The Thermomechanical Response of a Polyurea

Infrared thermography was carried out on a polyurea, stretched to failure, over four dec­ades of strain rates (0.026 – 400 s⁻¹). A convective heat transfer correction was developed so that the thermal response of slow and fast measurements could be compared. The deformation was mainly exothermic, but in contrast to simpler elastomers, the behavior was a complicated function of strain and rate. The larg­est temperature increase was 20 °C, about twice that at failure and 5–10 times that at com­parable strains, of both neat and filled rubber systems (e.g., natural and styrene butadiene rubber). At low rates (< 1 s⁻¹), the tem­perature increased up to a strain of ca. 3; the decline afterward was corre­lated to the stress upturn. At high rates (> 1 s⁻¹), the temperature increased monotonically. The temperature rise mostly increased with rate, but at 1.2 s⁻¹ there was a local maximum in the response. A ther­moelastic inversion point was identified at 4% strain: this coincided with the thermal expansion coef­ficient, and attested that the elasticity of this portion was rubbery, i.e., en­tropic. At higher strain the deformation departed from this behavior, as endothermic pro­cesses were identified to begin shortly after the ‘yield’ point. These processes were tentatively correlated to the plastic deformation and breakup of the hard domains. An energy balance showed that, in spite of the large temperature increase, structural changes accounted for the largest share of the strain energy. The temperature change caused a one-half decade shift toward higher frequency in the mechanical response of the soft segments.