A Novel Method for Synthesis of Sulfonated SBS Ionomers by Ring-Opening Reaction of Epoxidized SBS, Their Characterization, Properties, and Blends

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A novel method for synthesis of sulfonated ionomer of (styrene-butadiene-styrene) triblock copolymer (SBS) is developed. SBS is first epoxidized by performic acid formed in situ, followed by ring-opening reaction with an aqueous solution of NaHSO3. The optimum conditions for ring-opening reaction of the epoxidized SBS with aqueous solution of NaHSO3 and some properties of the ionomer are studied. It has been found that during the ring-opening reaction phase transfer catalyst, ring-opening catalyst, and a pH regulator are necessary to raise the conversion of epoxy groups to ionic groups. The product is characterized with FTIR spectrophotometry and transmission electron microscopy (TEM) to be the sulfonated ionomer with ionic domains about 5 nm in diameter. With increasing ionic groups, the water absorbency of the ionomer and the dilute solution viscosity of the ionomer increase, whereas the oil absorbency decreases. The ionomer behaves as thermoplastic elastomer in the presence of zinc stearate as ionic plasticizer. The tensile strength and ultimate elongation of the sodium ionomers are higher than those of the original SBS and both increase with zinc stearate below 10 wt% and with ionic group content up to about 1 mmol/g. When the ionomer is blended with crystalline polypropylene, a synergistic effect occurs with respect to the tensile strength. The ionomer can act as a compatibilizer for blending equal weight of SBS and oil-resistant chlorohydrin rubber. In the presence of 3% ionomer, the blend exhibits much better mechanical properties and solvent resistance than the blend without the ionomer. The blend of equal weight of SBS and CHR compatibilized by the ionomer behaves as an oil resistant thermoplastic elastomer. Scanning electron micrographs of the blends with or without the ionomer indicate the increased compatibility between the two components in the presence of the ionomer.