Elastomers, Fibers and Plastics

The chemist classifies polymers in several ways. There are thermosetting plastics such as Bakelite and melamine and the much larger category of thermoplastic materials, which can be molded, blown, and formed after polymerization. There are the arbitrary distinctions made among plastics, elastomers, and fibers. And there are the two broad categories formed by the polymerization reaction itself (1) addition polymers (e.g., vinyl polymerizations), in which a double bond of a monomer is transformed into a single bond between monomers, (2) condensation poly-... 

Because most synthetic plastics, elastomers, and fibers are prepared by free-radical chain polymerizations, this method will be discussed here. Initiation can occur through decomposition of an initiator such as azobisisobutyronitrile (AIBN), light, heat, sonics, or other technique to form active free radicals. Here initiation will be considered as occurring by decomposition of an initiator, I, and is described as follows. 

The nomenclature rules say that the trivial names of common polymers need not necessarily be replaced by structural names. Therefore, both trivial and structural names of polymers will be used in this book. Standard abbreviations of trivial names (see Table A1 in the Appendix to this book) will generally only be used in diagrammatic illustrations. For further information, a special table (Table A2 in the Appendix) gives trade names of some important plastics, elastomers, and fibers. 

Plastics, elastomers, and fibers thus differ essentially in their modulus of elasticity and by their extensibility (Table 1-5). Tensile strength and elongation at break can be used for further distinction. 

Polymers come in many forms including plastics, elastomers, and fibers. Plastics are stiffer than elastomers yet have reduced low-temperature properties. Generally, a plastic differs from an elastic material because the location of its glass transition temperature (Tg). A plastic has a Tg above room temperature, whereas rubber will have a Tg below room temperature. 

Polymers are found in nature and may also be produced by laboratory synthesis. Important examples of naturally occurring macromolecules, or biopolymers, are proteins, polysaccharides, terpenes, and nucleic acids. General representations of these substances are provided by structures 3-6, respectively, in which the monomeric subunits of an a-amino acid, 3, a pyranose, 4, an isoprene, 5, and a ribonucleotide phosphate, 6, are seen. Synthetic, or man-made, polymers are represented by the myriad plastics, elastomers, and fibers that are commonplace in contemporary society. 

This chapter provides a brief overview of several medical applications that polymers have made seminal contributions to over the years. Many of the polymers discussed here are initially developed as plastics, elastomers, and fibers for nonmedical industrial applications. They were borrowed by the smgeons post-World War II to address medical problems. Since then, they have led to the development of biomedical-specific materials. Currently, with the rapid growth in modem biology and the collaborative effort, cross-disciplines such as materials science, engineering, chemistry, biology, and medicine, polymeric biomaterials are now being fashioned into bioactive, biomimetic, and most importantly, with excellent biocompatibility. Examples of this newer generation of polymeric biomaterials are also included in this chapter. 

Biopolymers are an essential element in improving human health and quality of life. The wide spectrum of physical, mechanical, and chemical properties provided by polymers has increased the extensive research, development, and applications of polymeric biomaterials. The significance of polymers as biomaterials is reflected in the market of medical polymers, estimated to be approximately 1 billion. Many of these polymers were initially developed as plastics, elastomers, and fibers for nonmedical industrial applications. However, they were later developed as biomedical-specific materials. Currently, with rapid growth in modern biology and interdisciplinary collaborative research, polymeric biomaterials are being widely used in pharmaceutical technology with excellent biocompatibility [33]. 

Polymers are viscoelastic materials meaning they can act as liquids, the visco portion, and as solids, the elastic portion. Descriptions of the viscoelastic properties of materials generally falls within the area called rheology. Determination of the viscoelastic behavior of materials generally occurs through stress-strain and related measurements. Whether a material behaves as a viscous or elastic material depends on temperature, the particular polymer and its prior treatment, polymer structure, and the particular measurement or conditions applied to the material. The particular property demonstrated by a material under given conditions allows polymers to act as solid or viscous liquids, as plastics, elastomers, or fibers, etc. This chapter deals with the viscoelastic properties of polymers. 

Only about 5% of the fossil fuels consumed today are used as feedstocks for the production of synthetic carbon-based products. This includes the products of the chemical and drug industries with a major portion acting as the feedstocks for plastics, elastomers, coatings, fibers, etc. 

Materials with properties intermediate between those of elastomers and fibers are grouped together under the term plastics. Thus plastics exhibit some flexibility and hardness with varying degrees of crystallinity. The molecular requirements for a plastic are that (1) if it is linear or branched, with little or no cross-linking, it be below its T (2) if it is amorphous and/or crystalline, it be used below its Tm, or (3) if it is cross-linked, the cross-linking be sufficient to severely restrict molecular motion. 

By varying the nature of the side chain, R, various elastomers, plastics, films, and fibers have been obtained. These materials tend to be flexible at low temperatures, and water and fire resistant. Some fluoroalkoxy-substituted polymers (R = CHXFJ are so water repellent that they do not interact with living tissues and promise to be useful in fabrication of artificial blood vessels and prosthetic devices. 

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. 

Pseudocommodities are differentiated products produced in large quantities. They are not characterized simply by composition specification, inasmuch as manufacturer promise to meet an in-use performance specification for each customer. Synthetic fibers, resins and plastics, elastomers, and carbon blacks are good examples of pseudocommodities. 

Plastics. Plastics reqnire properties that are intermediate between elastomers and fibers. Engineering plastics can be readily machined, cnt, and drilled. Condensation polymers are typically engineering plastics while vinyl polymers are typically plastics. Table 4 contains a fisting of the most common engineering plastics and plastic materials and Table 5 the volnme of engineering plastics and plastics produced in the United States. Table 2. 

Advanced composite n. Polymer, resin, or other matrix-material system in which reinforcement is accomphshed via high-strength, high-modulus materials in continuous filament form or is discontinuous form such as staple fibers, filberts, and in-situ dispersions. Harper CA (2002) Handbook of plastics, elastomers, and composites, 4th edn. McGraw-Hill, New York. 

A beneficial property of elastomers is that they can be compounded or joined with other materials to strengthen certain characteristics. Other kinds of polymers may be installed next to various other materials, such as metal, hard plastic, or different kinds of rubber, with excellent adhesion. Harper CA (ed) (2002) Handbook of plastics, elastomers, and composites, 4th edn. McGraw-Hill, New York. Harper CA (ed) (2002) Handbook of plastics, elastomers, and composites, 4th edn. McGraw-Hill, New York. Elias HG et al. (1983) Abbreviations for thermoplastics, thermosets, fibers, elastomers, and additives. Polym News 9 101-110. James F (ed) (1993) Whittington s dictionary of plastics. Technomic Publishing Co. Inc., Carley. Harper CA (ed) (2002) Handbook of plastics, elastomers, and composites, 4th edn. McGraw-Hill, New York. Skeist I (ed) (1990) Handbook of adhesives. Van Nostrand Reinhold, New York. 

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