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.

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.

Here we report on a novel type of smart material that is capable of specifically responding to the changing rate of an environmental signal. This is shown on the example of lightly cross-linked syndiotactic polypropylene that reacts to a temperature increase by adapting its shape change according to the applied heating rate. In general, a material with such properties can be used to predict a system failure when used in a defined environment and is therefore called "predictive material".