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© 2016 American Chemical Society.Proton (1H) field-cycling (FC) NMR relaxometry is applied to monitor the crossover in the segmental subdiffusion from the Rouse to the constrained Rouse regime in an entangled linear polymer melt. The method probes the dispersion of the spin-lattice relaxation rate R1(ω). Via Fourier transformation the segmental mean square displacement 〈r2(t)〉 is calculated from the intermolecular relaxation contribution R1inter(ω) to the total 1H spin-lattice relaxation dispersion R1(ω). As an example we chose poly(ethylene propylene) (M = 200k), and R1inter(ω) is singled out by performing an isotope dilution experiment. The 〈r2(t)〉 data obtained by FC NMR is directly compared to such of neutron scattering (NS) available from the literature. Because of different experimental time windows the NS data is converted to a reference temperature assuming frequency-temperature superposition. Absolute agreement is revealed between FC NMR and NS. The data on 〈r2(t)〉 confirm the predictions of the tube-reptation model; i.e., the crossover from Rouse regime to constraint Rouse regime is identified, and the tube diameter is estimated to d ≈ (4.6 ± 0.2) nm. Thus, 1H FC NMR has established itself as an alternative route to access subdiffusion.

© 2016 American Chemical Society.The dynamics of melts of linear poly(ethylene-alt-propylene) (PEP) of different molar masses (M) is investigated by 1H field-cycling (FC) NMR relaxometry. Employing a commercial and a home-built relaxometer the spin-lattice relaxation rate R1(ω) is measured in the frequency range of 200 Hz to 30 MHz and the temperature range of 200-400 K. Transforming the FC NMR relaxation data to the susceptibility representation and applying frequency-temperature superposition, master curves for the dipolar correlation function CDD(t/τα) (containing intra- and intermolecular contributions) are constructed which extend up to six decades in amplitude and eight in time. Here, τα is the time scale of the structural (α-) relaxation, which is obtained over several decades. Comparison with previously reported FC data for polybutadiene (PB) discloses very similar CDD(t). Depending on M, all the five relaxation regimes of a polymer melt are covered: in addition to the α-process (0) and the terminal relaxation (IV), which are immanent to all liquids, three polymer-specific power-law regimes (Rouse, I; constraint Rouse, II; and reptation, III) are found, i.e. CDD(t) ∝ t-ϵ. The corresponding exponents (ϵI-III) are close to those predicted by the tube-reptation (TR) model for the segmental translation. In contrast to previous interpretation the intermolecular relaxation dominates CDD(t), in particular in regime II and beyond. The decomposition into intra- (mediated by segmental reorientation) and intermolecular relaxation (mediated by segmental translation) via isotope dilution experiments yields Cinter(t) = Ctrans(t) ∝ t-0.28±0.05 concerning PEP and Cinter(t) ∝ t-0.30±0.05 concerning PB for regime II (high-M limit). For the reorientational correlation function Cintra(t) = C2(t) ∝ t-0.50±0.05 (PEP) and C2(t) ∝ t-0.45±0.05 (PB) are obtained. These exponents ϵIIintra are at variance with ϵIITR = 0.25 predicted by the TR model. The fact that translation conforms to the TR model, while reorientation does not, now confirmed for the two polymers PEP and PB, challenges de Gennes' return-to-origin hypothesis which assumes strong translational-rotational coupling in the TR model.

© 2014 American Chemical Society. 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(v) = R1(v) 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 ∈ in the entanglement regime is directly determined from the susceptibility representation χNMR ′(ω) = ω/T1(ω) ω∈ by a derivative method. Correspondingly, a power-law t-∈ 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 ∈total = 0.31 ± 0.03 is found. An isotope dilution experiment, which yields the intramolecular relaxation reflecting solely segmental reorientation, provides an exponent ∈intra = 0.44 ± 0.03. It agrees with that of FC 2H NMR (∈Q = 0.42 ± 0.03) probing only segmental reorientation. The fact that ∈intra > ∈total demonstrates the relevance of intermolecular relaxation in the entanglement regime (but not in the Rouse regime), and ∈intra is significantly higher than predicted by the tube-reptation (TR) model (∈TR = 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 ∈intra = 0.66 ± 0.03 down to ∈cent = 0.41 ± 0.03 for M = 29k which is very close to the high-M value ∈intra. 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 > Me.

We report on an experimental study of single particle properties and interactions of poly(ethylene-alt-propylene)-poly(ethylene oxide) (PEP-PEO) starlike micelles. The starlike regime is achieved by an extremely asymmetric block ratio (1:20) and the number of arms (functionality) is changed by varying the composition of the solvent (the interfacial tension). Small angle neutron scattering (SANS) data in the dilute regime can be modeled by assuming a constant density profile in the micellar core (compact core) and a starlike density profile in the corona (starlike shell). The starlike morphology of the corona is confirmed by a direct comparison with SANS measurements of dilute poly butadiene star solutions. Comparison of structure factors obtained by SANS measurements in the concentrated regime shows in addition that the interactions in the two systems are equivalent. Micellar structure factors at several packing fractions can be modeled by using the ultrasoft potential recently proposed for star polymers [Likos , Phys. Rev. Lett. 80, 4450 (1998)]. The experimental phase diagram of PEP-PEO micelles is quantitatively compared to theoretical expectations, finding good agreement for the location of the liquid-solid boundary and excellent agreement for the critical packing fraction where the liquid-to-bcc crystal transition takes place for f < 70. The functionality, i.e., the coronal density, strongly influences the nature of the solid phase: for f < 70 the system crystallizes into a bcc phase, high f>70 formation of amorphous arrested states prevents crystallization.

In order to study the mechanisms limiting the topological chain confinement in polymer melts, we have performed neutron-spin-echo investigations of the single-chain dynamic-structure factor from polyethylene melts over a large range of chain lengths. While at high molecular weight the reptation model is corroborated, a systematic loosening of the confinement with decreasing chain length is found. The dynamic-structure factors are quantitatively described by the effect of contour-length fluctuations on the confining tube, establishing this mechanism on a molecular level in space and time. \ud \ud \ud \ud

The dielectric behavior of polybutadiene copolymers with different compositions of 1,2 and 1,4 units is analyzed to study the influence of the microstructure on the relaxation spectrum, especially the β relaxation. The experimental data were obtained between 10-2 and 106 Hz and temperatures above and below Tg. Above Tg, the relaxation spectra are described by the superposition of two processes, the main α relaxation at low frequencies and a β relaxation at high frequencies. The α relaxation has the typical Vogel-Fulcher-Tammann behavior and exhibits a quasi-linear shift of time scale upon change of the vinyl content. The high-frequency process shows Arrhenius behavior only in the case of the homopolymers. For the other samples with a mixture of microstructures, spectral broadening occurs and deviation from the Arrhenius behavior is observed. In these samples, the relaxation can be modeled by the simple assumption that the high-frequency process consists of a superposition of two separate processes, each with the properties of the β relaxation of one of the homopolymers. This suggests that secondary relaxations can relax quite independently from the segmental relaxation and are therefore restricted to a very local environment influenced by intramolecular interaction.