Structural materials, polymers

Higher molecular weight has different meanings to users of polymeric materials. For structural materials, polymers have molecular weights of tens to hundreds of thousands. Materials nsed in plastics have molecnlar weights of... 

In this chapter we examine the flow behavior of bulk polymers in the liquid state. Such substances are characterized by very high viscosities, a property which is directly traceable to the chain structure of the molecules. All substances are viscous, even low molecular weight gases. The enhancement of this property due to the molecular structure of polymers is one of the most striking features of these materials. 

All polymer molecules have unique features of one sort or another at the level of individual repeat units. Occasional head-to-head or tail-to-tail orientations, random branching, and the distinctiveness of chain ends are all examples of such details. In this chapter we shall focus attention on two other situations which introduce variation in structure into polymers at the level of the repeat unit the presence of two different monomers or the regulation of configuration of successive repeat units. In the former case copolymers are produced, and in the latter polymers with differences in tacticity. Although the products are quite different materials, their microstructure can be discussed in very similar terms. Hence it is convenient to discuss the two topics in the same chapter. 

Fibrous proteins can serve as structural materials for the same reason that other polymers do they are long-chain molecules. By cross-linking, interleaving and intertwining the proper combination of individual long-chain molecules, bulk properties are obtained that can serve many different functions. Fibrous proteins are usually divided in three different groups dependent on the secondary structure of the individual molecules coiled-coil a helices present in keratin and myosin, the triple helix in collagen, and P sheets in amyloid fibers and silks. 

Fibrous proteins are long-chain polymers that are used as structural materials. Most contain specific repetitive amino acid sequences and fall into one of three groups coiled-coil a helices as in keratin and myosin triple helices as in collagen and p sheets as in silk and amyloid fibrils. 

Polymers have come a long way from parkesine, celluloid and bakelite they have become functional as well as structural materials. Indeed, they have become both at the same time one novel use for polymers depends upon precision micro-embossing of polymers, with precise pressure and temperature control, for replicating electronic chips containing microchannels for capillary electrophoresis and for microfluidics devices or micro-optical components. 

Lotz, B. and Wittmann, J.-C. (1993) Structure of Polymer Single Crystals, in Structure and Properties of Polymers, ed. Thomas, E.L. Materials Science and Technology, A Comprehensive Treatment, eds. Cahn, R.W., Haasen, P. and Kramer, E.J. (VCH, Weinheim) p. 79. 

So far the structure of polymers has been described with reference to the material with the simplest molecular structure, i.e. polyethylene. The general principles described also apply to other polymers and the structures of several of the more common polymers are given below. 

The mechanical properties of polymers are of interest in all applications where they are used as structural materials. The analysis of the mechanical behavior involves the deformation of a material under the influence of applied forces, and the most important and characteristic mechanical property is the modulus. A modulus is the ratio between the applied stress and the corresponding deformation, the nature of the modulus depending on that of the deformation. Polymers are viscoelastic materials and the high frequencies of most adiabatic techniques do not allow equilibrium to be reached in viscoelastic materials. Therefore, values of moduli obtained by different techniques do not always agree in the literature. 

Cellulose is an important part of woody plants, occurring in cell walls and making up part of the structural material of stems and trunks. Cotton and flax are almost pure cellulose. Chemically, cellulose is a polysaccharide—a polymer made by successive reaction of many glucose molecules giving a high molecular weight (molecular weight ->- 600,000). This polymer is not basically different from the polymers that were discussed in Section 18-6 ... 

Waste products from a number of commercial processes can be used as cheap and readily available fillers for PCM. For example, lightweight structural materials may be obtained by filling various low-viscous resins with waste materials [4, 5]. Also by adding fillers to reprocessed polymers it is possible to improve their properties considerably and thus return them to service [6]. This method of waste utilization is not only economically feasible but also serves an ecological purpose, since it will help to protect the environment from contamination. The maximum percentage of the filler should in these cases be such as to assure reliable service of the article made from the PCM under specified conditions for a specified period of time. 

In view of these constraints, we recently suggested a different strategy for the improvement of the material properties of synthetic poly (amino acids) (12). Our approach is based on the replacement of the peptide bonds in the backbone of synthetic poly(amino acids) by a variety of "nonamide" Linkages. "Backbone modification," as opposed to "side chain modification," represents a fundamentally different approach that has not yet been explored in detail and that can potentially be used to prepare a whole family of structurally new polymers. 

DNA is ideally suited as a structural material in supramolecular chemistry. It has sticky ends and simple rules of assembly, arbitrary sequences can be obtained, and there is a profusion of enzymes for modification. The molecule is stiff and stable and encodes information. Chapter 10 surveys its varied applications in nanobiotechnology. The emphasis of Chapter 11 is on DNA nanoensembles, condensed by polymer interactions and electrostatic forces for gene transfer. Chapter 12 focuses on proteins as building blocks for nanostructures. 

Selection of Corrosion-Resistant Materials The concentrated sofutions of acids, alkalies, or salts, salt melts, and the like used as electrolytes in reactors as a rule are highly corrosive, particularly so at elevated temperatures. Hence, the design materials, both metallic and nonmetallic, should have a sufficiently high corrosion and chemical resistance. Low-alloy steels are a universal structural material for reactors with alkaline solutions, whereas for reactors with acidic solutions, high-alloy steels and other expensive materials must be used. Polymers, including highly stable fluoropolymers such as PTFE, become more and more common as structural materials for reactors. Corrosion problems are of particular importance, of course, when materials for nonconsumable electrodes (and especially anodes) are selected, which must be sufficiently stable and at the same time catalytically active. 

These two examples show that regular patterns can evolve but, by definition, dissipative structures disappear once the thermodynamic equilibrium has been reached. When one wants to use dissipative structures for patterning of materials, the dissipative structure has to be fixed. Then, even though the thermodynamic instability that led to and supported the pattern has ceased, the structure would remain. Here, polymers play an important role. Since many polymers are amorphous, there is the possibility to freeze temporal patterns. Furthermore, polymer solutions are nonlinear with respect to viscosity and thus strong effects are expected to be seen in evaporating polymer solutions. Since a macromolecule is a nanoscale object, conformational entropy will also play a role in nanoscale ordered structures of polymers. 

The four largest classes of synthetic polymers (PE, PP, PVC and PET) make up about 80 % of the world market. About 60% of the production of polymers supplies structural materials to the market (packaging 41 %, building components 20 %, electric insulation 9 %, automobile parts 7 %, agriculture 2 %, miscellaneous... 

One of the main advantages of the polymer route to ceramics is the preparation of ceramic fibers, a shape difficult to achieve by other methods. Ceramic fiber-based composites are becoming an increasingly important group of structural materials (12, 13). 

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