Supramolecular reinforcement of styrene-butadiene rubber composites

Abstract

A strategy of supramolecular reinforcement of elastomers is conceptualized and demonstrated. Amphiphile A consisting of a β-alanine oleophobic motif and a pentadecyl oleophilic motif is designed to self-assemble in a continuous rubber phase to form discrete micelle-type structures under high-temperature processing conditions and crystallize upon cooling to form supramolecular reinforcing particles dispersed in the rubber phase. Styrene-butadiene rubber (SBR) composites reinforced by A are prepared and vulcanized using conventional rubber processing methods. The hierarchical structure and morphology of the assemblies of A in the vulcanized SBR composites are studied by Fourier-transform infrared spectroscopy, differential scanning calorimetry, wide-angle X-ray diffraction, and transmission electron microscopy. In the SBR continuous phase, A forms β-sheets, and the β-sheets stack to form crystals. At a filler loading of 5 phr (parts per hundred rubber), the β-sheet crystalline domains of A are whisker-like with the short dimensions on the order of a few to a few tens of nanometers and the long dimension on the order of a few hundred nanometers. The whisker-like crystalline domains aggregate to form globules of the size of hundreds of nanometers to micrometers at higher filler loadings (10–20 phr). Tensile test shows that A does improve strength at break, strain at break, and toughness of the reinforced SBR composites in comparison to gum SBR. The best reinforcement, as measured by toughness of the vulcanized rubber composite, is achieved at 5 phr filler loading, where the toughness is increased to nearly 3 times as high as that of the vulcanized gum SBR.