Synthesis and mechanical properties of poly(styrene-b-isobutylene-b-styrene) block copolymer ionomers

Linear and three-arm star poly(styrene-b-isobutylene-b-styrene) (PS-PIB-PS) block copolymer ionomers possessing various counterions and levels of sulfonation were synthesized by sulfonating the polystyrene blocks of PS-PIB-PS block copolymers. Analysis of compression-molded films of the ionomers showed that the incorporation of sulfonate groups into the polystyrene blocks of these materials resulted in an increase in tensile strength, a decrease in elongation at break, and a persistence of elastic properties to much higher temperatures as compared with unsulfonated precursors. Among all counterions studied, zinc resulted in the strongest ionomers and potassium yielded the most easily processed ionomers. Dynamic mechanical analysis showed that the block copolymer ionomers possessed a phase-separated morphology; however, anomalous relaxations observed during the first heating cycle, but that were substantially reduced or completely absent in subsequent cycles, implied that strong ionic interactions were causing reduced process ability and the formation of nonequilibrium morphologies. The observed relaxations were interpreted to be domain rearrangements brought about by the thermal energy supplied by the dynamic experiment, Annealing of films of relatively low ionic contents yielded viscoelastic behavior that was consistent with an equilibrium morphology characterized by phase-separated, partially sulfonated polystyrene domains of the same density and size as the polystyrene domains of the unsulfonated precursor. Compression molded films of high ionic content yielded a higher rubbery plateau modulus than the unsulfonated precursor, suggesting a different morphology. Solution-cast films of zinc ionomers exhibited two values for the rubbery modulus, a higher value at temperature below the T-g of polystyrene and a lower value at temperature above the T-g of polystyrene. Thermogravimetric analysis revealed the major mass-loss process of the parent, linear block copolymer at 417 degrees C (mid-point) and of the tetramethyl ammonium ionomer at 431 degrees C.