Hard domains are found to be effective reinforcing agents provided their
volume fraction exceeds 0.20, their size is less than 0.1/~m and their softening
temperature is substantially above the test temperature. 2°2 They are more effective
than particulate fillers because they are deformable and uniformly distributed
throughout the sample volume. Plastic deformation of hard domains reduces
stress concentration, delays formation of microcracks and blunts crack tips,
increasing the strength and toughness of elastomers. In a study of mechanical
properties of polyurethanes the strength and extensibility of a series of polyurethanes
with different SSC was compared. It has been shown that ultimate
tensile properties depend markedly on temperature and extension rate. 2°3'2°6
Figure 34 illustrates the effect of temperature on extension at break in a single
phase polyurethane. Approaching Tg, elongation at break as well as tensile
strength increase due to a slowing down of the relaxation process.
Apart from structural parameters, tensile strength depends also on experimental
parameters such as the rate of extension and temperature. Smith 2°4 has
found that with some elastomers the elongation at break increases from 100%
to 230% and the true stress at break increases six-fold with a 1000-fold increase
in extension rate. If stress-strain curves measured at different temperatures and
extension rates are represented on a single diagram having as ordinate 6To/T
and e, where 6 is stress, To is some reference temperature, T is test temperature
and e is relative extension, then the ends of the curves indicated by arrows define
the curve known as the failure envelope, as shown in Fig. 35. With an increase
of either temperature or time the fracture moves clockwise along the failure
envelope.