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Polymer Science and Engineering: The Shifting Research Frontiers
To obtain high elasticity and the desirable properties it imparts, polymers are needed that have high chain flexibility and mobility. This need has led both nature and industry to choose polymers with small side chains, little polarity, and a reluctance to crystallize in the undeformed state. Rubberlike elasticity arises from the flexible chains interconnecting the cross-linking of polymer chains. The cross-linking carried out in nature is more sophisticated than the cross-linking used in the production of elastomers in the laboratory. In biological systems, cross-links are introduced at specific amino acid repeat units and are thus restricted both in their number and in their locations along the chain. Furthermore, they may be carefully positioned spatially as well, by being preceded and succeeded along the chain by rigid alpha-helical sequences. If we had nature's ability to control network structure, it would be possible for us to design materials with better mechanical properties. For example, many bioelastomers have relatively high efficiencies for storing elastic energy through the precise control of cross-link structure. A desirable advanced material would be an elastomer with low energy loss. Such a material would have the advantages of energy efficiency and fewer problems from degradation resulting from the heat buildup associated with incomplete recovery of elastic energy. Another desirable advanced material would have high toughness, which may be obtained by exploiting non-Gaussian effects that increase the modulus of an elastomer near its rupture point. Some work on bioelastomers suggests that toughness may be controlled by the average network chain length and the distribution about this average. There have been attempts to mimic this synthetically by end-linking chains of carefully controlled length distributions, but much more should be done along these lines.
Biocomposites are usually composed of an inorganic phase that is reinforced by a polymeric network. The various types of biocomposites found in nature, such as bone, teeth, ivory, and sea shells, differ from synthetic analogs in one or more important respects. First, the hard reinforcing phase in biocomposites is frequently present to a very great extent, in some cases exceeding 96 percent by weight. Second, the relative amounts of crystallinity, morphology, and crystallite size and distribution are carefully controlled. Moreover, the orientation of crystalline regions is generally fixed, frequently by the use of polymeric templates or epitaxial growth. Third, instead of a continuous homogeneous phase, a gradation of properties in the material is obtained by either continuous changes in chemical composition or physical structure. Finally, larger-scale ordering is often present, for example, in complex laminated structures, with various roles being delegated to the different layers present.