Orientational Autocorrelation Function Research Articles

Simulating Microswimmers Under Confinement With Dissipative Particle (Hydro) Dynamics

In this work we study microwimmers, whether colloids or polymers, embedded in bulk or in confinement. We explicitly consider hydrodynamic interactions and simulate the swimmers via an implementation inspired by the squirmer model. Concerning the surrounding fluid, we employ a Dissipative Particle Dynamics scheme. Differently from the Lattice-Boltzmann technique, on the one side this approach allows us to properly deal not only with hydrodynamics but also with thermal fluctuations. On the other side, this approach enables us to study microwimmers with complex shapes, ranging from spherical colloids to polymers. To start with, we study a simple spherical colloid. We analyze the features of the velocity fields of the surrounding solvent, when the colloid is a pusher, a puller or a neutral swimmer either in bulk or confined in a cylindrical channel. Next, we characterise its dynamical behaviour by computing the mean square displacement and the long time diffusion when the active colloid is in bulk or in a channel (varying its radius) and analyze the orientation autocorrelation function in the latter case. While the three studied squirmer types are characterised by the same bulk diffusion, the cylindrical confinement considerably modulates the diffusion and the orientation autocorrelation function. Finally, we focus our attention on a more complex shape: an active polymer. We first characterise the structural features computing its radius of gyration when in bulk or in cylindrical confinement, and compare to known results obtained without hydrodynamics. Next, we characterise the dynamical behaviour of the active polymer by computing its mean square displacement and the long time diffusion. On the one hand, both diffusion and radius of gyration decrease due to the hydrodynamic interaction when the system is in bulk. On the other hand, the effect of confinement is to decrease the radius of gyration, disturbing the motion of the polymer and thus reducing its diffusion.

Structure and rotational dynamics of water around hydrogen peroxide

The density functional theory-based molecular dynamics simulations were induced to study various properties of water around different solvation pockets around hydrogen peroxide(H2O2). Our study aims to understand water behavior in the vicinity of H2O2 molecule through analyzing structure, hydrogen bond statistics, orientational and non-diffusive jump profile. A water-like peroxide molecule can form four hydrogen bonds with nearby water molecules: two as a proton donor and two as a proton acceptor, which must disturb the local structure and dynamics of the surrounding water. The presence of two different peak maxima in radial distribution function (RDF) allows us to resolve the solvation environment into three solvation sites: solvation shell I, solvation shell II, and bulk. The probability distribution of the average distance of central water molecule from 4 nearest water molecules shows a shift in the peak for solvation shell I water molecules towards a ∼0.3Å higher value compared to two other solvation shells. This further affects tetrahedral order parameters as water molecules shift from the regular tetrahedron. For the first solvation layer, the water molecules lose their tetrahedrality to a significant extent as qtet probability distribution peaks at 0.5 and broaden in the higher values of qtet without any shoulder. For the second solvation layer, we find the qtet peak at 0.87 with a minor shoulder appearing at qtet = 0.55, a sign of slight loss of tetrahedrality in the unstructured water compared to undisturbed water. Loss of tetrahedrality hints at the change of orientation of H2O molecules in the vicinity of H2O2 and affects the water-water hydrogen bond (HB) formation. A noteworthy reduction in the HB donor/acceptor account and per water HB count shows interference of H2O2. The water molecules prefer H2O2 compared to neighboring water molecules for HB formation. The orientation autocorrelation function provides the rotational pace of water molecules in the different solvation layers. The reorientation time constant difference in the solvation shells I and II fluctuate by mere 0.20 ps, showcasing the trivial nature of hydrogen peroxide and water HBs. Using the HB descriptors, we also report the non-diffusive jump profile involving vicinal water molecules of H2O2. The jump angle profile of solvation shell water molecules is in line with bulk water.

Chain dynamics in partially cross‐linked polyethylene by combined rheology and NMR‐based molecular rheology

AbstractIn recent years, 1H double‐quantum NMR (DQ NMR) was established as a suitable molecular‐rheology technique to elucidate chain dynamics and to determine entanglement or crosslink densities in linear entangled polymer melts and permanent as well as transient networks. In this work, industrial grade high‐density polyethylene, partially cross‐linked via electron beam irradiation in the semicrystalline state, is probed in the melt state by low‐field DQ NMR and shear rheology. The DQ NMR data is analyzed by two approaches, one assuming the presence of a permanent network and the other considering the potentially complex relaxation spectrum of the studied inhomogeneous systems. A correlation between the DQ NMR results and extent of cross‐linking is found. By direct comparison of the rheological results and the NMR‐based segmental orientation autocorrelation functions (OACF) via time–temperature superposition (TTS), qualitative consistency between the microscopic and macroscopic observables is established. In this way, the frequency range of shear rheology can be extended by about two decades into the 10 krad/s range. The NMR method is thus a valuable extension of the toolbox of characterization techniques, where gel content measurements by solvent extraction proved to be the least sensitive.

Dynamics of Ionic Liquid through Intrinsic Vibrational Probes Using the Dispersion-Corrected DFT Functionals.

First principles molecular dynamics simulations have been utilized to study the spectral properties of the protic ionic liquid, methylammonium formate (MAF). All simulations were performed using density functional theory (DFT) and various van der Waals-corrected exchange-correlation functionals. We calculated the vibrational stretch frequency distributions, determined the time-frequency correlations of the intrinsic vibrational probes, the N-H and C-O modes in MAF, and the frequency-structure correlations. We also estimated the average hydrogen-bond lifetimes and orientation dynamics to capture the ultrafast spectral response. The spectroscopic signature of the N-H stretching vibrations using the Becke-Lee-Yang-Parr (BLYP) and Perdew-Burke-Ernzerhof (PBE) functionals displays a spectral shift in the lower frequency side, suggesting stronger hydrogen-bonding interactions represented by the gradient approximation functionals than the van der Waals (vdW)-corrected simulations. The carboxylate frequency profiles with the dispersion-corrected representations are almost similar without a significant difference in the normalized distributions. Besides, the COO stretching frequencies at the peak maxima positions of the PBE functionals exhibit a lesser deviation from the experimental data. Spectral diffusion dynamics of the intrinsic vibrational probes on the cationic and anionic sites of the ionic liquid proceed through a short time relaxation of the intact hydrogen bonds followed by an intermediate time constant and a longer time decay indicating the switchover of hydrogen bonds. Dispersion-corrected atom-centered one-electron potential (DCACP) correction added to the BLYP system slows down the picosecond time scales of frequency correlation and the time constants of rotational motion, lengthening the overall system dynamics. The observed trends in the time-dependent decays of frequency fluctuations and the orientation autocorrelation functions correlate with the structural interactions in liquid MAF and hydrogen-bond dynamics. In this study, we examine the predictions made by different density functional treatments comparing the results of the uncorrected BLYP and PBE representations with the semiempirical vdW methods of Grimme and matching our calculated data with the experimental observations.

Glass-Transition Temperature of Cyclic Polystyrene: A Computational Study

Molecular-dynamics simulations are employed to study the glass transition of cyclic polystyrene melts. Gibbs and DiMarzio’s theory predict an increase in glass transition temperature, Tg upon lowering the length of cyclic polymer chains, which is opposite to the well-known trend for linear polymers. Their theory has been confirmed by some experiments; however, others observe a decrease in Tg upon lowering the chain length instead. When volumetric methods are employed to obtain the glass transition temperature in simulated cyclic polystyrene, a slight increase with decreasing cyclic polystyrene chain length is obtained. This increase is more pronounced when glass transition temperatures are obtained from dynamics. Both the glass transition temperature Tg obtained from diffusion data and the ideal glass transition temperature T0 obtained from the decay of the orientational autocorrelation function of the phenyl bond show a clear upturn.

Time-dependent inertia of self-propelled particles: The Langevin rocket.

Many self-propelled objects are large enough to exhibit inertial effects but still suffer from environmental fluctuations. The corresponding basic equations of motion are governed by active Langevin dynamics, which involve inertia, friction, and stochastic noise for both the translational and orientational degrees of freedom coupled via the self-propulsion along the particle orientation. In this paper, we generalize the active Langevin model to time-dependent parameters and explicitly discuss the effect of time-dependent inertia for achiral and chiral particles. Realizations of this situation are manifold, ranging from minirockets (which are self-propelled by burning their own mass), to dust particles in plasma (which lose mass by evaporating material), to walkers with expiring activity. Here we present analytical solutions for several dynamical correlation functions, such as mean-square displacement and orientational and velocity autocorrelation functions. If the parameters exhibit a slow power law in time, we obtain anomalous superdiffusion with a nontrivial dynamical exponent. Finally, we constitute the "Langevin rocket" model by including orientational fluctuations in the traditional Tsiolkovsky rocket equation. We calculate the mean reach of the Langevin rocket and discuss different mass ejection strategies to maximize it. Our results can be tested in experiments on macroscopic robotic or living particles or in self-propelled mesoscopic objects moving in media of low viscosity, such as complex plasma.

Diffusion of nano-rods

Diffusion behaviors in nano-rods suspension in the fluid are investigated by Stochastic Eulerian Lagrangian Methods, where the degrees of freedom for coarse-grained microstructure are coupled with continuum stochastic field to capture both the relaxation of hydrodynamic modes and thermal fluctuations. The diffusion characteristics of a single rod and multi-rods are examined with hydrodynamic interaction in three dimensions. For a single rod, the mean square displacement is verified by comparing with prior experimental results. The effects of hydrodynamic interactions on the translational and rotational diffusivity with different aspect ratios are represented by velocity auto-correlation function and the orientational auto-correlation functions. For multi-rods cases, we obtained their diffusion in the ballistic and the short-time regime. The comparison between the SELM and the Langevin method confirms the hydrodynamic effect.

Chain Heterogeneity in Simulated Polymer Melts: Segment Orientational Autocorrelation Function

The dynamic heterogeneity of polymer chains is characterized by the lower correlation times of its end Kuhn segments with respect to those of the middle segments. In this study, the heterogeneity was investigated using molecular dynamics simulation of freely jointed chains of various lengths in the melt. Our characterization of the segmental dynamics along the chain additionally revealed the heterogeneity of rotational anisotropy, which was also lower for chain-end segments compared to those for chain-middle ones. This heterogeneity can be quantitatively characterized by intra-segmental residual dipolar coupling, which influences the transverse magnetic relaxation rate of the various chain segments and, ultimately, defines the free induction decay shape for chains of any length. Further, in the second part of this report, the origin of this relationship is discussed in detail.

Molecular Dynamics and Spin-Lattice NMR Relaxation in $$\alpha$$- and $$\varepsilon$$-Polylysine

The NMR relaxation method is widely used in various biomedical applications. Investigation of model homopeptides is an important step for understanding of structure and NMR properties of more complex branched peptides based on lysine monomers, for example, dendrimers, dendrigrafts and dendritic polymer brushes. In this paper, we perform molecular dynamics simulation of two linear lysine peptides with the same number of lysine monomers but with different connection between them through $$\alpha$$ - or $$\varepsilon$$ -peptide bonds. We obtained that the end-to-end distance and radius of gyration are smaller and radial density near the center of mass is essentially larger and decrease faster with radial distance for $$\alpha$$ -lysine peptide than for $$\varepsilon$$ -lysine peptide. Orientational mobility of $$\hbox {CH}_2$$ groups in both peptides could be described by second order orientational autocorrelation function and by spin-lattice NMR relaxation time. We calculated both functions and found that the relaxation of vector in side chains of $$\alpha$$ -lysine peptide is slightly faster in comparison with mobility of $$\hbox {CH}_2$$ groups in main chain of $$\varepsilon$$ -lysine peptide. Thus the big difference between the relaxation rates of these two types of $$\hbox {CH}_2$$ groups in lysine dendrimers obtained recently both in NMR and in simulation is mainly due to dendrimer effect and not due to difference in position (side or main chain) of $$\hbox {CH}_2$$ group in short linear lysine fragments. This result allows to use NMR for discrimination between $$\alpha$$ -lysine and $$\varepsilon$$ -lysine peptides as well as between linear lysine peptides (or their mixtures) and lysine dendrimers.

Scaling law of Brownian rotation in dense hard-rod suspensions.

Self-diffusion in dense rod suspensions are subject to strong geometric constraints because of steric interactions. This topological effect is essentially anisotropic when rods are nematically aligned with their neighbors, raising considerable challenges in understanding and analyzing their impacts on the bulk physical properties. Via a classical Doi-Onsager kinetic model with the Maier-Saupe potential, we characterize the long-time rotational Brownian diffusivity for dense suspensions of hard rods of finite aspect ratios, based on quadratic orientation autocorrelation functions. Furthermore, we show that the computed nonmonotonic scalings of the diffusivity as a function of volume fraction can be accurately predicted by an alternative tube model in the nematic phase.

Cavity molecular dynamics simulations of liquid water under vibrational ultrastrong coupling.

We simulate vibrational strong coupling (VSC) and vibrational ultrastrong coupling (V-USC) for liquid water with classical molecular dynamics simulations. When the cavity modes are resonantly coupled to the O-H stretch mode of liquid water, the infrared spectrum shows asymmetric Rabi splitting. The lower polariton (LP) may be suppressed or enhanced relative to the upper polariton (UP) depending on the frequency of the cavity mode. Moreover, although the static properties and the translational diffusion of water are not changed under VSC or V-USC, we do find the modification of the orientational autocorrelation function of H2O molecules especially under V-USC, which could play a role in ground-state chemistry.

Dynamics and Spectral Response of Water Molecules around Tetramethylammonium Cation.

Till now, there has been ambiguity about the structural heterogeneity inside a solute solvation shell and the dynamical response of the surrounding solvent molecules. To address the dynamics and spectral response of solvent molecules, we performed first-principles molecular dynamics simulations for the comprehensive study of water's hydroxyl stretch frequency evolution due to environmental variations (also called "spectral diffusion") in the vicinity of a hydrophobe, tetramethylammonium (TMA) cation. The N-Ow radial distribution function (RDF), spatial distribution function (SDF), and combined distribution function (CDF) were calculated to provide information about the arrangement of water molecules around TMA. In the probability distribution plot of the cosine of the angle (θ) between Ow-Hw and NTMA-Ow bond vectors, the hydrogen atoms are observed being oriented toward TMA and the oxygen atoms aligned away. The decaying dynamics of the orientation autocorrelation function (OACF) reveals the reorientation time is more inside the solvation shell (4.1 ps) as compared to bulk (2.8 ps), matching with the trend obtained from water's orientational dynamics in tetraalkylammonium salts. Wavelet transform of the obtained trajectory was used to calculate the time-dependent vibrational stretching frequencies of the OH modes of water molecules. The normalized frequency distribution in the aqueous solvation shell of TMA, tagging a particular water molecule within the N-Ow cutoff distance 5.5 Å, displays an intense peak at 3661 cm-1 representing non-hydrogen bonded or dangling or free OH modes. Simulations around aromatic solutes and Raman-MCR studies in the hydration shell of hydrophobic TBA reported a distinctive dangling OH band at 3660 cm-1 (range: 3661 ± 2 cm-1). Besides dangling water molecules in the first hydration layer of ammonium nitrogen, few OH modes are strongly hydrogen-bonded having an average frequency at 3300 cm-1. The predominance of dangling hydroxyl modes around apolar hydrophobic TMA was further explored by comparing the dangling lifetime (∼0.68 ps) with the lifespan of hydrogen bonded OH modes (∼0.48 ps). At our simulation temperature 330 K, a significant fraction of the water molecules in the vicinity of TMA ion are free or dangling, and a few of them form an ordered structure with enhanced hydrogen bonding. Structural analysis, orientation correlation, frequency fluctuations, dangling, and hole dynamics calculations provide the evidence of the existence of dangling OH modes dominating over highly ordered strong hydrogen bonded structure in the cationic TMA solvation shell at an elevated temperature.

Time-Domain NMR Observation of Entangled Polymer Dynamics: Focus on All Tube-Model Regimes, Chain Center, and Matrix Effects

Proton multiple-quantum time-domain NMR combined with time–temperature superposition is a powerful method to study entangled chain dynamics. Overcoming the previous limitation to regimes II–IV of the tube model, this study extends the method to regime I (localized Rouse motions) by use of a pulse sequence adapted to shorter times, thus covering all relevant regimes for the model case of poly(butadiene) with molecular weights (M) between 10 and 200 kDa. We determine a value for the entanglement time that is consistent with current rheological results and confirm a value below 1 for the time scaling exponent of the segmental orientation autocorrelation function (OACF) in regime I previously observed by other NMR techniques. The origins of deviations from tube-model predictions are assessed by focusing on the dynamics of the chain centers in end-chain deuterated triblock samples and by dilution of probe chains to 15% in a deuterated matrix with M of 2 MDa. The study is complemented by self-diffusion coeffici...

Microscopic observation of the segmental orientation autocorrelation function for entangled and constrained polymer chains

Previous work on probing the dynamics of reptating polymer chains in terms of the segmental orientation autocorrelation function (OACF) by multiple-quantum (MQ) NMR relied on the time-temperature superposition (TTS) principle as applied to normalized double-quantum (DQ) build-up curves. Alternatively, an initial-rise analysis of the latter is also possible. These approaches are subject to uncertainties related to the relevant segmental shift factor or parasitic signals and inhomogeneities distorting the build-up at short times, respectively. Here, we present a simple analytical fitting approach based upon a power-law model of the OACF, by the way of which an effective power-law time scaling exponent and the amplitude of the OACF can be estimated from MQ NMR data at any given temperature. This obviates the use of TTS and provides a robust and independent probe of the shape of the OACF. The approach is validated by application to polymer melts of variable molecular weight as well as elastomers. We anticipate a wide range of applications, including the study of physical networks with labile junctions.

Orientational Mobility in Dendrimer Melts: Molecular Dynamics Simulations

We have simulated the melt of poly(carbosilane) dendrimers using atomistic models and have reproduced the effect predicted by the analytical theory; i.e., orientational autocorrelation functions of a segment from the same layer (numbered from periphery) are practically identical and do not depend on dendrimer size. The frequency dependences of the dielectric and NMR relaxation were obtained and studied in detail. The main contribution to the maxima of these dependences is given by the pulsation process. It leads to a shift of the maxima to low frequencies for the core segment in comparison with the maxima for peripheral segments. The contribution of local reorientation can also be significant, and in some cases this contribution manifests as an additional maximum. The nontrivial scaling laws in the frequency dependences of dielectric permittivity and NMR relaxation rate averaged over all layers of a dendrimer macromolecule are found. A similar scaling law is observed in the experiments on NMR relaxation b...

Molecular simulations and density functional theory calculations of bromine in clathrate hydrate phases.

Bromine forms a tetragonal clathrate hydrate structure (TS-I) very rarely observed in clathrate hydrates of other guest substances. The detailed structure, energetics, and dynamics of Br2 and Cl2 in TS-I and cubic structure I (CS-I) clathrate hydrates are studied in this work using molecular dynamics and quantum chemical calculations. X-ray diffraction studies show that the halogen-water-oxygen distances in the cages of these structures are shorter than the sum of the van der Waals radii of halogen and oxygen atoms. This suggests that the stabilizing effects of halogen bonding or other non-covalent interactions (NCIs) may contribute to the formation of the unique tetragonal bromine hydrate structure. We performed molecular dynamics simulations of Br2 and Cl2 clathrate hydrates using our previously developed five-site charge models for the dihalogen molecules [Dureckova et al. Can. J. Chem. 93, 864 (2015)] which reproduce the computed electrostatic potentials of the dihalogens and account for the electropositive σ-hole of the halogen bond donor (the dihalogen). Analysis of the radial distribution functions, enthalpies of encapsulation, velocity and orientation autocorrelation functions, and polar angle distributions are carried out for Br2 and Cl2 guests in various cages to contrast the properties of these guests in the TS-I and CS-I phases. Quantum chemical partial geometry optimizations of Br2 and Cl2 guests in the hydrate cages using the M06-2X functional give short halogen-water distances compatible with values observed in X-ray diffraction experiments. NCI plots of guest-cage structures are generated to qualitatively show the relative strength of the non-bonding interactions between dihalogens and water molecules. The differences between behaviors of Br2 and Cl2 guests in the hydrate cages may explain why bromine forms the unique TS-I phase.

Multiscale simulations of PS-SiO2 nanocomposites: from melt to glassy state.

The interaction energetics, molecular packing, entanglement network properties, segmental dynamics, and elastic constants of atactic polystyrene-amorphous silica nanocomposites in the molten and the glassy state are studied via molecular simulations using two interconnected levels of representation: (a) a coarse-grained one, wherein each polystyrene repeat unit is mapped onto a single "superatom" and the silica nanoparticle is viewed as a solid sphere. Equilibration at all length scales at this level is achieved via connectivity-altering Monte Carlo simulations. (b) A united-atom (UA) level, wherein the polymer chains are represented in terms of a united-atom forcefield and the silica nanoparticle is represented in terms of a simplified, fully atomistic model. Initial configurations for UA molecular dynamics (MD) simulations are obtained by reverse mapping well-equilibrated coarse-grained configurations. By analysing microcanonical UA MD trajectories, the polymer density profile is studied and the polymer is found to exhibit layering in the vicinity of the nanoparticle surface. An estimate of the enthalpy of mixing between polymer and nanoparticles, derived from the UA simulations, compares favourably against available experimental values. The dynamical behaviour of polystyrene (in neat and filled melt systems) is characterized in terms of bond orientation and dihedral angle time autocorrelation functions. At low concentration in the molten polymer matrix, silica nanoparticles are found to cause a slight deceleration of the segmental dynamics close to their surface compared to the bulk polymer. Well-equilibrated coarse-grained long-chain configurations are reduced to entanglement networks via topological analysis with the CReTA algorithm, yielding a slightly lower density of entanglements in the filled than in the neat systems. UA melt configurations are glassified by MD cooling. The elastic moduli of the resulting glassy nanocomposites are computed through an analysis of strain fluctuations in the undeformed state and through explicit mechanical deformation by MD, showing a stiffening of the polymer in the presence of nanoparticles. UA simulation results for the elastic constants are compared to continuum micromechanical calculations invoked in homogenization models of the overall mechanical behaviour of heterogeneous materials. They can be interpreted in terms of the presence of an "interphase" of approximate thickness 2 nm around the nanoparticles, with elastic constants intermediate between those of the filler and the matrix.

Equivalence of Brownian dynamics and dynamic Monte Carlo simulations in multicomponent colloidal suspensions.

We propose a simple but powerful theoretical framework to quantitatively compare Brownian dynamics (BD) and dynamic Monte Carlo (DMC) simulations of multicomponent colloidal suspensions. By extending our previous study focusing on monodisperse systems of rodlike colloids, here we generalize the formalism described there to multicomponent colloidal mixtures and validate it by investigating the dynamics in isotropic and liquid crystalline phases containing spherical and rodlike particles. In order to investigate the dynamics of multicomponent colloidal systems by DMC simulations, it is key to determine the elementary time step of each species and establish a unique timescale. This is crucial to consistently study the dynamics of colloidal particles with different geometry. By analyzing the mean-square displacement, the orientation autocorrelation functions, and the self part of the van Hove correlation functions, we show that DMC simulation is a very convenient and reliable technique to describe the stochastic dynamics of any multicomponent colloidal system. Our theoretical formalism can be easily extended to any colloidal system containing size and/or shape polydisperse particles.

NMR relaxation of the orientation of single segments in semiflexible dendrimers.

We study the orientational properties of labeled segments in semiflexible dendrimers making use of the viscoelastic approach of Dolgushev and Blumen [J. Chem. Phys. 131, 044905 (2009)]. We focus on the segmental orientational autocorrelation functions (ACFs), which are fundamental for the frequency-dependent spin-lattice relaxation times T1(ω). We show that semiflexibility leads to an increase of the contribution of large-scale motions to the ACF. This fact influences the position of the maxima of the [1/T1]-functions. Thus, going from outer to inner segments, the maxima shift to lower frequencies. Remarkably, this feature is not obtained in the classical bead-spring model of flexible dendrimers, although many experiments on dendrimers manifest such a behavior.

NMR Observations of Entangled Polymer Dynamics: Focus on Tagged Chain Rotational Dynamics and Confirmation from a Simulation Model

Molecular-level insights into the entangled dynamics of high-molecular-weight chains, in particular of slower chain modes in regimes II–IV of the tube model, are still rare due to the lack of methods resolving the rather long associated time scales. On the theoretical side, new computer simulation methods are just reaching the relevant time scales in sufficiently large systems. Here, we confront results from a recent multiple-quantum proton NMR method with results from a novel lattice model. We address the concern that proton NMR, relying on the dipole–dipole couplings between nearby nuclei, is intrinsically sensitive not only to intrachain rotational motions which reflect the desired details of the tube model or possibly necessary modifications, but also to the translational diffusion of chains past each other via interchain dipole–dipole couplings. In order to critically assess the influence of the latter, we here present results of isotope-dilution experiments, in which the data reflect mainly tagged-chain dynamics. We find overall weak effects of interchain dipole–dipole couplings on the shape of the extracted orientation autocorrelation function and very good agreement of the experimental and the computer simulation data. We conclude that the NMR method as well as the novel lattice model faithfully reflects the universal features of entangled chain dynamics and in particular the deviations from simple tube-model predictions on a microscopic level.