From an elastomer transition state point of view, all movements of the elastomer molecules are fixed in the glassy region of the elastomer’s character (below its glass transition temperature). When the temperature is increased, the rotation surrounding the elastomer bonds also increases approaching the rubbery region, indicating that the elastomer molecules are close to entangled. Most of the macromolecules form a highly coiled conformation with the maximum entropy based on the Boltzmann equation [63]. In the rubbery state, an elastomer with sufficient molecular weight can be stretched along the alignment of the external force. When the force is applied at a high speed or over short periods of time, the local entanglement of the elastomer molecules can block the movement of the molecule. Consequently, the elastomer will recover to its original shape once the applied force is taken away, exhibiting what is known as a viscoelastic response. In this way, the elastomer with a randomly coiled state possesses a type of memory of its initial state thereby exhibiting a memory effect. In contrast, when the applied force is applied at a low speed or over long periods of time, a relaxation process will take place. This relaxation is due to the slipping and disentangling of the elastomer chains from each other into new positions, which enables the molecules to form other random coil conformations with favourable entropy (Figure 2). The described slipping or flow of the elastomer molecules under force can be prevented by crosslinking the molecules. Chemically crosslinked elastomers form insoluble elastomers; their structure is linked during the crosslinking process and then cannot be transformed afterwards [64].

Therefore, molecular mobility is the main cause of the shape memory effect for polymers as mentioned before. For example, Oikonomou et al. (2021) [65] studied the relationship between free volume and molecular mobility using the Couchman−Karasz and Gordon−Taylor equations, which are numerical approaches to describing molecular mobility in poly(vinyl alcohol) and poly(vinyl pyrrolidone) blends. The results revealed that molecular mobility increased with the increasing free volume in the blend system.

During the transition of a shape memory elastomer from its original shape to a temporary shape and then its return to the original shape based on the stimuli-response environment, there are two types of phases or domains in the system [66]. This is related to the phase diagrams, which are a graphical representation of the various material domains of stability at equilibrium. Phase diagrams are most commonly constructed in temperature–pressure–composition space, or Gibbs free energy composition space. Other coordinate systems, though not yet as widely used, may find increasing practical applications, especially in predicting changes in the internal structure of shape memory. A system that is equilibrated in some initial domain on the map and then placed in another domain with a different equilibrium structure, such as by simply changing its temperature, undergoes a series of microstructure transformations that take it towards its new equilibrium state. Microstructures with optimum properties can be selected by interrupting this process and quenching (i.e., rapid cooling) in the structure, which is then the state in which the shape memory elastomer is used.

A system is considered to be unary (single phase) if for the range of states under study it consists of a single chemical component. Each of the elements forms a unary system over its full range of existence. Molecular compounds such as synthetic polyisoprene (cis–1,4–polyisoprene) may be treated as unary systems over most of the range of temperatures and pressures normally encountered in the laboratory. Under conditions in which it may decompose or change to form significant quantities of other molecules, it cannot be treated as a unary system. From a thermodynamic point of view, a system is homogeneous if it consists of a single phase. More specifically, when a system consists of a single phase, its intensive properties are similar. A heterogeneous system consists of more than one phase. Some of its intensive properties exhibit discontinuities at the boundaries between the phases in a heterogeneous system. In the case of SMEs, the thermodynamic equilibrium is not always reached with regard to compatibility (as opposed to miscibility). Miscibility always relates to small molecules. The definition of compatibility is linked to the properties of the mixture as natural latex (cis–1,4–polyisoprene molecules with non–rubber components); if the response of a mixture to the input force is one model or one characteristic, the mixture is compatible. Therefore, thermodynamics is a significant property-governing character in the investigation and development of shape memory elastomers [67].