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

Abstract

This 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.

Cross‐linked Poly(Octadecyl Acrylate)/Polybutadiene Shape Memory Polymer Blends Prepared by Simultaneous Free Radical Cross‐linking, Grafting and Polymerization of Octadecyl Acrylate/Polybutadiene Blends

“…Previous investigation of crystalline fatty acid/natural rubber SMPs showed that the modulus of fatty acid network scaled with the weight fraction of crystalline fatty acid in the network. [ 21 ] Therefore, if the 2–5 wt% samples form similar crystal morphologies in terms of the size and shape of the crystalline domains, it is likely that their mechanical properties, such and modulus and yield strength would scale with the degree of crystallinity of the PODA.…”

“…The second approach is to blend natural rubber with a crystalline small molecule, such as a fatty acid or paraffin wax, which is able to form a thermally responsive solid network. [ 18–21 ] Compared to the first approach, which relies on the inherent strain‐induced crystallinity of natural rubber, this blending approach decouples the elastomer network and fixing network. This decoupling has been applied to the fabrication of SMPs from a range of polyolefins including sulfonated ethylene propylene diene copolymer, [ 22,23 ] polystyrene/polydiene block copolymers, [ 24–26 ] and polyolefin block copolymers.…”

“…While this blend approach can be broadly applied to convert conventional elastomers to SMPs and tune the shape memory properties through the choice of molecular crystal, one challenge it faces is the lack of connectivity between the elastomer and the small molecule, which can lead to blooming of the small molecule on aging, its extraction into solvent, and its expulsion at high rates of deformation during programming and recovery. [ 21,29 ] To address this issue, the present work has investigated a new approach for fabricating a SMP built off the elastomer/small molecule blends using a blend of PB, n ‐octadecyl acrylate (ODA), and dicumyl peroxide (DCP).…”

“…[ 33 ] It has previously been used to prepare structure‐fixing networks in copolymers, such as with acrylic acid, methyl acrylate, and a urethane diacrylate [ 34–37 ] or from the foamed homopolymer. [ 38 ] Much like the properties of SMPs prepared from blends of an elastomer and small molecule varying with the choice of small molecule, [ 21,26 ] this approach using side‐chain crystalline polymers retains that versatility, where the melting point may be tuned by varying the side‐chain length of the monomer or copolymerization through its neat melting point and solubility. [ 39,40 ]…”

Effect of electron beam irradiation on the properties of EVA/EPDM blends

“…The modification using EB irradiation of such polyolefin is widely used as materials for heat shrinkable tubes and wire or cable insulation. 34,35 Thus, it is reasonable and interesting to modify EVA/EPDM blends using EB irradiation to further increase its properties.…”