Polyisobutylene stress relaxation curve

FIGURE 24.9. Stress relaxation curves—as explained in the text—for polyisoprene (natural rubber, 1), oriented low density polyethylene (LDPE) with the draw ratio A = 1.8 (curve 2), indium (3), unoriented LDPE (4), cadmium (5), polyisobutylene (6), and lead (7). 

Figure 3-12. Stress relaxation master curve for N.B.S. polyisobutylene at 25°C (dotted line) and corresponding box-and-wedge distribution (solid lines). [Adapted from A. V. Tobolsky, Properties and Structure of Polymers, p. 128 and 151, by permission of John Wiley Sons, Inc.]...

Time-temperature superposition is applicable to a wide variety of viscoelastic response tests, as are creep and stress relaxation. We illustrate the principle by considering stress relaxation test data. As a result of time-temperature correspondence, relaxation curves obtained at different temperatures can be superimposed on data at a reference temperature by horizontal shifts along the time scale. This generates a simple relaxation curve outside a time range easily accessible in laboratory experiments. This is illustrated in Figure 14.13 for polyisobutylene. Here, the reference temperature has been chosen arbitrarily to be 25°C. Data obtained at temperature above 25°C are shifted to the right, while those obtained below 25°C are shifted to the left. 

FIGURE 13.25 Illustration of the time-temperature superposition principle using stress-relaxation data for polyisobutylene. Curves are shifted along the axis by an amount represented by O.J as shown in the inset. The reference temperature in this instance is 298 K. (Adapted from Castiff, E. and Tobolsky, A.V., J. Colloid Sci., 10, 375, 1955.)... 

Example 16.6 The master curve for the polyisobutylene in Figure 16.12 indicates that stress relaxes to a modulus of 10 dyn/cm in about 10 h at 25 °C. Using the WLF equation, estimate the time it will take to reach the same modulus at a temperature of —20 °C. For PIB, Tg = -10°C. 

FIGURE 9.11 Master curve for polyisobutylene as determined by superposition of experimental stress relaxation data (dashed line) and from the insertion of the wedge and box models in Equation 9.46. (Data from Tobolsky, A. V., Properties and Structure of Polymers, chap. 3, Wiley, New York, 1960.)... 

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