New Insights into the Dynamics of Poly(amidoamine) Dendrimers with Amino Surface Groups from Segmental Relaxation and Ionic Conductivity

Abstract

In this work, the glassy dynamic features of poly(amidoamine) (PAMAM) dendrimers were determined explicitly through their dielectric segmental relaxation behaviors, and the structure−dynamic relationships were revealed via the decoupling behavior of ionic conductivity from segmental relaxation. By means of broadband dielectric spectroscopy, the relaxation behaviors of PAMAM dendrimers of generations 0 through 4 were investigated in a broad frequency and temperature range. Three secondary relaxations were observed in the glassy state, including a typical Johari−Goldstein β relaxation, which is considered a precursor of the α relaxation and also signifies the glass transition, a γ relaxation, and a δ relaxation in order of increasing frequency. Above the glass transition temperature, the segmental α relaxation and a slow mode are distinctly disclosed in dielectric spectra. The slow mode was found to arise from interfacial polarization connected with ionic conductivity. The glass transition temperature and fragility of the dendrimers were determined to be in line with the VFT behavior of the α relaxation time. The former is basically consistent with that determined in DSC measurement, and the latter suggests that PAMAM dendrimers are moderately fragile glass formers. Decoupling of ionic conductivity from segmental relaxation was identified for all generations, but the decoupling pattern of generations 0 and 1 was found to be obviously different from that in generations 2 through 4, evidencing a fundamental divergence in the microscopic structure of these two groups as a result of different interpenetration behavior. Furthermore, it is advised that the ionic conduction in PAMAM dendrimers is because glassy dynamics resisted proton hopping in a hydrogen-bonding network.