Shape‐memory polymers (SMPs) are smart, responsive materials with numerous potential applications. Based on previously introduced shape‐memory natural rubber (SMNR), which shows exceptional properties such as strain storage of 1000%, cold storage, cold programmability, and mechanical and thermal triggers tunable both during and after programming, different SMNRs regarding their shape‐memory parameters are investigated. Furthermore, their energy‐storage capability and their mechanical properties are explored. SMNRs show fixity ratios of up to 94% and excellent recovery ratios of up to 100% whereas strains even above 1000% can be stored. Energies of up to 4.88 J g−1 can be stored with efficiencies of up to 53.30%. Further, the Young's modulus of SMNR can be switched by two orders of magnitude upon triggering or programming.

Supercooled liquids are still considered to be magic materials that can store heat, which is easily to be released with a suited trigger. Storage of cold is much harder to achieve, e.g., by using superheated liquids or liquifi ed gases, which are naturally hard to handle and require laborious equipment. Storing cold with solid materials is not yet realized because superheated solids are generally assumed to be impossible. [ 1 ] Here we report on designed natural rubber (NR) networks that can store cold and release it on a trigger. The unique base of this effect in the NR networks is the ability to form stable crystals upon strain and to stabilize them at the stretching temperature after the stress is released, also retaining up to 1000% elongation. This is a 5-10 times greater strain storage capacity compared to recently described high-tech shape memory materials. [2][3][4] Increasing the temperature to a certain point, called the trigger temperature, results in a spontaneous collapse of the crystals that affords absorption of a signifi cant amount of heat originating from the disruption of the crystals and the relaxation of the rubber itself. This is the fi rst example of a solid material that is capable of storing a signifi cant amount of cold by bringing rubber in a superheated state. Furthermore, we found that the trigger temperature is tunable for one sample in a broad range by varying the stretching conditions.Energy storage is one of the greatest current scientifi c issues. Most systems store energy in the form of electron transfer processes by the heating of matter or pressurizing of gases. Only a few examples are known that use the enthalpy of phase transition (latent heat) for this purpose. The most popular example is the supercooled liquid sodium acetate trihydrate, which does not crystallize until a nucleus is added as trigger. The resulting crystallization releases heat into the environment. In order to have a cold storage system, one would need a material that conserves a superheated state, i.e., the material stores cold that can be released spontaneously via an external trigger. Up to now, such solid materials have been considered impossible. [ 1 ] An interesting way to cool a material is to release the strain of an elastomer. Because the internal energy Δ U of a rubber remains nearly unchanged during deformation ( Δ U = Δ Q -Δ W = 0), the performed work, Δ W , is directly released as heat, Δ Q , during the stretching process and is taken up during its relaxation. This behavior is well-known for NR, which shows a supercooling of up to 10 K by releasing an elongation of λ = 5 (400% strain). [ 5 ] In contrast to common entropy-elastic materials, NR forms crystals upon strain (strain-induced crystallization (SIC)), which additionally contribute to the cooling of the rubber during relaxation because the necessary heat of fusion Δ h f is taken up from the surrounding. [6][7][8][9][10][11] Altogether the stored cold corresponds to the sum of Δ h f and Δ W .So far only a few materials are known that can s...

In contrast to all known shape memory polymers, the melting temperature of crystals in shape memory natural rubber (SMNR) can be greatly manipulated by the application of external mechanical stress. As shown previously, stress perpendicular to the prior programming direction decreases the melting temperature by up to 40 K. In this study, we investigated the influence of mechanical stress parallel to prior stretching direction during programming on the stability of the elongation-stabilizing crystals. It was found that parallel stress stabilizes the crystals, which is indicated by linear increase of the trigger temperature by up to 17 K. The crystal melting temperature can be increased up to 126.5 °C under constrained conditions as shown by X-ray diffraction measurements.

Lightly cross-linked natural rubber (NR, cis-1,4-polyisoprene) was found to be an exceptional cold programmable shape memory polymer (SMP) with strain storage of up to 1000%. These networks are stabilized by strain-induced crystals. Here, we explore the influence of mechanical stress applied perpendicular to the elongation direction of the network on the stability of these crystals. We found that the material recovers its original shape at a critical transverse stress. It could be shown that this is due to a disruption of the strain-stabilizing crystals, which represents a completely new trigger for SMPs. The variation of transverse stress allows tuning of the trigger temperature T(trig) (σ) in a range of 45 to 0 °C, which is the first example of manipulating the transition of a crystal-stabilized SMP after programming.

The paper describes a new apparatus to measure tensile creep curves of polymer/volatile organic compound (VOC) systems, especially designed for measurements of small VOC loadings in glassy polymers. For the first time creep curves for glassy polymer/VOC systems are recorded. The measurements were performed for the system polystyrene/toluene at different toluene loads up to w toluene = 0.13 and at temperatures of 30, 50, and 70 °C. It was found that increasing VOC mass fractions qualitatively influence the mechanical properties of a polymer in the same way like increasing temperature does. Since at isothermal conditions these properties are affected by the glass transition of the system, this information for the polystyrene/toluene mixtures was used to modify and to verify the correlation of Kelly and Bueche to predict the glass-transition temperature of polymer/solvent systems.