DIRECT-DETECTION OF IONIC CLUSTERING IN TELECHELIC IONOMERS BY DSC AND ESR

Complexes of amine-terminated telechelic polybutadiene with Cu2+ have been studied by electron spin resonance (ESR) and differential scanning calorimetry (DSC). In addition to the glass transition temperature at 200 K, an exothermic peak was detected by DSC in the temperature range 350-450 K. This peak, which is absent in the telechelic polymer without metal cations, becomes more intense and shifts to higher temperatures as the amount of cation in the sample increases, is not detected in a second run taken immediately after the first measurement, and reappears in samples left at ambient temperature for several days. The glass transition region is not affected by heating the samples to 450 K. We suggest that the exothermic peak is due to the organization of the cation into ionic clusters, even when the Cu2+/amine molar ratio is only 0.02. The reappearance of the high-temperature DSC peak in samples left at ambient temperature after measuring the exothermic transition suggests that the cluster formation is reversible. ESR spectra of Cu2+-polymer complexes provide additional support for this interpretation: ESR signals from complexed Cu2+ cations decrease in intensity on annealing above ambient temperature, disappear almost completely on heating at 523 K for 1 h, and gradually reappear in samples left at ambient temperature. These results are interpreted in terms of the formation of ESR-silent dismagnetic or paramagnetic species with very broad ESR lines upon cluster formation and support the suggestion that the clusters form on heating and disappear at ambient temperature. A narrow (6 G) ESR singlet detected in the annealed samples is tentatively assigned to small metallic copper particles with a diameter <100 angstrom.