Polymer architecture via abstraction of a hydrogen, can create a mid-chain growth site. Branching can be quantified by the branching index. An effect related to branching is chemical crosslinking - the formation of covalent bonds between chains. Crosslinking tends to increase T and increase strength and toughness. Among other applications, this process is used to strengthen rubbers in a process known as vulcanization, which is based on crosslinking by sulfur. Car tires, for example, are highly crosslinked in order to reduce the leaking of air out of the tire and to toughen their durability. Eraser rubber, on the other hand, is not
Rubber toughening itself, but furthermore initiation of shear bands also aids in craze arrest. The occurrence of cavitation is important to shear yielding theory because it acts to lower the yield stress. Cavitation precedes shear yielding, however shear yielding accounts for a much larger increase in toughness than does the cavitation itself. Cavitation is common in epoxy resins and other craze resistant toughened polymers, and is prerequisite to shearing in Izod impact strength testing. During the deformation and fracture of a toughened polymer, cavitation of the strained rubber particles occurs in crazing-prone and non-crazing-prone plastics, including, ABS, PVC, nylon, high impact polystyrene,