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Chapter 14 Synthetic Polymers Hasan A Al-Muallem 14.1 Basic Concepts and Definitions 511 14.2 Classification of Polymers 513 14.3 Polymers Industry 520 14.4 Polymer Structure 520 14.5 Polymer Structure-Property Relationships 541 14.5.1 Thermal properties 541 14.5.2 Mechanical properties 546 14.5.3 Solubility 548 14.5.4 Viscosity 14.6 Rheology 14.7 Molecular Weight of Polymers 14.8 The Synthesis of High Polymers 14.9 554 556 560 564 14.8.1 Condensation or step-reaction polymerization 569 14.8.2 Addition or chain-reaction polymerization 572 14.8.3 Free radical polymerization 573 14.8.4 Ionic polymerization 582 Polymerization Techniques 594 14.10 Copolymerization 600 14.11 Modification of Synthetic Polymers 607 14.12 Degradation, Stability, and Environmental Issues 611 14.13 Polymer Additives 616 References 618 14.1 Basic Concepts and Definitions A polymer is a giant molecule made up of a large number of repeating units joined together by covalent bonds The simple compounds from which polymers are made are called monomers The word polymer is derived from the Greek words poly (many) and meros (parts) Polymer molecules have molecular weight in the range of several thousands or more, and therefore, are also referred to as macro molecules This is illustrated by the following equation, which shows the formation of the polymer polystyrene Styrene (monomer) Polystyrene (polymer) (D The styrene molecule is the monomer, and the resulting structure, enclosed in square brackets, is the polymer polystyrene The unit in square brackets is called the repeating unit Some polymers are derived from the mutual reaction of two or more monomers For example, poly(hexamethylene adipamide) or nylon-6,6 is made from the reaction of hexamethylenediamine and adipic acid, as shown in the following equation: Hexamethylenediamine Adipic acid Poly(hexamethylene adipamide) (nylon-6,6) (2) For a molecule to be a monomer, it must be at least bifunctional The functionality of a molecule refers to its interlinking capacity, or the number of sites it has available for bonding with other molecules Reactions between monofunctional molecules use up the reactive groups completely and render the product incapable of further reactions, whereas the presence of two condensable groups in both hexamethylenediamine (-NH2) and adipic acid (-COOH) makes each of these monomers bifunctional with the ability to form polymers In this respect, styrene is also a bifunctional monomer because the extra pair of electrons in the double bond can form two bonds with vinyl groups in other molecules The number of repeating units in the polystyrene structure (1) is indicated by the index n This is known as the degree of polymerization (DP) It specifies the length of the polymer chain Oligomer is a very low Figure 14.1 Schematic structure of fully stretched polyethylene molecular weight polymer usually with less than 10 repeating units The word oligomer is derived from the Greek word oligos meaning a few Oligomers exhibit different thermal and mechanical properties compared to the corresponding high-molecular weight polymer It is sometimes useful to prepare oligomers with certain functional groups at the end that can be used in further chemistry The degree of polymerization represents one way of quantifying the molecular weight or size of a polymer For example, a linear polyethylene consisting of one thousand ethylene units will have a molecular weight of 28,000, and an extended length of 2520 angstroms (A) (Fig 14.1) However, because of rotation of the carbon-carbon bonds, the polymer chains are seldom extended to their full contour length but are present in many different shapes or conformations 14.2 Classification of Polymers Polymers can be classified in many ways according to various criteria such as: a Origin of the polymer: Polymers can be classified as being natural or synthetic based on the origin of the polymer Certain polymers, such as nucleic acids, proteins, cellulose, natural rubber, wool, and silk are found in nature Clays, sands, graphite, and diamond are also naturally occurring polymers On the other hand, thousands of polymers have been synthesized and more are likely to be produced in the future In some cases, naturally occurring polymers can be produced by synthetic routes For example, polyisoprene is the synthetic version of natural rubber (Hevea) b Functional groups present in the repeating unit: In this respect, polymers can be grouped in families like polyesters, polyamides, polyimides, polycarbonates, polyurethanes, polyureas, polyethers, polysulfides, and so on c Polymer structure: A polymer can be described as: Linear, branched, cross-linked, ladder, star-shaped, combshaped, dendritic, and the like (Fig 14.2) linear branched comb star crossl inked (network) ladder dendritic Figure 14.2 Schematic representation of different polymer structures Amorphous or crystalline based on absence or presence of longrange ordered pattern among polymer chains Homopolymer or copolymer with different types of copolymer Fibers, plastics, or elastomers Polymers (synthetic or natural) can be divided into various families; fibers, elastomers, plastics, adhesives, and each family itself has subgroups d Polymerization mechanism: Based on the polymer-forming reaction; condensation versus addition or step-growth versus chaingrowth polymerization reactions e Preparative technique: Bulk, solution, suspension, emulsion, or precipitation / Thermal behavior: Thermoplastics or thermosets g End use: Such as diene polymers (rubber industry); olefin polymers (sheet, film, and fiber industries); and acrylics (coating and decorative materials) Tension Random Release of Oriented coil tension (b) (a) Figure 14.3 Illustration of rubbery elastomeric property, (a) Relaxed: high entropy; (b) Stretched: low entropy Elastomers Elastomers are polymeric materials with irregular structure and weak intermolecular attractive forces Elastomers are capable of high extension (up to 1000%) under ambient conditions That is, they have the particular kind of elasticity characteristic of rubber The elasticity is attributed to the presence of chemical and/or physical crosslinks in these materials In their normal state, elastomers are amorphous, and as the material is stretched, the random chains are forced to occupy more ordered positions Releasing the applied force allows the elongated chains to return to a more random state Thus, the restoring force after elongation is largely because of entropy (Fig 14.3) In addition to natural rubber, there are synthetic elastomers such as diene elastomers (e.g., polybutadiene, polyisoprene, polychloroprene, and so on,), nondiene elastomers (e.g., polyisobutylene, polysiloxanes, polyurethanes), and nitrile and butyl rubber Elastomers can also be made from block copolymers containing hard or rigid segments of polyurethane and soft or flexible segments derived from the polyester or polyether diols with degrees of polymerization generally above 15 Polyurethane elastomeric materials exhibit good abrasion resistance, chemical resistance, and good tear strength with a wide variation of flexibility available These polyurethanes are also used in fabrics and sporting goods (Fig 14.4) Fibers A fiber is often defined as an object with a length-to-diameter ratio of at least 100 Fibers (synthetic or natural) are polymers with high molecular symmetry and strong cohesive energies between chains that usually result from the presence of polar groups Fibers possess a high degree of crystallinity characterized by the presence of stiffening groups in the polymer backbone, and of intermolecular hydrogen bonds Also, they are characterized by the absence of branching or irregularly spaced Isoprene Polyether or polyester cis-1,4-polyisoprene (natural rubber) Aromatic diisocyanate (excess) (unreacted) hard segment soft segment Figure 14.4 Examples of synthetic elastomers, (a) polyisoprene, and (6) polyurethane elastomer pendant groups that will otherwise disrupt the crystalline formation Fibers are normally linear and drawn in one direction to make them long, thin, and threadlike, with great strength along the fiber These characteristics permit formation of this type of polymer into long fibers suitable for textile applications Typical examples of fibers include polyesters, nylons, and acrylic polymers such as polyacrylonitrile, and naturally occurring polymers such as cotton, wool, and silk (Fig 14.5) Plastics Plastics are the polymeric materials with properties intermediate between elastomers and fibers In spite of the possible differences in chemical structure, the demarcation between fibers and plastics may sometimes be blurred Polymers such as polypropylene and polyamides can be used as fibers and plastics by a proper choice of processing conditions Plastics can be extruded as sheets or pipes, painted on surfaces, or molded to form countless objects A typical commercial plastic resin may contain two or more polymers in addition to various additives and fillers Additives and fillers are used to improve some property such as the processability, thermal or environmental stability, and mechanical properties of the final product Thermoplastics and thermosets All polymers can be divided into two major groups {thermoplastics and thermosets) based on their thermal processing behavior Thermoplastic polymers soften and flow under the action Poly(ethylene terephthalate) (polyester) Hexamethylenediamine Sebacoyl chloride Acrylonitrile Poly(hexamethylsebacamide) (nylon-6,10) Poly(acrylo nitrile) Figure 14.5 Examples of synthetic fibers: polyester, nylon and poly (acrylonitrile) of heat and pressure Upon cooling, the polymer hardens and assumes the shape of the mold (container) Thermoplastics, when compounded with appropriate ingredients, can usually withstand several heating and cooling cycles without suffering any structural breakdown Examples of commercial thermoplastics are polystyrene, polyolefins (e.g., polyethylene and polypropylene), nylon, poly(vinyl chloride), and poly(ethylene terephthalate) (Fig 14.6) Thermoplastics are used for a wide range of Ethylene Polyethylene Propylene Polypropylene Vinyl chloride Poly(vinyl chloride) Figure 14.6 Examples of commercial thermoplastics applications, such as film for packaging, photographic, and magnetic tape, beverage and trash containers, and a variety of automotive parts and upholstery Advantageously, waste thermoplastics can be recovered and refabricated by application of heat and pressure Thermosets are polymers whose individual chains have been chemically linked by covalent bonds during polymerization or by subsequent chemical or thermal treatment during fabrication The thermosets usually exist initially as liquids called prepolymers; they can be shaped into desired forms by the application of heat and pressure Once formed, these cross-linked networks resist heat softening, creep and solvent attack, and cannot be thermally processed or recycled Such properties make thermosets suitable materials for composites, coatings, and adhesive applications Principal examples of thermosets include epoxies, phenol-formaldehyde resins, and unsaturated polyesters Vulcanized rubber used in the tire industry is also an example of thermosetting polymers Thermosetting polymers are usually insoluble because the crosslinking causes a tremendous increase in molecular weight At most, thermosetting polymers only swell in the presence of solvents, as solvent molecules penetrate the network Examples of the reactions of phenol and formaldehyde to yield phenol-formaldehyde resins are shown in Fig 14.7 Properties of a specific polymer can often be varied by means of controlling molecular weight, end groups, processing, cross-linking, additives, and so on Therefore, it is possible to classify a single polymer in more than one category For example, nylon can be produced as fibers in the crystalline forms, or as plastics in the less crystalline forms Also, poly(vinyl chloride) and siloxanes can be processed to act as plastics or elastomers Commodity and engineering polymers On the basis of end use and economic considerations, polymers can be divided into two major classes: commodity plastics and engineering polymers Commodity plastics are characterized by high volume and low cost They are used frequently in the form of disposable items such as packaging film, but also find application in durable goods Commodity plastics comprise principally of four major thermoplastic polymers: polystyrene, polyethylene, polypropylene, and poly (vinyl chloride) Engineering plastics refer to those polymers that are used in the manufacture of premium plastic products where high temperature resistance, high impact strength, chemical resistance, or other special properties are required They compete with metals, ceramics, and glass in a variety of applications Engineering plastics are designed to replace metals Compared to commodity plastics, engineering plastics are specialty polymers that provide outstanding properties such as superior mechanical heat Phenol Formaldehyde (a) Novolac (b) Resole Figure 14.7 Representative structures of phenol-formaldehyde resins: (a) novolac (formed under acidic conditions), and (b) resole (formed under basic conditions) properties, high thermal stability, excellent chemical resistance, low creep compliance, and high tensile, flexural, and impact strength In contrast to commodity plastics, however, engineering and specialty polymers are produced at lower volume and higher cost Examples of engineering plastics include aliphatic polyamides (such as nylon-6,6), aromatic polyamides, acrylonitrile-butadiene-styrene (ABS) resin, polyacetal, polycarbonate, polysulfones, poly(phenylene oxide) resins, poly(phenylene sulfide), and fluoroplastics such as teflon Structures of some of these polymers are shown in Fig 14.8 Specialty polymers achieve very high performance and find limited but critical use in aerospace composites, in electronic industries, as membranes for gas and liquid separations, as fire-retardant textile fabrics for firefighters and race-car drivers, and for biomedical applications (as sutures and surgical implants) The most important class of specialty plastics is polyimides Other specialty polymers include polyetherimide, poly(amide-imide), polybismaleimides, ionic polymers, polyphosphazenes, poly(aryl ether ketones), polyarylates and related aromatic polyesters, and ultrahigh-molecular-weight polyethylene (Fig 14.9) 14.3 Polymers Industry Today, polymeric materials are used in nearly all areas of daily life that it is right to claim that we live in a polymer age The evidence is all around us Polymers are used in shelter, clothing, health, food, transportation, and almost every facet of our lives In fact, it is hard to imagine what the world would be like without synthetic polymers Production and fabrication of polymers are major worldwide industries The data presented in Tables 14.1 to 14.8 indicate the size of the polymers' marketplace in the 1992 to 2002 decade [I] 14.4 Polymer Structure The properties of polymers are strongly influenced by details of the chain structure The structural parameters that determine properties of a polymer include the overall chemical composition and the sequence of monomer units in the case of copolymers, the stereochemistry or tacticity of the chain, and geometric isomerization in the case of diene-type polymers Homopolymer It is a macromolecule consisting of only one type of repeating unit The repeating unit may consist of a single species as in PS (1), or may contain more species of monomer unit Polymers such as nylon-6,6 (2) that have repeating units composed of more than one monomer are considered homopolymers From the thermodynamic point of view, the solution process is governed by the free energy relationship (Eq 4) AG = AH - TAS (4) where AG = the change in Gibbs free energy AH = the change in enthalpy T = the absolute temperature AS = the change in entropy For a polymer to dissolve spontaneously, it is essential that the free energy of solution, AG, is negative The absolute temperature must be positive, and the entropy of solution, AS, is generally positive because of the increase in conformational mobility of the polymer chains Therefore, the magnitude of the enthalpy of solution, AH, determines the sign of AG A positive AH means the pure materials of solvent and the polymers are each in a lower energy state and hence dissolution is disfavored, whereas a negative AH indicates that the solution is the lower energy state In general, the solution process is an endothermic process, and then AH must be less than TAS if the polymer is to be soluble However, negative AH's are usually encountered when specific interactions such as hydrogen bondings are possible between polymer and solvent molecules The best one can expect in a good solvent is that AHsolution should be close to zero (it may not be negative because dissolution of a polymer is usually an endothermic process); therefore, (S1 - S2)2 must also be small Hildebrand showed that the polymer will dissolve in a solvent or a mixture of solvents having solubility parameters,