Effect of degree of neutralization on the properties of ionic thermoplastic elastomer based on sulfonated maleated styrene-ethylene/butylene-styrene-block copolymer

Novel block copolymer ionomers having both carboxylate and sulfonate groups in the same polymer backbone were obtained by sulfonating maleated poly (styrene-(ethylene-co-butylene)-styrene) block copolymer (m-SEBS), followed by its neutralization with barium acetate. The effects of the degree of neutralization on the properties of the ionomers were studied. Infrared spectroscopic studies confirm the formation of both carboxylate and sulfonate salts and the occurrence of the sulfonation reaction in the para position of the benzene rings of polystyrene segments. Dynamic mechanical thermal analyses show the presence of a high temperature rubbery plateau, particularly at high degree of neutralization, in addition to the glass-rubber transitions due to the polystyrene (PS) and ethylene-butylene (EB) phases. T-g of the PS block remains unchanged and tandelta at the T-g gradually decreases with increasing the degree of neutralization. But T-g of the EB block gradually increasesg and tandelta at the Tg marginally increases. The decrease in tandelta at T-g of the PS block is ascribed to the stiffening of the polystyrene segments on neutralization. Increase in the degree of neutralization causes formation of ionic crosslinks which increases the T-g of the EB block. Dielectric thermal analyses show that incorporation of ionic groups plays an important role in the temperature dependence of the dielectric constant of the ionomers. At lower degrees of neutralization (25 and 50%), the ionomers behave like the base polymer in the sense that dielectric constant increases at elevated temperature(>120degreesC). At higher degrees of neutralization, however, the increase in dielectric constant in the high temperature region is restricted due to the formation of the rubbery network arising out of the ionic aggregates. Transmission electron micrographs reveal that on neutralization the domain shape changes from predominantly cylindrical type to exclusively spherical one and at 100% neutralization the diameter of the spheres ranges from 15 to 20 nm. Processability studies show that as the degree of neutralization increases, the melt viscosity of the ionomers increases. The tensile strength, hardness, modulus and tear strength increase but the elongation at break and tension set decrease with increase in the degree of neutralization. Thermal stability of the polymers increases with the degree of neutralization. Reprocessability studies show that the ionomer can be recycled without fall in properties. The barium salt of sulfonated maleated SEBS at higher degrees of neutralization (75 and 100%) behaves as an ionic thermoplastic elastomer.