Strength and Related Properties of Elastomeric Block Copolymers

Stress-strain curves of single-phase noncrystallizable elastomers over extended ranges of temperature and extension rate are considered qualitatively along with phenomenological and mechanistic aspects of fracture. Data are presented to show that single-phase noncrystallizable elastomers lack toughness except when segmental mobility is sufficiently low so that viscoelastic processes near the tip of a slowly growing crack effectively retard its growth. Highly effective strengthening mechanisms are imparted by plastic domains which result from phase separation in elastomeric block copolymers and from strain-induced crystallization in certain elastomers. The stiffness, tensile strength, and extensibility of a poly(urea-urethane) and three polyurethane elastomers over a broad temperature range are discussed in terms of the type, size, and concentration of the domain-forming segments. These elastomers, and particularly their true stress-at-break, are compared with other block copolymers and with polyurethane elastomers devoid of plastic domains.