ABSTRACT
Several elastomer types are capable of crystallization, the principal of which are poly-isoprene (natural rubber) and polychloroprene (neoprene). At high strains crystallization can be a desirable feature of elastomers for engineering applications since it can enhance resistance to fracture and fatigue. Its occurrence is related to the level of strain and will be most rapid at the tip of a crack where strains are high. This provides a natural reinforcing mechanism and crystallizing elastomers show better fatigue resistance than other elastomers at high strains. Crystallization can also occur in these elastomers at low temperatures, even without strain, when the consequences (increased modulus and enhanced stress relation rates) can be deleterious for engineering applications. It is important to clearly differentiate between crystallization and glass transition. An elastomer below glass transition will be brittle and may still have an amorphous structure, with a modulus 1000 times that in the rubbery state. The transition to the glassy state occurs when the local temperature is low enough. Crystallization, on the other hand, is a slow process where crystallites grow from within the amorphous matrix over an extended period of time. At equilibrium, less than half the material is crystalline and the elastic modulus increases by a factor of about 100. Rubber is not brittle but tough in this semicrystalline state and shows some characteristics of semicrystalline plastics (e.g., a yield point).
