Polymers

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Polymers :

Polymers

Polymers :

Polymers INTRODUCTION 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 this group of materials, and the development of numerous polymers , which are synthesized from small organic molecules.

Polymers :

Polymers Since most polymers are organic in origin, we briefly review some of the basic concepts relating to the structure of their molecules . First , many organic materials are hydrocarbons; that is, they are composed of hydrogen and carbon . Furthermore, the intramolecular bonds are covalent. Each carbon atom has four electrons that may participate in covalent bonding, whereas every hydrogen atom has only one bonding electron .

Polymers :

Polymers In ethylene which has the chemical formula C2H4, the two carbon atoms are doubly bonded together , and each is also singly bonded to two hydrogen atoms, as represented by the structural formula. Molecules that have double and triple covalent bonds are termed unsaturated. That is, each carbon atom is not bonded to the maximum (four) other atoms; as such , it is possible for another atom or group of atoms to become attached to the original molecule. Furthermore , for a saturated hydrocarbon, all bonds are single ones , and no new atoms may be joined without the removal of others that are already bonded .

Polymers :

Polymers Some of the simple hydrocarbons belong to the paraffin family; the chain like paraffin molecules include methane (CH4), ethane (C2H6), propane (C3H8), and butane (C4H10 ). Compositions and molecular structures for paraffin molecules are contained in Table 14.1 Hydrocarbon compounds with the same composition may have different atomic arrangements , a phenomenon termed isomerism.

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THERMOPLASTIC AND THERMOSETTING POLYMERS

Thermosetting :

Thermosetting The response of a polymer to mechanical forces at elevated temperatures is related to its dominant molecular structure. In fact, one classification scheme for these materials is according to behavior with rising temperature. Thermoplastics ( or thermoplastic polymers) and thermosets (or thermosetting polymers) are the two subdivisions. 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

Thermosetting :

Thermosetting 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 too high of a temperature. In addition, thermoplastics are relatively soft. Most linear polymers and those having some branched structures with flexible chains are thermoplastic . These materials are normally fabricated by the simultaneous application of heat and pressure. Most linear polymers are thermoplastics. Examples of common thermoplastic polymers include polyethylene, polystyrene, poly(ethylene terephthalate ), and poly(vinyl chloride).

Thermosetting :

Thermosetting Thermosetting polymers are network polymers . They become permanently hard during their formation, and do not soften upon heating . Network polymers have covalent cross links 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 .

Thermosetting :

Thermosetting 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, and phenolics and some polyester resins, are thermosetting.

Thermosetting :

Thermosetting POLYMER CRYSTALLINITY The crystalline state may exist in polymeric materials. However , since it involves molecules instead of just atoms or ions, as with metals and ceramics, the atomic arrangements will be more complex for polymers . We think of polymer crystallinity as the packing of molecular chains to produce an ordered atomic array. Crystal structures may be specified in terms of unit cells, which are often quite complex.

Thermosetting :

Thermosetting POLYMER CRYSTALS It has been proposed that a semicrystalline polymer consists of small crystalline regions (crystallites), each having a precise alignment, which are interspersed with amorphous regions composed of randomly oriented molecules . The structure of the crystalline regions may be deduced by examination of polymer single crystals, which may be grown from dilute solutions . These crystals are regularly shaped, thin platelets (or lamellae), approximately 10 to 20 nm thick, and on the order of 10 m long.

Thermosetting :

Thermosetting 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 14.12. Each platelet will consist of a number of molecules ; however, the average chain length will be much greater than the thickness of the platelet.

Thermosetting :

Thermosetting DEFECTS IN POLYMERS The point defect concept is different in polymers than in metals and ceramics as a consequence of the chain-like macromolecules and the nature of the crystalline state for polymers. Point defects similar to those found in metals have been observed in crystalline regions of polymeric materials; these include vacancies and interstitial atoms and ions .

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