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.

Shape memory polymers (SMPs) are an important class of smart materials. So far the focus of such polymers was to find suited triggers for various application fields. Thus, the potential of most of these macromolecular networks regarding their maximally storable strain capability was not explored. In this study, the polyethylenes HDPE, LDPE, and ethylene-1-octene (EOC) were systematically investigated with respect to their strain storage potential. To achieve maximum strains, the polymers were chemically cross-linked in such a way that they are at the borderline between thermoplastics and elastomers. All investigated polymers showed higher strain storage than literature reported systems and exhibited excellent shape memory parameters. The highest stored strain was found for networks of EOC with fully recoverable 1400%. Interestingly, this value could not be enlarged by using EOCs with higher molecular weight, which is probably due to increasing content of entanglements as confirmed by Mooney-Rivlin.

We found that constrained shape memory natural rubber (SMNR) generates mechanical stress when exposed to solvent vapor. When the solvent vapor is removed, the material reprograms itself. This process is reversible and the stress answer is proportional to the solvent vapor concentration. Further, the stress answer is specific to the solvent.

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.

Shape memory polymers (SMPs) are an important class of smart materials. Usually, these polymers can be switched between two shapes. Recently, the possibility of switching more than two shapes was introduced for SMPs with relatively low strain storage capability. In this work, a lightly cross-linked polyethylene blend comprising 80 wt% EOC, 15 wt% LDPE, and 5 wt% HDPE is prepared in order to obtain a tunable multiple-shape memory polymer with high strain storage capacity. It is found that depending on the programming procedure, this SMP obtains a dual-, triple-, or quadruple-shape memory effect, with well-defi ned intermediate temporary shapes (retraction < 0.5% K −1 ) over a signifi cantly broad temperature range (up to 30 K), large storable strains (up to 1700%), and nearly full recovery of all shapes ( > 98.9%). transition range. [14][15][16][17][18] For instance, Xie [ 19 ] obtained a tunable multiple-shape memory for PFSA (perfl uorosulfonic acid ionomer), which possesses a broad glass transition from 55 to 130 ° C, by correlating certain temporary shapes to different temperatures within its glass transition range. Kolesov and Radusch [ 20 ] showed a triple-shape memory for cross-linked polyethylene blends of HDPE and two different ethylene-1-octene copolymers (EOC), by correlating different temporary shapes to distinct temperatures within a broad melting range. With this system, they were able to store strains of up to 100% with an intermediate shape at 44%. In general, multiple-shape memory polymers store strains of up to 240%, which limit the relative retraction response. We have recently found that the stored strain of PE-SMPs can be greatly enhanced by lowering the degree of cross-linking in a way that the resulting network is at the borderline between thermoplastics and elastomers. [ 21 ] In the present manuscript, this concept is extended to polyethylene blends in order to obtain a tunable multiple-shape memory polymer with large strain storage capability. Experimental Section MaterialsHigh density polyethylene (HDPE) (Lupolen 6021D, M w = 220 000 g mol −1 , PDI = 10, 0.5 branches per 1000 C atoms) [ 22 ]

In this work, syndiotactic polypropylene (sPP) as well as isotactic polypropylene (iPP) are cross-linked to gain a shape memory effect. Both prepared PP networks exhibit maximum strains of 700%, stored strains of up to 680%, and recoveries of nearly 100%. While x-iPP is stable for many cycles, x-sPP ruptures after the first shape-memory cycle. It is shown by wide-angle X-ray scattering (WAXS) experiments that cross-linked iPP exhibits homoepitaxy in the temporary, stretched shape but in contrast to previous reports it contains a higher amount of daughter than mother crystals.