Field-Cycling NMR Relaxometry Probing the Microscopic Dynamics in Polymer Melts

Abstract

Field-cycling (FC) 1H and 2H NMR relaxometry is applied to linear polybutadiene (PB) of different molar mass (M) in order to test current polymer theories. Applying earth field compensation, five decades in the frequency dependence of the spin–lattice relaxation rate T1–1(ν) = R1(ν) are accessed (200 Hz - 30 MHz), and we focus on the crossover from Rouse to entanglement dynamics. A refined evaluation is presented, which avoids application of frequency–temperature superposition as well as Fourier transformation. Instead, the power-law exponent e in the entanglement regime is directly determined from the susceptibility representation χNMR″(ω) = ω/T1(ω) ∝ ωe by a derivative method. Correspondingly, a power-law t–e characterizes the decay in the time domain, i.e., the dipolar correlation function. For the total 1H relaxation, comprising intra- and intermolecular relaxation, a high-M exponent etotal = 0.31 ± 0.03 is found. An isotope dilution experiment, which yields the intramolecular relaxation reflecting so...