Rubber thermoelastic behavior

These deductions from basic facts of observation interpreted according to the rigorous laws of thermodynamics do not alone offer an insight into the structural mechanism of rubber elasticity. Supplemented by cautious exercise of intuition in regard to the molecular nature of rubberlike materials, however, they provide a sound basis from which to proceed toward the elucidation of the elasticity mechanism. The gap between the cold logic of thermodynamics applied to the thermoelastic behavior of rubber and the implications of its... 

FIGURE 14.3 Thermoelastic behavior of a rubber sample. Stress-temperature (O-T) curves for a series of extension values. The percentage strain is shown against each curve. (Adapted from Beevers, R.B., Experiments in Fibre Physics, Butterworths, 1970.)... 

The foregoing state of affairs appears to hold for living muscle. As myosin muscle is and must be extensible, but, as a living machine, it is contractile only. We are forced, therefore, to question the work of Hill (35) and others in which it was shown that muscle in the relaxed state resembles rubber, that the stretch-strain curves of the two are similar, that the stretching forces are of the same order of magnitude, and that both rubber and muscle exhibit the same anomalous thermoelastic behavior (at... 

Finally, we turn from solutions to the bulk state of amorphous polymers, specifically the thermoelastic properties of the rubbery state. The contrasting behavior of rubber, as compared with other solids, such as the temperature decrease upon adiabatic extension, the contraction upon heating under load, and the positive temperature coefficient of stress under constant elongation, had been observed in the nineteenth century by Gough and Joule. The latter was able to interpret these experiments in terms of the second law of thermodynamics, which revealed the connection between the different phenomena observed. One could conclude the primary effect to be a reduction of entropy... 

From the dynamic mechanical investigations we have derived a discontinuous jump of G and G" at the phase transformation isotropic to l.c. Additional information about the mechanical properties of the elastomers can be obtained by measurements of the retractive force of a strained sample. In Fig. 40 the retractive force divided by the cross-sectional area of the unstrained sample at the corresponding temperature, a° is measured at constant length of the sample as function of temperature. In the upper temperature range, T > T0 (Tc is indicated by the dashed line), the typical behavior of rubbers is observed, where the (nominal) stress depends linearly on temperature. Because of the small elongation of the sample, however, a decrease of ct° with increasing temperature is observed for X < 1.1. This indicates that the thermal expansion of the material predominates the retractive force due to entropy elasticity. Fork = 1.1 the nominal stress o° is independent on T, which is the so-called thermoelastic inversion point. In contrast to this normal behavior of the l.c. elastomer... 

The stress-temperature behavior of natural rubber at various extension ratios has been measured by Shen et al. (1967) and shown in Figure 14.12. Compare trends between data at varying extension ratios with those shown in Figure 14.3 and provide an explanahon for the changes at low elongations (a phenomenon that is termed thermoelastic inversion). 

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