Dynamics of PPI dendrimers: A study by dielectric and ²H NMR spectroscopy and by field-cycling ¹H NMR relaxometry

Abstract

© 2015 American Chemical Society. We investigate bulk poly(propyleneimine) dendrimers of generation (G) 2-5 by dielectric spectroscopy (DS), solid-state ²H NMR, and field-cycling ¹H NMR relaxometry (FC ¹H NMR) in a large temperature range (120-400 K). Three relaxation processes are identified by DS: a main (α-) relaxation (T > T<inf>g</inf>) and two secondary processes (T < T<inf>g</inf>). The α-process exhibits a super-Arrhenius temperature dependence typical of glass-forming liquids and changes only weakly with G, yielding T<inf>g</inf> ∼ 200 K. The temperature dependence of the secondary relaxations is governed by an Arrhenius law. While one secondary process exhibits features characteristic for glasses, the other is atypical. Its time constant is virtually independent of G, and its spectral width does not increase with lowering temperature as is usually observed for sub-T<inf>g</inf> relaxations. Regarding FC ¹H NMR probing the dispersion of the spin-lattice rate R<inf>1</inf> in the frequency range 200 Hz-30 MHz, transformation to the susceptibility representation, π″(ω) ωR<inf>1</inf>(ω), and applying frequency-temperature superposition, an effective frequency range of 9 decades is covered by a master curve πPrime;(ωτ<inf>α</inf>). In addition to the segmental time τ<inf>α</inf>(T), which complements the results from DS up to high temperatures, a longer terminal relaxation τ<inf>t</inf>(T) is identified. In between, an intermediate power-law regime is observed in πPrime;(ωτ<inf>α</inf>) with an exponent of about 0.8. The broad relaxation spectrum is attributed to local dynamics, breathing modes, and overall tumbling and diffusion of the dendrimer molecule. In the low-frequency limit, R<inf>1</inf>(ω) is determined by intermolecular relaxation from which the molar mass dependence of the translational diffusion coefficient can be estimated. We find D(M) ∝ M^-1.2±0.2.