Engineering Materials - Polymers PDF

CHEME - Chemistry for Engineers Engineering Materials – Polymer A polymer is a molecular compound distinguished by a high molar mass, ranging into thousands and millions of grams, and is constructed by the sequential stringing together of smaller molecules ca...

CHEME - Chemistry for Engineers Engineering Materials – Polymer A polymer is a molecular compound distinguished by a high molar mass, ranging into thousands and millions of grams, and is constructed by the sequential stringing together of smaller molecules called monomers. CLASSIFICATION OF POLYMERS ACCORDING TO ORIGIN 1. NATURAL: occur in nature. Examples: cellulose, natural rubber, cotton, natural silk, wool, starch, protein 2. SEMISYNTHETIC: modified natural polymers. Examples: hydrogenated natural rubber, cellulose nitrate, methylcellulose 3. SYNTHETIC: those that are synthesized in the laboratory. Examples: plastics, Nylon, Dacron, polyvinyl chloride **C-C bonds are exceptionally strong and stable; it is advantageous for them to serve as the backbone of most synthetic polymers available. STRUCTURE OF ORGANIC COMPOUNDS Carbon has four valence electrons ([He]2s2,2p2); hence, it forms four bonds in virtually all its compounds. Because the valence shell of C—H can hold only two electrons, hydrogen forms only one covalent bond. As a result, hydrogen atoms are always located on the surface of organic molecules whereas the bonds form the backbone, or skeleton, of the molecule. CHEME - Chemistry for Engineers Engineering Materials – Polymer CHEME - Chemistry for Engineers Engineering Materials – Polymer CHEME - Chemistry for Engineers Engineering Materials – Polymer  CHEME - Chemistry for Engineers Engineering Materials – Polymer CLASSIFICATION OF POLYMERS ACCORDING TO MODE OF FORMATION: 1. ADDITION POLYMERS: They are formed from the simple addition of monomer molecules to each other in quick succession by a chain mechanism without the liberation of small molecules. Examples: polyethylene, polypropylene, polystyrene 2. CONDENSATION POLYMERS: Occurs between monomers containing polar groups which form polymers along with the elimination of small molecules. The reactions usually take place through the functional groups. Examples: polyurethane, polycarbonate, polyesters 3. CO-POLYMERIZATION: Two or more molecules undergo joint polymerization. Example: SBR CLASSIFICATION OF POLYMERS ACCORDING TO CHAIN STRUCTURE: 1. Homopolymer–when all the repeating units along a chain are of the same type. 2. Copolymer –when all the repeating units along a chain are different. CLASSIFICATION OF POLYMERS ACCORDING TO MOLECULAR CONFIGURATION: TYPES OF POLYMERIC STEREOISOMERS Stereoisomerism denotes the situation in which atoms are linked together in the same order (head-to-tail) but differ in their spatial arrangements. 1. Isotactic configuration –the R groups are situated on the same side of the chain 2. Syndiotactic configuration–the R groups are situated at the alternate sides of the chain. 3. Atactic configuration–the R groups are situated randomly at the alternate sides of the chain. CLASSIFICATION OF POLYMERS ACCORDING TO MOLECULAR CONFIGURATION: TYPES OF GEOMETRIC ISOMERS 1. Cis-structure –CH3 group and H are positioned on the same side of the double bond. Example: Natural rubber (cis polyisoprene) 2. Trans-structure–CH3 group and H are positioned on opposite sides of the double bond. Example: Guttapercha (trans-polyisoprene) *Trans is less elastic because of the more ordered structure and more close packing of the molecule. CLASSIFICATION OF POLYMERS ACCORDING TO MOLECULAR STRUCTURE: The physical characteristics of a polymer depend not only on its molecular weight and shape but also on differences in the structure of the molecular chains. 1. Linear –those in which the repeat units are joined together end to end in a single chain. The long chains are flexible and may exhibit extensive van de Waals and hydrogen bonding between the chains. Examples: polyethylene, poly (vinyl chloride), polystyrene, poly(methyl methacrylate) and nylon. 2. Branched polymers –those that have side-branch chains that are connected to the main ones. The chain packing efficiency is reduced with the formation of the side branches which results in a lowering of the polymer density. 3. Cross-linked –adjacent linear chains are joined one to another at various positions by covalent bonds. Often, this cross-linking is accomplished by additive atoms or molecules that are covalently bonded to the chains. 4. Network –a three-dimensional network polymer that is formed from three or more active covalent bonds of multifunctional monomers. Examples: epoxies, polyurethanes, and phenol-formaldehyde. CLASSIFICATION ACCORDING TO THERMAL RESPONSE 1. Thermoplastics (thermoplastic polymers) –soften when heated and harden when processes that are totally reversible and may be repeated. 2. Thermosets (thermosetting polymers) –become permanently hard during formation and do not soften upon heating.  CHEME - Chemistry for Engineers Engineering Materials – Polymer INFLUENCE OF STRUCTURE ON POLYMER PROPERTIES 1. Strength of polymer a. Cross-linked polymers are strong and tough due to the restricted movement of the intermolecular chains that are linked through covalent bonds. b. Polymers of low molecular weights are soft and gummy but brittle. Those with higher chain groups are tougher and have higher resistance to heat. c. Presence of polar groups along the polymer chain length has a higher strength due to the increased intermolecular interactions. 2. Plastic deformation a. Polymers consisting of linear-chain molecules are thermoplastics even with high molecular weight and are soluble. b. Three-dimensional polymer molecules are insoluble and are thermosets. c. Cross-linking converts thermoplastic materials into thermosetting. 3. Solubility and chemical resistance a. Polymers containing polar groups are more soluble in polar solvents but are chemically resistant to non-polar solvents. b. Nonpolar groups containing polymers are soluble in nonpolar solvents but are chemically resistant to polar solvents. 4. Mechanical properties a. The strength of a polymer is controlled by the length of the chain and its cross-linking. COMPOUNDING: The process of mixing additives to plastics COMMON ADDITIVES 1. Resins: bind all additives together 2. Fillers: modify the properties of plastic to make the final product better in hardness, tensile strength, and workability. 3. Plasticizers: small molecules that penetrate the polymer matrix and neutralize a part of the intermolecular forces of attraction between macromolecules and increase the mobility of the polymer segments so that chains can slide over each other. 4. Coloring materials: added to improve the aesthetic effect of the material. 5. Catalysts: added to accelerate the cross-linking of the thermosetting. 6. Stabilizers: added to improve the thermal stability of polymers during processing. 7. Antistatic: added to dissipate the electrical charge developed by conducting it away. POLYMER MOLECULAR WEIGHT 1. Number average molecular weight, Mn–obtained by dividing the chains into a series of size ranges and determining the number fraction of chains within each size range 𝑀𝑛 = ∑ 𝑥𝑖 𝑀𝑖 where xi = the fraction of the total number of chains within the corresponding size range Mi = the mean molecular weight of size range i 2. Weight-average molecular weight, Mw –based on the weight fraction of molecules within the various size ranges 𝑀𝑤 = ∑ 𝑤𝑖 𝑀𝑖 where wi= the weight fraction of molecules within the same size interval Mi = the mean molecular weight of the size range  CHEME - Chemistry for Engineers Engineering Materials – Polymer 3. Degree of Polymerization, DP represents the average number of repeat units in a chain. 𝑀𝑛 𝐷𝑃 = 𝑚 where m = repeat unit molecular weight Mn= the number average molecular weight Sample Problem: Calculate the number-average molecular weight of a random poly(isobutylene-isoprene) copolymer in which the fraction of isobutylene repeat units is 0.25. Assume that this concentration corresponds to a degree of polymerization of 1500.