Linear Viscoelasticity and Fourier Transform Infrared Spectroscopy of Polyether-Ester-Sulfonate Copolymer Ionomers

Fourier transform infrared spectroscopy (FTIR) and linear viscoelasticity (LVE) were used to characterize amorphous copolyester ionomers synthesized via condensation of sulfonated phthalates with mixtures of poly(ethylene glycol) with M = 600 g/mol and poly(tetramethylene glycol) with M = 650 g/mol. The copolymer ionomers exhibited microdomain separation, as confirmed in previous X-ray scattering measurements. Since PEO has superior ion solvating ability compared with PTMO, the ions near the interface reside preferentially in the PEO microdomain. FTIR measurements were used to quantify fractions of ions in different association states, in turn quantifying the fractions in the PEO-rich domains, in the PTMO-rich domains, and at the interface between these domains. FTIR shows that the structure of the interfacial ion aggregates is quite different for the copolymers with different counterions; at the interface Na+ aggregates into open string structures while Li+ aggregates into denser sheets of ions, as depicted schematically at the far right. Ionic conductivity is dominated by ions in the PEO domain, due to superior cation solvation by PEO; in the PTMO-rich microdomain both Na+ and Li+ form dense aggregates with of order 15 ion pairs. The temperature dependence of viscoelastic properties depends primarily on the PEO segmental dynamics, due to much higher T-g for the PEO-rich microdomains that are continuous at all copolymer compositions studied. Increasing the PTMO fraction increases the ionic association lifetime and delays the LVE terminal relaxation, creating an extended rubbery plateau, despite the fact that the chains are quite short.