From simple liquid to polymer melt. Glassy and polymer dynamics studied by fast field cycling NMR relaxometry: Low and high molecular weight limit

Fast field cycling (FFC) NMR is applied to study the dispersion of the ¹H spin-lattice relaxation in the low molecular weight glass-formers o-terphenyl, tristyrene, and oligomeric polybutadiene (PB, with M/gmol ^-1 = 355 and 466) over a broad temperature range (203-401 K). Differing from previous FFC NMR works, we analyze the relaxation data in the susceptibility form ω/T₁(ω), and applying frequency-temperature superposition, master spectra are obtained covering up to 8 decades in frequency. In all cases solely the glassy dynamics (α-process) determines the relaxation behavior, and the Rouse unit is estimated to M_R ≅ 500 g/mol. The time constant τ_α(T) in the range 10^-11-10^-6 s is extracted, which agrees well with those measured at the same time by dielectric spectroscopy. For the high molecular weight PB (M/gmol^-1 = 56 500, 87 000, 314 000, and 817 000) pronounced polymer effects are observed at low frequencies (ωτ_α ≪ 1) which are isolated from the total spectrum by subtracting the "glass spectrum" as obtained from low molecular PB. We argue that unless the underlying α-relaxation is properly accounted for, the apparent power law spectrum does not reflect the actual polymer dynamics.