Dynamics of Telechelic Ionomers with Distribution of Number of Ionic Stickers at Chain Ends

A new type of ionic telechelic polymer, characterized by a distribution of the number x of ionic groups (sulfonated styrene neutralized by sodium) at the chain ends, was synthesized by a two-step reversible addition–fragmentation chain transfer (RAFT) polymerization with a bifunctional agent. The chain backbone was polystyrene (PS) having a molecular weight of ∼3000 g/mol, and the average number of ionic groups per chain end, m = ⟨x⟩, varied from 0.22 to 1.3. These ionic groups associated with each other to form flower-like micelles, as revealed from small-angle X-ray scattering, and those micelles were organized into a network for m > 0.53. Correspondingly, the linear viscoelastic modulus of the ionomer showed the sol-to-gel transition with increasing m > 0.53, and slow viscoelastic relaxation activated by the ionic dissociation was noted for samples well above the gel point (for m ≥ 0.70). This relaxation had a broad mode distribution (power-law type distribution accompanied by undetectably slow terminal relaxation), which made a strong contrast to a narrow, almost single Maxwellian mode distribution observed for a model telechelic ionomer having exactly one ionic group per chain end. Thus, the broad relaxation mode distribution seen for the new type of ionomer was attributed to the distribution of the ionic group number x at the chain end: This distribution resulted in a multibranched sol structure on a time scale longer than the dissociation time for the chain end with x = 1 but shorter than the time for the chain ends with x ≥ 2, and the power-law type mode distribution reflected motion of the sol chains activated by dissociation of the chain ends with x = 1.