Classically, rubber-like elasticity is described as an entropic phenomenon, which leads to heat production during loading and heat absorption during unloading. This was first observed by Refs. [16] and [17]. In spite of these results, no further experimental study was dedicated to the thermomechanical characterization of elastomers, although this is a key point for the understanding of the above mentioned phenomena as well as the phenomena involved in fatigue damage [18], [19], [20] and [21].