Molecular variation
If we look at the fundamental building blocks in a metallic and a polymeric material, we can begin to appreciate the reasons for the differences in behavior. At a molecular level, the basic units that make up a metallic structure are relatively small and uniform in size. Consequently, they readily arrange into a structure that is very regular and predictable. Material scientists identify these structures as crystalline solids. This type of structure is influenced very little by changes in temperature until the materials in this class reach their respective melting points. Metals are also not significantly affected by sustained loading until the stresses reach a point close to the yield strength of the material. In other words, they are not generally susceptible to creep or cold flow.
The smallest unit in a polymeric material is a very large molecule with an extended chain shape. Even the smallest polymer molecule will be about 50 times more massive than the heaviest naturally occurring unit in a metallic structure. These polymer molecules can twist, turn, fold, and entangle in almost unlimited variations. In addition, not all of these molecules are of the same size. The smallest molecule in any random sample of plastic is often a thousand times smaller than the largest, which increases the possibilities for local variation in the structure of a material.
Figure 1. Melting Behavior of Indium Metal and Polypropylene by DSC
This has important implications for the mechanical and thermal properties of the two classes of materials. Figure 1 shows temperature scans for polypropylene and a low-melting elemental metal known as indium. The two materials have comparable melting points. If you check the literature, you will find that the melting point of indium is 156.61°C, give or take a couple of thousandths of a degree. This property is so consistent and so easily measured that the metal is used to perform temperature calibrations on thermal analysis instruments such as
differential scanning calorimeters (DSC) and
thermogravimetric analyzers (TGA). Polypropylene has an approximate melting point of 165°C. But if we compare the process by which these two materials melt, we can see a
Mechanical performance and stresses
Figure 2 attempts to give a conceptual view of what a crystalline structure looks like in a polymer. Areas of the structure organize in fairly well-defined patterns. These represent the crystallized regions. In some polymers these well-organized regions may constitute as much as 90% of the material. However, in others this level can be so low that it is insignificant for all practical purposes and a melting point cannot even be detected.
Figure 2. Schematic of Semicrystalline Structure
That portion of the material that fails to organize into crystals is referred to as amorphous. These amorphous areas achieve a substantial level of molecular mobility at a much lower temperature than the measured melting point. We perceive this mobility as a reduction in the strength and stiffness of the material. In addition, when placed under load, these amorphous regions tend to deform at stresses much lower than any measured upper limit such as the yield point. Metals, by contrast, contain little or no disorganized material. Therefore, when they are placed under load, their behavior is much more predictable.
http://www.ides.com/articles/design/2008/sepe_02.asp
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има и калкулатор
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