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 T 1 –1(ν) = R 1(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 ε in the entanglement regime is directly determined from the susceptibility representation χNMR ″(ω) = ω/T 1(ω) ∝ ωε by a derivative method. Correspondingly, a power-law t –ε 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 εtotal = 0.31 ± 0.03 is found. An isotope dilution experiment, which yields the intramolecular relaxation reflecting solely segmental reorientation, provides an exponent εintra = 0.44 ± 0.03. It agrees with that of FC 2H NMR (ε Q = 0.42 ± 0.03) probing only segmental reorientation. The fact that εintra > εtotal demonstrates the relevance of intermolecular relaxation in the entanglement regime (but not in the Rouse regime), and εintra is significantly higher than predicted by the tube-reptation (TR) model (εTR = 0.25) and, the latter being supported also by recent simulations. The ratio of inter- to intramolecular relaxation grows with decreasing frequency, again in contradiction to the TR model and results from double quantum 1H NMR. We conclude that no clear evidence of a tube is found on the microscopic level and the so-called return-to-origin hypothesis is not confirmed. Studying the influence of chain end dynamics by FC 1H NMR we compare differently chain end deuterated PB. For the dynamics of the central part of the polymer the exponent drops from εintra = 0.66 ± 0.03 down to εcent = 0.41 ± 0.03 for M = 29k which is very close to the high-M value εintra. Thus, the protracted transition to entanglement dynamics reported before is not found when the polymer center is probed; instead full entanglement dynamics appears to set in directly with M > M e .

Dynamics of poly(vinyl butyral) studied using dielectric spectroscopy and 1H NMR relaxometry

“…Departures of w 0 0 NMR trends from this function, observed at temperatures much higher than T g , have been ascribed to contributions to relaxation arising from slower collective polymer dynamics. 21,[52][53][54][55][56] Hence, in our case the obtained values of a indicate that the collective dynamics does not significantly contribute to relaxation, most probably because the measurements were performed only up to 50 K above the glass transition. Since glassy dynamics is not influenced by the chain length, 21 the polymer polydispersity is expected to play a minor role.…”

Glassy and Polymer Dynamics of Elastomers by 1H-Field-Cycling NMR Relaxometry: Effects of Fillers

“…At 303 K, all samples show NMRD curves with a power law dependence on frequency ( R 1 (ω) ∝ ω –γ ) at lower frequencies (γ = 0.16–0.18) and a dispersion at higher frequencies. The observed exponents are similar to those reported in the literature for high-molar-mass ( M > M e ) polyisoprene 64 − 66 , 68 − 70 , 85 and IR rubbers, 73 and can be ascribed to the Rouse regime (regime I of the TR model), while the dispersion corresponds to glassy dynamics (regime 0). Small differences were found between the NMRD curves of filled rubbers and that of IR_S2.…”

“… 46 − 48 , 61 Since glassy dynamics is strongly prevalent over polymer dynamics, 44 the latter can only be correctly investigated after separation of these two components in FC NMR data, as pointed out by Rössler and co-workers. 45 , 47 , 62 − 68 …”

Field-Cycling Relaxometry as a Molecular Rheology Technique: Common Analysis of NMR, Shear Modulus and Dielectric Loss Data of Polymers vs Dendrimers

“…4,13,23 Applying these procedures for polymers a picture emerged which confirms the tube-reptation model only in parts. 8,13 Recently, we also investigated the dynamics of poly-(propyleneimine) (PPI) dendrimer of different generations by different techniques. 19 The bulk dendrimer constitutes highly viscous liquids at room temperature that undergo a glass transition with a glass transition temperature T g of around 200 K, virtually independent of the number of generations.…”

“…We note that, neglecting the intermolecular term, eq 4 was tested previously, 22,24 but succeeding experiments revealed that the intermolecular relaxation has to be taken into account, in particular, when low-frequency dispersion is discussed. 8,12,13 Next, we define the cumulative mode ratio F NMR (N):…”