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.

Generally reversible stimuli-responsive materials do not memorize the stimulus. In this study we describe an example in which stretched and constrained semi-crystalline polymer networks respond to solvent gases with stress and simultaneously memorize the concentration and the chemical nature of the solvent itself in their microstructure. This written solvent signature can even be deleted by temperature.

Sorption of volatile organic compound (VOC) vapors in natural rubber (NR) was measured at 20°C using a magnetic suspension balance. Experiments were performed with non-cross-linked NR, as well as NR cross-linked with dicumyl peroxide. Stretching the cross-linked NR samples leads to crystal formation and therefore to a constraint to volume swelling of the amorphous domains. To investigate the influence of NR stretching and therefore of crystal formation on the VOC sorption, measurements of nonstretched NR were compared to data for cross-linked NR samples stretched with different extension ratios. Analysis of the VOC sorption data revealed a reduced VOC sorption in the stretched NR compared to fully amorphous, nonstretched NR. The sorption data were modeled using the perturbed-chain statistical associating fluid theory (PC-SAFT) accounting for network elasticity by an additional Helmholtz-energy contribution. Influence of crystallinity content on VOC solubility in stretched NR was finally accounted for following a recently proposed approach and thus accounting for an additional contribution to pressure in the amorphous phase as a result of constraint imposed by crystalline domains. Comparison of modeling results with measured solubility for several VOCs in NR confirmed the overall consistency of the modeling approach used. ■ INTRODUCTIONAlready since the beginning of the 19th century, natural rubber (NR) played a significant role in the life of modern mankind. Discovery of the vulcanization process increased the relevance of NR for the industry, which ultimately stimulated a comprehensive investigation of the physicochemical and thermodynamic properties of NR. Indeed, until today, NR is one of the most-used and best-investigated polymers.Among peculiar characteristics of NR is the ability to sustain a large deformation in elastic range, storing free energy as a result of change in the configuration entropy of polymer chains. The representation of this kind of mechanical behavior was first provided through the theory of rubberlike elasticity, 1 by P. J. Flory, later extended by the same author to the description of sorption and swelling data of volatile organic compounds (VOC) in NR. Flory was also the first one who thermodynamically described strain-induced crystallization, 2 which was revealed as the main reason for the high tensile strength of stretched NR.Toki and Hsiao 3−6 and Tosaka et al. 7 investigated straininduced crystallization of stretched, cross-linked NR. The investigations were performed using wide-angle X-ray diffraction measurements. They showed that the crystallinity of NR cross-linked with sulfur 3−7 and with dicumyl peroxide (DCP) 5 was up to 20% for the highest extension ratios (8 and 6). 5,7 Due to this strain-induced crystallization, cross-linked NR shows a remarkable shape-memory effect as recently discovered by Katzenberg et al. 8 This effect allows storing large amounts of strain and deformation energy. Further investigation of this shape-memory NR showed a reversible response of stretche...