# The Effect of the End Segments on the Dynamics of a Polymer Melt: The Frequency Nature of the Effect and Possibility of Experimental Observation in the Free Induction Decay of Deuterons

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Abstract

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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 t...

Signature of reptation in the long-time behavior of the deuteron NMR Free Induction Decay in high molecular mass polymer melts

Abstract The classical tube-reptation (TR) model is based on the assumption, that in entangled polymer melts diffusion takes place inside the effective tubes having a conformation of ideal chains and being stable on the time scale of the terminal relaxation time τ t , the latter being strongly molar mass ( M ) dependent. We argue, that this assumption leads for high M to a characteristic, strongly non-exponential time dependence of the deuteron Free Induction Decay, or likewise of the Hahn Echo ...

Abstract We consider the role of local orientational mobility for the NMR relaxation of hyperbranched macromolecules modeled in form of Vicsek fractals. The spectral density J ( ω ) , that is relevant for the NMR relaxation, is studied for labeled segments of flexible and semiflexible Vicsek fractals. We show that the semiflexibility leads to strong effects. For flexible Vicsek fractals the function ω J ( ω ) has a single maximum, whose position is determined by the local-scale modes and therefo...

Self-diffusion studies by intra- and inter-molecular spin-lattice relaxometry using field-cycling: Liquids, plastic crystals, porous media, and polymer segments

Abstract Field-cycling NMR relaxometry is a well-established technique for probing molecular dynamics in a frequency range from typically a few kHz up to several tens of MHz. For the interpretation of relaxometry data, it is quite often assumed that the spin-lattice relaxation process is of an intra-molecular nature so that rotational fluctuations dominate. However, dipolar interactions as the main type of couplings between protons and other dipolar species without quadrupole moments can imply a...

Abstract This review focuses on recent advances in the theory of local orientational mobility in dendrimers that reveals in the NMR relaxation. In particular, we summarize recent results of analytic theory, computer simulations, and NMR relaxation experiments. The analytic theory provides basic means for the analysis of the simulations and experiments by predicting the existence of two dominating processes: overall branch relaxation and local vibrations. On the other hand, the results of simulat...

We study the dynamics of local bond orientation in regular hyperbranched polymers modeled by Vicsek fractals. The local dynamics is investigated through the temporal autocorrelation functions of single bonds and the corresponding relaxation forms of the complex dielectric susceptibility. We show that the dynamic behavior of single segments depends on their remoteness from the periphery rather than on the size of the whole macromolecule. Remarkably, the dynamics of the core segments (which are mo...

Proton spin dynamics in polymer melts: New perspectives for experimental investigations of polymer dynamics

Abstract The proton spin dynamics in polymer melts is determined by intramolecular and intermolecular magnetic dipole-dipole interactions among the proton spins. During many decades it was postulated that the main contribution is a result of intramolecular magnetic dipole-dipole interactions of protons belonging to the same polymer segment. This postulate is far from reality. The relative weights of intra- and intermolecular contributions are time (or frequency) dependent and sensitive to detail...

On the theory of the proton free induction decay and Hahn echo in polymer systems: The role of intermolecular magnetic dipole-dipole interactions and the modified Anderson–Weiss approximation

The influence of the intermolecular magnetic dipole-dipole interaction on the free induction decay (FID) as well as on the Hahn-echo of proton spins in polymer melts is investigated. It is shown that for isotropic models of polymer dynamics, when polymer segment displacements do not correlate with an initial chain conformation, the influence of the intermolecular magnetic dipole-dipole interactions to the FID and Hahn echo is increasing more rapidly with evolution time than the corresponding inf...

Why does the rouse model fairly describe the dynamic characteristics of polymer melts at molecular masses below critical mass

Generalization of the Rouse model without any use of the postulates concerning the Gaussian distribution of the vector connecting the ends of segments is advanced. In the initial (in general, nonlinear) Langevin equations, self-averaging over continuous fragments of a macromolecule naturally defines a linear term for the tagged chain, and this term differs from the entropy term of the classical Rouse model only by the numerical coefficient. According to the inertia-free approximation, the initia...

The universal features of polymer dynamics are specifically represented by laws for (anomalous) segment diffusion and chain relaxation modes. Nuclear magnetic resonance (NMR)-based techniques provide direct access to these phenomena. This in particular refers to NMR relaxation and diffusion studies. Methods suitable for this purpose are described in detail. Three basic classes of polymer dynamics models, namely the Rouse model, the tube/reptation model, and the renormalized Rouse models are outl...