Structure–property relationships of fatty acid swollen, crosslinked natural rubber shape memory polymers

Abstract

ABSTRACTThis article investigates shape memory polymers (SMPs) fabricated by swelling sulfur crosslinked natural rubber with four different molten fatty acids: lauric, myristic, palmitic, and stearic acid. As inexpensive additives, they allow commodity natural rubber to be directly converted to SMPs. The shape memory properties are investigated as a function of wt% fatty acid, the choice of fatty acid, and the applied load during shape memory programming. It is found that increasing the wt% acid improves the shape fixity up to ca. 97% at ≥50 wt% fatty acid, at which point the recovery starts to decline with increasing wt% acid due to network failure during shape programming. The shape fixity is found to depend on the yield stress and modulus of the fatty acid network, which both increase with increasing wt% acid. The choice of fatty acid also varies the trigger temperature for shape memory, which scales with the melting point of the fatty acid. Serendipitously, it is found that alignment of the fatty acid crystals during programming produces stiffer networks whose modulus increase with applied load, which counterbalances the higher elastic energy stored in the rubber network to produce lower sensitivity of the shape fixity to the applied load. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys.2018,56, 673–687