Relation between Polymer Science and Mechanics

As discussed briefly in the next section, polymers have a unique response to mechanical loads and are properly treated as materials which in some instances behave as elastic solids and in some instances as viscous fluids. As such their properties (mechanical, electrical, optical, etc.) are time dependent and cannot be treated mathematically by the laws of either solids or fluids. The study of such materials began long before the macromolecu-lar nature of polymers was understood. Indeed, as will be evident in later chapters on viscoelasticity, James Clerk Maxwell (1831-79), a Scottish physicist and the first professor of experimental physics at Cambridge, developed one of the very first mathematical models to explain such peculiar behavior. Lord Kelvin (Sir William Thomson, (1824-1907)), another Scottish physicist, also developed a similar mathematical model. Undoubtedly, each had observed the creep and/or relaxation behavior of natural materials such as pitch, tar, bread dough, etc. and was intrigued to explain such behavior. Of course, these observations were only a minor portion of their overall contributions to the physics of matter. 

Ludwig Boltzmann (1844-1906), an Austrian physicist, correctly conceived the hereditary nature of materials which we now describe as viscoelastic in a series of publications throughout his career. Such ideas were hotly debated at the time by Boltzmann, Ostwald and others but it is now clear that Boltzmann s view was the correct approach. For an excellent discussion of Boltzmann s contributions and their significance, see Mark-ovitz (1975, 1977). 

In 1812, even before Maxwell, Kelvin and Boltzman, the Scottish scientist Sir David Brewster (1781-1868) discovered that certain transparent optically isotropic solids (e.g., glass) when loaded developed optical characteristics of natural crystals. That is, he found that such a solid when loaded exhibited birefringence or double refraction and thus behaved as a temporary crystal. His discovery was the beginning of the well-known photoelastic method by which it is possible to experimentally determine the state of stress or strain on the interior of a loaded elastic body using polarized light. Maxwell (as well as F. E. Neumann at an earlier date) also studied the technique and deduced the relationship between stress and the optic effect now known as the Maxwell-Neumann stress-optic law. The impor- 

In recent years, epoxy resins have become the polymer of choice for three-dimensional photoelastic investigations. Further, the phenomena of birefringence has been used to study plasticity and viscoelasticity effects in materials through the use of extensions to the photoelastic method called photoplasticity and photoviscoelasticity (see Brill (1965) and Brinson (1965, 1968), respectively). Brill used polycarbonate, a thermoplastic polymer, as a model material for his work on photoplasticity and Brinson used an epoxy, a thermosetting polymer, as a model material for his work on photoviscoelasticity. Later, it will become clear why thermoplastic materials are used for photoplasticity while thermosetting materials are used for photoviscoelasticity. 

In addition to the use of polymers to study fundamental concepts in mechanics, another driving force for the critical link between polymer science and mechanics has been use of polymers in applications. As the understanding of the physical nature of polymers increased and synthesis techniques matured, many polymers of widespread usage were developed. As these materials were employed in devices and structures, it was essential to analyze and understand from an engineering perspective the response of polymers to load and other environmental variables, such as temperature and moisture. As indicated earlier, today high performance polymer composites are used for critical load bearing applications as diverse as alpine skis and airframe parts, and thus the study of the mechanics of polymers as a structural material is an active and important area of research. Later sections in this text will deal explicitly with the viscoelastic nature of polymeric response and mathematical methods to analyze this behavior. 

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