FIGURE 1 Schematic representation of a polymer chain adsorbed on a surface. The black dots represent polymer-surface bonding.

in composites, and the special properties that can be achieved from polymer blends.

Figure 1 shows one aspect of the sometimes unique behavior of polymers. It depicts a typical long, concatenated polymer chain adsorbed on a surface. The chain consists of similar chain units (monomers) and is flexible. This illustration is important because it shows that, to desorb the entire polymer molecule, all of the little “feet” in the long molecule that are attached to points on the surface must be lifted from the surface simultaneously. From a probability point of view this is difficult even when the occasional attachments involve relatively weak van der Waals forces, and it is still more difficult if the attachments involve chemical bonds. The figure thus indicates one reason that polymers make such good surface coatings: Once adsorbed, they can be very difficult to detach. Polymer molecules will adsorb on a surface in seconds but may take weeks to detach fully, even in the presence of pure solvent; in poor solvents or in the absence of solvents, they virtually never detach. For short chains this is not the case; from a statistical point of view they come off rapidly in solvents because of the relatively low number of attachments. This simple illustration shows that the treatment of polymers deals with a property not ordinarily thought of as an important materials parameter, namely, molecular length, or, in more customary terms, molecular weight.

Crystalline Polymers

In the field of organic polymers, a wide range of chemical structures is readily available. As a beginning, consider the remarkable variety of properties and morphologies one can obtain with a specific single polymer chain, i.e., with “constant chemistry.” For this purpose we emphasize polyethylene, —(CH2—CH2)n—, which is a very simple chain. The examples will be single crystals, lamellar spherulitic structures, and high-strength fibers— all with the same molecule (common polyethylene), but with different processing.

Consider the following experiment: in ordinary xylene at, say, 135 to 138°C, a small amount of linear polyethylene (0.001 to 0.01 percent) is dissolved, and the solution is cooled to around 70 to 80°C. Chains with a

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