Vulcanization is used to produce cross-linking between molecules in natural rubber to improve the physical properties of rubber.  When cross-linking agents join one linear polymer chain to another, a three- dimensional network is created; this reduces the mobility of the polymer chains.  Cross-linking agents used for this purpose include sulfur, oxygen, heat, gamma rays, beta rays, ultraviolet light, and energetic electrons.

Chemists have also discovered that high-energy beams of oxygen, carbon, nitrogen, boron, or argon ions have remarkable effects on the surfaces of some polymers.  Their surfaces become harder than steel and more resistant to wear and abrasion than steel.   Such ion-irradiated polymers could lend themselves to applications such as wear-resistant gears and bearings, artificial hip and knee joints, scratch-resistant plastic sunglasses and airplane windows, and lighter weight materials for automobiles and spacecraft.

In addition to increased hardness and wear resistance, ion-irradiated polymers are also resistant to many solvents and less permeable to gases.  They also resist oxidation, a serious degrading problem for many polymers; ion-bombardment increases electrical conductivity of polymers.

Chemists have found that the polymer's surface hardness is directly proportional to the energy of the ion beam striking the polymer.  The degree of ion-penetration and resulting extent of cross-linking vary inversely with the sizes of the ions.    Researchers don't understand fully how ion beams modify polymers to improve their hardness.  Cross-linking plays an important role, but many other changes accompany cross-linking.  Chemists hope that a better understanding of what happens at the molecular level will lead to even more interesting and exciting research as well as to new polymers.

Whitten/Davis/Peck:  General Chemistry with Qualitative Analysis,  5/e,  p. 1045