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T. Carter Gilmer* and Matthew Williams Department of Chemistry, University of Michigan-Dearborn, 4901 Evergreen Road, Dearborn, MI 48128-1491
Polymers, or macromolecules, are gaining interest in the technical and scientific communities. There is an abundance of new and useful polymeric structures that generate corresponding new, interesting, and potentially useful (mechanical) properties. Table 1 shows a way of categorizing natural and synthetic polymers (1). The subcategorization that will be emphasized divides solid material oriented polymers into:
* fibers (low elongation, high stiffness and modulus)
* plastics (intermediate elongation, generally intermediate stiffness)
* elastomers or rubbers (high elongation, low modulus)
Examples of natural fibers are wool, silk, and wood. Common synthetic fibers are Nylon (polyamides) and Rayon. Rubbers are more readily identifiable. Natural rubber (polyisoprene) comes from trees. Rubber bands and tires are common examples of (hybrid) synthetic elastomers. But plastics are all synthetic, originating with Bakelite discovered by Leo Bakeland in the early 1900s. Synthetic plastics are ubiquitous now. Eyeglass lenses are often made of polycarbonates. Trash bags are often polyethylene. Cookware is coated with a nonstick, high surface energy Teflon or tetrafluoropolyethylene.
Many demands are placed on each specific polymer, polymer blend, or polymeric composite. Essentially all materials, be they substitutes for brick, wood, glass, or other structurally demanding materials, must have and reasonably maintain specific mechanical properties. One way of measuring a range of some mechanical properties is to perform tensile and/or compression testing.
Discussions here will refer to determination of tensile properties in a uniaxial mode. That is, stress is applied along one axis, as when you stretch a rubber band by extending your hands. Figure 1 shows three representative stress/strain curves of a typical fiber, a plastic and an elastomer, which have undergone tensile testing (2, 3). As described in the attached Tensile Properties Experiment, five common parameters are obtained from each curve:
1. Tensile strength (in lb/sq in. or MPa) (force/area) 2. Elongation-to-break (% increase in length) 3. Elastic modulus or Young's modulus or modulus (measure of stiffness, or stress/strain-same units as tensile strength)
4. Yield (applies primarily to plastics but also to rubbers, where stress/strain slope equals zero; polymer chains are disentangling and rearranging causing no additional stress with increasing strain), determined at a specific stress or strain point 5. Toughness...