Molecular Rearrangements

In order to understand the behavior of polymers (macromolecules), molecular size needs to be understood. Molecular size is the key to most of the properties that make polymers useful, like their strength and ability to stretch and recover. These properties depend on the interactions between molecules that are orders of magnitude larger than familiar ones such as water or ammonia.

The behavior of small molecules can be understood in terms of 3 states: solid, liquid and gas. The behavior of polymers is much more complex. A few types of polymers can exhibit long-range order. That is, portions of the chains can arrange themselves into small 3-dimensional structures. Because large polymers are difficult to accommodate in such restricted spaces, the crystallites (small crystals) formed are microscopic. Most polymers, however, cannot assume the close-range packing needed to form stable crystals. Most polymers are disordered, chaotic, entangled masses of chains with no long-range order (once again, think of the bowl of spaghetti).

The response of this entangled mass of chains to an outside stress depends on the ability of the individual chains to move about. When the temperature of a polymer is raised, there results an increase in internal energy. The increased energy allows the polymer segments to rotate and slip past one another, which results in large-scale molecular rearrangements. Larger molecules rearrange more slowly than smaller ones.

Each type of polymer has a characteristic point called the Glass Transition Temperature (abbreviated as Tg). Below this temperature, there is insufficient energy for bond rotation, and the polymer chains cannot rearrange. Below Tg, the polymers are rigid and quite brittle. This phenomenon can be observed in the winter time, when sometimes a plastic garbage can or sled shatters in the sub-zero temperature. This rigid state is called a glass. Above the Tg, molecular rearrangements can occur and the material is called a melt.

Cross-linked or network polymers can also rearrange (see figure below). The segments between the cross-links move around as temperature increases. However, when the outside stress (in this case the higher temperature) is removed, the cross-linked points return to their normal positions, and the segments return back to their unstressed locations. Materials that exhibit this type of behavior are typically called rubbers.