- Polymers and society
- Polymer chemistry
Topics in polymer physics
Plasticity and Elasticity
“Plastic” is somewhat of a misnomer, because the engineering definition of “plastic” only touches upon one property of polymers: their ability to permanently deform without breaking. Plasticity is exemplified in silly putty stretching or a car bumper denting. In these instances, the material deforms, generating heat as molecules slide past one another. Plasticity is incredibly important because it enables the kinetic energy of a collision, for example, to be partially released as heat. Elasticity, however, is the property that enables a polymer to undergo deformation, then return to its original shape. Elasticity enables a rubber band to snap back, or a car bumper to bounce back from a low-impact collision. In these instances, molecules are able to stretch during the deformation, but not slip past one another, because parts of the polymer chain are physically or chemically bound to nearby chains. The polymer chains stretch in-between bonds, but return to their original shape when the force is relaxed.
One of my favorite examples of plastic polymer deformation is the stretching of an ubiquitous grocery carrier bag, made of low density polyethylene. Once stretched, the material does not return to its original shape, nor does it break, indicating a plastic deformation. Something else happens too, the polymer changes from being completely translucent to exhibiting opaque streaks. These streaks are actually polymer crystals that form when the polymer chains are pulled past one another in the same direction. The crystals that form range in size from ~10 – 1000 nm, large enough to disperse light (hence the opaque appearance) but too small to form a single, macroscopic crystal.