Thermodynamic and Mechanical Characterization of Shape-Memory Natural Rubber

Natural rubber (NR) is one of the most widely used and investigated materials in modern industry and science. The recent discovery of shape-memory natural rubber (SMNR), however, arises numerous new applications and research possibilities. In this work, the thermodynamic behavior of NR, cross-linked with dicumyl peroxide (DCP) and stretched at different extension ratios was investigated. The vapor sorption of different volatile organic compounds (VOC) was measured using a magnetic suspension balance, while the swelling of NR in liquid toluene was measured gravimetrically. The perturbed-chain statistical association fluid theory (PC-SAFT) was used to model the swelling equilibrium and vapor sorption data. An additional contribution to the residual Helmholtz energy was used in order to account for the cross-linking of NR, and was further modified to consider the influence of stretching on vapor sorption in NR. The influence of crystallinity on VOC-vapor sorption in stretched NR was addressed using the “constraint pressure” hypothesis, suggesting that the crystals impose an additional pressure on the amorphous parts of the polymer, causing a reduction in sorption. The modeling showed very good results for toluene vapor sorption and was also successfully used to predict the vapor sorption of chloroform and acetone in very good agreement with experimental data. Further, the influence of VOC-vapor on the shape-memory properties of SMNR cross-linked with DCP was investigated in this work. For this purpose the stress answer of programmed and non-programmed SMNR samples was measured using cyclic VOC-vapor sorption-desorption experiments in a creep apparatus. The results showed that SMNR can act as readable, rewritable and erasable memory-material that is specific to VOC-type and its partial pressure.