melting behavior is a function of the rate of heating; increasing this rate results in an elevation of the melting temperature. Polymeric materials are responsive to heat treatments that produce structural and property alterations An increase in lamellar thickness may be induced by annealing just below the melting temperature. Annealing also raises the melting temperature by decreasing the vacancies and other imperfections in polymer crystals and increasing crystallite thickness  Glass Transition Temperature The glass transition occurs in amorphous (or glassy) and semicrystalline polymers and is due to a reduction in motion of large segments of molecular chains with decreasing temperature. Upon cooling, the glass transition corresponds to the gradual transformation from a liquid into a rubbery material and finally into a rigid solid The temperature at which the polymer experiences the transition from rubbery into rigid states is termed the glass transition temperature, Tg. This sequence of events occurs in the reverse order when a rigid glass at a temperature below Tg is heated Melting and Glass Transition Temperature Melting and glass transition temperatures are important parameters relative to in-service applications of polymers Melting and glass transition temperatures define, respectively, the upper and lower temperature limits for numerous applications, especially for semicrystalline polymers A plot of specific volume (the reciprocal of density) versus temperature is shown in figure where curves A and C, for amorphous and crystalline polymers, respectively For the crystalline material, there is a discontinuous change in specific volume at the melting temperature Tm The curve for the totally amorphous material is continuous but experiences a slight decrease in slope at the glass transition temperature, Tg The behavior is intermediate between these extremes for a semicrystalline polymer (curve B), in that both melting and glass transition phenomena are observed; Tm and Tg are properties of the respective crystalline and amorphous phases in this semicrystalline material Classification of polymers based on their end use The polymer types based on their end use include plastics, elastomers (or rubbers), fibers, coatings, adhesives, foams and films Plastics Plastics are materials that have some structural rigidity under load and are used in general-purpose applications Polyethylene, polypropylene, poly(vinyl chloride),polystyrene, and the fluorocarbons, epoxies, phenolic, and polyesters may all be classified as plastics They have a wide variety of combinations of properties. Some plastics are very rigid and brittle. Others are flexible, exhibiting both elastic and plastic deformations when stressed and sometimes experiencing considerable deformation before fracture Elastomers Elastomer, any rubbery material composed of long chainlike molecules, or polymers, that are capable of recovering their original shape after being stretched to great extents—hence the name elastomer, from “elastic polymer.” Under normal conditions the long molecules making up an elastomeric material are irregularly coiled. With the application of force, however, the molecules straighten out in the direction in which they are being pulled. Upon release, the molecules spontaneously return to their normal compact, random arrangement. Classification of polymers based on their end use Following are examples of elastomers with their applications: 1.Natural rubber: These are used in the automotive industry and in the manufacture of medical tubes, balloons, adhesives 2.Polyurethanes: These are used in the textile industry for manufacturing elastic clothing 3.Polybutadiene: These are used for providing wear resistance in wheels of vehicles. 4.Silicone: These are used in the manufacture of medical prostheses and lubricants as they have excellent chemical and thermal resistance 5.Neoprene: These are used in the manufacture of wet-suits and in industrial belts Fibers Fiber polymers are capable of being drawn into long filaments having at least a 100:1 length-to-diameter ratio. Most commercial fiber polymers are used in the textile industry, being woven or knit into cloth or fabric. In addition, the aramid fibers are employed in composite materials. To be useful as a textile material, a fiber polymer must have a host of rather restrictive physical and chemical properties. While in use, fibers may be subjected to a variety of mechanical deformations—stretching, twisting, shearing, and abrasion. Consequently, they must have a high tensile strength (over a relatively wide temperature range) and a high modulus of elasticity, as well as abrasion resistance. These properties are governed by the chemistry of the polymer chains and also by the fiber-drawing process. Classification of polymers based on their end use Coatings Coatings are frequently applied to the surface of materials to serve one or more of the following functions: (1) to protect the item from the environment, which may produce corrosive or deteriorative reactions; (2) to improve the item’s appearance; and (3) to provide electrical insulation. Many of the ingredients in coating materials are polymers, most of which are organic in origin. These organic coatings fall into several different classifications: paint, varnish, enamel, lacquer, and shellac Films Polymeric materials have found widespread use in the form of thin films. Films having thicknesses between 0.025 and 0.125 mm (0.001 and 0.005 in.) are fabricated and used extensively as bags for packaging food products and other merchandise, as textile products, and in a host of other uses. Important characteristics of the materials produced and used as films include low density, a high degree of flexibility, high tensile and tear strengths, resistance to attack by moisture and other chemicals, and low permeability to some gases, especially water vapor. Some of the polymers that meet these criteria and are manufactured in film form are polyethylene, polypropylene, cellophane, and cellulose acetate. Classification of polymers based on their end use Adhesives An adhesive is a substance used to bond together the surfaces of two solid materials (termed adherends). There are two types of bonding mechanisms: mechanical and chemical. In mechanical bonding there is actual penetration of the adhesive into surface pores and crevices Chemical bonding involves intermolecular forces between the adhesive and adherend, which forces may be covalent and/or van der Waals; the degree of van der Waals bonding is enhanced when the adhesive material contains polar groups. Although natural adhesives (animal glue, casein, starch, and rosin) are still used for many applications, a host of new adhesive materials based on synthetic polymers have been developed; these include polyurethanes, polysiloxanes (silicones), epoxies, polyimides, acrylics, and rubber materials. Adhesives may be used to join a large variety of materials—metals, ceramics, polymers, composites, skin, and so on—and the choice of which adhesive to use will depend on such factors as (1) the materials to be bonded and their porosities; (2) the required adhesive properties (i.e., whether the bond is to be temporary or permanent); (3) maximum/minimum exposure temperatures; and (4) processing conditions Classification of polymers based on their end use Foams Foams are plastic materials that contain a relatively high volume percentage of small pores and trapped gas bubbles. Both thermoplastic and thermosetting materials are used as foams; these include polyurethane, rubber, polystyrene, and poly(vinyl chloride). Foams are commonly used as cushions in automobiles and furniture, as well as in packaging and thermal insulation. The foaming process is often carried out by incorporating into the batch of material a blowing agent that, upon heating, decomposes with the liberation of a gas. Gas bubbles are generated throughout the now-fluid mass, which remain in the solid upon cooling and give rise to a spongelike structure. The same effect is produced by dissolving an inert gas into a molten polymer under high pressure. When the pressure is rapidly reduced, the gas comes out of solution and forms bubbles and pores that remain in the solid as it cools.