Molecular shape and the way molecules are arranged in a solid are important factors in determining the properties of polymers. From polymers that crumble to the touch to those used in bullet proof vests, the molecular structure, conformation and orientation of the polymers can have a major effect on the macroscopic properties of the material. The general concept of self-assembly enters into the organization of molecules on the micro and macroscopic scale as they aggregate into more ordered structures. Crystallization, discussed below, is an example of the self-assembly process as is the orientational organization of liquid crystals to be discussed later.
We need to distinguish here, between crystalline and amorphous materials and then show how these forms coexist in polymers. Consider a comparison between glass, an amorphous material, and ice which is crystalline. Despite their common appearance as hard, clear material, capable of being melted, a difference is apparent when viewed between crossed polarizers, as illustrated below:
|Photo courtesy of Geon Corp.|
The highly ordered crystalline structure of ice changes the apparent properties of the polarized light, and the ice appears bright. Glass and water, lacking that highly ordered structure, both appear dark.
The amorphous morphology of glass leads to very different properties from crystalline solids. This is illustrated in the heating process where the application of heat to glass turns it from a brittle solid-like material at room temperature to a viscous liquid, as discussed later in more detail under Thermal Properties of Polymers. In contrast, the application of heat to ice turns it from solid to liquid. Crystalline melting leads to striking changes in optical properties during the melting process when observed through crossed polarizers. This is illustrated in the following movie of the melting of an organic crystalline material. Note that while the temperatures are not recorded, the entire process occurs over a very narrow temperature range.
The reasons for the differing behaviors lie mainly in the structure of the solids. Crystalline materials have their molecules arranged in repeating patterns. Table salt has one of the simplest atomic structures with its component atoms, Na+ and Cl-, arranged in alternating rows and the structure of a small cube. Salt, sugar, ice and most metals are crystalline materials. As such, they all tend to have highly ordered and regular structures. Amorphous materials, by contrast, have their molecules arranged randomly and in long chains which twist and curve around one-another, making large regions of highly structured morphology unlikely.
The morphology of most polymers is semi-crystalline. That is, they form mixtures of small crystals and amorphous material and melt over a range of temperature instead of at a single melting point. The crystalline material shows a high degree of order formed by folding and stacking of the polymer chains. The amorphous or glass-like structure shows no long range order, and the chains are tangled as illustrated below.
There are some polymers that are completely amorphous, but most are a combination with the tangled and disordered regions surrounding the crystalline areas. Such a combination is shown in the following diagram.
An amorphous solid is formed when the chains have little orientation throughout the bulk polymer. The glass transition temperature is the point at which the polymer hardens into an amorphous solid. This term is used because the amorphous solid has properties similar to glass.
In the crystallization process, it has been observed that relatively short chains organize themselves into crystalline structures more readily than longer molecules. Therefore, the degree of polymerization (DP) is an important factor in determining the crystallinity of a polymer. Polymers with a high DP have difficulty organizing into layers because they tend to become tangled.
The cooling rate also influences the amount of crystallinity. Slow cooling provides time for greater amounts of crystallization to occur. Fast rates, on the other hand, such as rapid quenches, yield highly amorphous materials. For a more complete discussion, see the section on thermal properties. Subsequent annealing (heating and holding at an appropriate temperature below the crystalline melting point, followed by slow cooling) will produce a significant increase in crystallinity in most polymers, as well as relieving stresses.
Low molecular weight polymers (short chains) are generally weaker in strength. Although they are crystalline, only weak Van der Waals forces hold the lattice together. This allows the crystalline layers to slip past one another causing a break in the material. High DP (amorphous) polymers, however, have greater strength because the molecules become tangled between layers. For uses and examples of high and low DP polymers, see the section on Polymer Applications. In the case of fibers, stretching to 3 or more times their original length when in a semi-crystalline state produces increased chain alignment, crystallinity and strength.
In most polymers, the combination of crystalline and amorphous structures forms a material with advantageous properties of strength and stiffness.
Also influencing the polymer morphology is the size and shape of the monomers' substituent groups. If the monomers are large and irregular, it is difficult for the polymer chains to arrange themselves in an ordered manner, resulting in a more amorphous solid. Likewise, smaller monomers, and monomers that have a very regular structure (e.g. rod-like) will form more crystalline polymers.