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

摘要

Fast field cycling (FFC) NMR is applied to study the dispersion of the H-1 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 omega/T-1(omega), and applying frequency-temperature superposition, master spectra are obtained covering up to 8 decades in frequency. In all cases solely the glassy dynamics (alpha-process) determines the relaxation behavior, and the Rouse unit is estimated to M-R congruent to 500 g/mol. The time constant tau(alpha)(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 (omega tau(alpha) 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 a-relaxation is properly accounted for, the apparent power law spectrum does not reflect the actual polymer dynamics.