Neutron Spin Echo Spectroscopy Research Articles

The dynamics of polymer melts as seen by neutron spin echo spectroscopy

AbstractUsing the neutron spin echo spectroscopy, the internal segmental diffusion of chain molecules in polymer melts and concentrated solutions was studied. These investigations show that beyond a characteristic length dt and after a cross over time τe(dt) the segmental diffusion of the single chains is strongly impeded and deviates from the Rouse dynamics. dt is polymer specific and depends on the temperature as well as on the polymer concentration.Within the framework of the reptation concept, where dt is identified with the mean distance between intermolecular entanglements or with the tube diameter, the microscopically determined dt‐values agree quite well with those derived from related macroscopic measurements of the plateau modulus. A similar good agreement is also found with respect to the segmental friction coefficients obtained either from the Rouse regime of the NSE spectra or from Theological data of corresponding short chain systems, where entanglements are not yet effective.

Polymer Motion at the Crossover from Rouse to Reptation Dynamics

Employing neutron spin-echo spectroscopy, we have studied the dynamic structure factors for the relaxation of a single chain in polymer melts. We have varied the molecular weights through the transition region from unrestricted Rouse dynamics to entanglement controlled behavior. Investigating the dependence of the dynamic structure factor on the momentum transfer Q, it is possible to access the different relaxation modes separately. We found that, depending on their spatial extension in relation to the entanglement distance, larger scale relaxations are successively slowed down compared to Rouse relaxation. A comparison with macroscopic diffusion and viscosity data yields excellent internal consistency

H-bond and glassy state formation. A structural and dynamic study applied to aqueous solutions

Aqueous electrolytic solutions MX·RH 2O, especially with LiCl as solute, are good at forming a glass. At some specific concentrations, compound formation can be bypassed on cooling the system. This occurs when the hydration number is such that the solution has lost the characteristic behaviour of water. Structural relaxation can be analysed easily in the three thermodynamic states: liquid, metastable (supercooled liquid) and out of equilibrium (glass). The local structural relaxation is given by neutron spin echo spectroscopy. The structural evolution can be followed efficiently also using neutron scattering. Indeed using isotopic substitution on nearly all the elements of the material Li, Cl and D we can derive the evolution of the short, medium and long range order in the three thermodynamic states. Correlated to the strong slowing down of the structural relaxation, is the ability of the supercooled and glassy states to construct characteristic H-bonded local configurations.

Neutron spin echo studies on the segmental diffusion behavior in the different chain sections of high molecular weight poly(dimethylsiloxane) melts

AbstractUsing neutron spin echo spectroscopy, the dynamics of high molecular weight poly(dimethylsiloxane) in the melt (T=473 K) was investigated. In order to decide whether the lateral constraints are uniform or nonuniform with respect to the contour of a test chain, chains were labeled either completely or partially by a corresponding proton‐deuterium exchange. It was found that the central and the terminal chain sequences experience the same amount of lateral confinement, which–in the framework of the reptation or tube model–can be characterized by a tube diameter dt of about 70 Å. Nearly the same dt value is obtained from the plateau modulus of rheological measurements, too. Thus, these experiments show unambiguously that the lateral constraints, responsible for the reptational chain dynamics, are quite homogeneous, even in the case of extremely large tube dimensions.

Onset of topological constraints in polymer melts: A mode analysis by neutron spin echo spectroscopy.

By neutron spin echo spectroscopy we have investigated the dynamic structure factors for single chain relaxation in polymer melts varying the molecular weight through the transition from unrestricted Rouse motion to entanglement controlled behavior. From an analysis of the structure factors with respect to the different relaxation modes we found that, depending on their spatial extension in relation to the entanglement length, large-scale relaxations are successively suppressed with increasing molecular weight. A comparison with macroscopic diffusion and viscosity data yields excellent internal consistency.

Collective dynamics of tethered chains: Breathing modes.

Polystyrene (PS)-polyisoprene (PI) diblock copolymers dissolved in n-decane aggregate into spherical micelles with a PS core and a corona of tethered PI chains. The PI corona may be considered as a brush on the surface of a sphere or as the outer part of a many-armed polymer star. The thermal density fluctuations of the PI corona have been observed by neutron spin-echo spectroscopy. The peculiar multi-decay time relaxation behavior can be described remarkably well by a model based on an idea of de Gennes for treating the corona as a semidilute polymer solution with varying concentration profile.

Entanglement constraints in polymer melts. A neutron spin echo study

The melt dynamics of polyisoprene (PI), saturated polybutadiene (PEB-2), and poly(ethylene-propylene) alternating copolymer (PEP) was studied using neutron spin echo spectroscopy. The experiments on all three polymers show that beyond a characteristic length d and after a crossover time τ e the relaxational density fluctuations within a given chain are strongly impeded. The presence of an intermediate dynamic length scale in the chain dynamics establishes the microscopic existence of a well-defined entanglement distance confirming thereby one of the essential assumptions of the reptation concept

Neutron spin echo investigations on molecular motion in polymers

Neutron spin echo spectroscopy allows the observation of long range internal relaxation mechanisms of macromolecules, simultaneously in space and time. Thereby, it facilitates a microscopic study of the molecular origins of the macroscopic viscoelastic properties of polymer materials. The molecular understanding of polymer dynamics bases on concepts of entropic forces and topological and hydrodynamic interactions. I shall briefly outline the different concepts and then discuss experimental results on these different aspects of polymer motion on a molecular scale.

Comparative study of the segmental relaxation in polyisoprene by quasi-elastic neutron scattering and dielectric spectroscopy

We have used neutron spin echo and dielectric spectroscopy to investigate the relaxational properties of polyisoprene. NSE data reveal a relaxation similar to what was found previously in polybutadiene. The comparison to the dielectric relaxation and rheological data exhibits that all time scales shift with the same temperature dependent factor.

Structure and topology of silica aerogels

Neutron spin-echo spectroscopy is used to study the topology of aerogels. Topology or connectivity is varied through precursor chemistry and thermal annealing. Topology is characterized using the concept of fractons (the vibrational excitations of a fractal network). A qualitative difference is observed in the spectrum of polymeric vs. colloidal aerogels, the latter showing a peak in the density of vibrational states. For colloidal aerogels whose structure appears to arise from phase separation in the solution precursor, low-energy excitations were only observed in the lowest density material studied. Finally, a transition from fractal to colloidal microstructure was observed during the sintering of polymeric aerogels. This transformation revealed itself as a transition from a fracton-like to a peaked density of states function.

On the dynamics of dense polymer systems

In this short review we discuss experimental results on molecular motion in polymer melts, obtained by neutron spin echo spectroscopy (NSE). We show that in the short-time regime the assumption of entropic restoring forces (Rouse model) describes perfectly well the space and time dependence of self- and pair-correlation functions of one chain molecule. For longer times or stronger geometrical constraints we observe systematic deviations from the Rouse model revealing the presence of a well-defined intermediate dynamical length scale beyond which density fluctuations within a given chain are strongly reduced. Its value is found to be in excellent agreement with the entanglement distance obtained from rheological measurements. Measurements of the temperature dependence of the entanglement distance, the radius of gyration and the plateau modulus give first insight into the molecular origin of entanglement constraints. The data favour packing models and contradict the topological approach.

Dynamics of weakly connected solids: Silica aerogels.

Apply neutron spin-echo spectroscopy, we investigate low-energy vibrational excitations in a series of silica aerogels of varying connectivity. Connectivity is adjusted both by precursor solution chemistry and by heat treatment. Polymeric aerogels show fractonlike densities of states, whereas colloidal materials exhibit a peak in the \ensuremath{\mu}ev regime. On sintering, the low-frequency excitations disappear. We conclude that neutron spin-echo spectroscopy provides a measure of the topological aspects of disordered materials.

Direct microscopic observation of the entanglement distance in a polymer melt.

Using neutron spin-echo spectroscopy and rheometry, we investigated the melt dynamics of poly(ethylene-propylene) alternating copolymers. The experiments show that beyond a certain length scale the relaxation of density fluctuations within a given chain is strongly reduced. The presence of this intermediate dynamic length scale implies the existence of a well-defined entanglement distance confirming the essential assumption of the reptation concept. Its value is found to be in excellent agreement with the entanglement distance obtained from the rheological measurement.

Neutron Spin Echo Investigations on the Dynamics of Polymers

Neutron spin-echo spectroscopy allows the observation of long-range internal relaxation mechanisms of macromolecules simultaneously in space and time. Thereby, it facilitates a microscopic study of molecular models applied for the explanation of macroscopic viscoelastic properties of polymer materials. After a short outline of the method, this paper discusses experimental results on chain relaxation in dilute solution of linear as well as of branched polymer chains choosing the star polymer as a prototype. Thereafter, chain motion in dense polymer systems under varying topological constraints is addressed. Results on concentrated solutions, melts and chains trapped in networks with different mesh sizes are presented and compared with theoretical models. Finally aspects of the dynamics of polymer networks are discussed and spin-echo results on the motion of labelled cross links are presented.

Dynamics of Star-Branched Polymers in Solution

ABSTRACTWe have studied star-branched polymers in solution under various labelling and contrast matching conditions by small angle neutron scattering and neutron spin echo spectroscopy. We have investigated the relation between the static and dynamic partial structure factors of the variously labeled star polymers. The system of interest is a set of solutions containing 12-arm star branched polyisoprene of arm-molecular weight of 8000, and two polystyrene 12-arm stars, one fully labelled, the other with one arm labelled. The polyisoprene star polymer arms consisted of diblock copolymers, half-deuterated, and half-protonated. Measurements were performed on a center-deuterated star in a contrasted matching deuterated solvent (shell contrast), and on a reverse labelled star with core contrast. We have also studied a solution with the solvent matched the average star scattering length density and solutions containing identical stars with protonated homopolymers. It is found that in the frame-work of the random phase approximations, the initial relaxation rate of the partial dynamic structure factors can be quantitatively accounted for by a function of the eigenmodes of the mobility matrix weighted by an appropriate combinations of the various partial static structure factors.

Polarized neutron scattering methods and studies involving artificial superlattices

The creation of magnetic, thin-film multilayered structures has led to a number of valuable applications in polarized neutron scattering. Perhaps the best known example is the use of supermirrors in neutron spin echo spectroscopy. Supermirrors and novel epitaxially grown, single-crytal multilayer polarizers are described as well as the results of some interesting polarized neutron diffraction studies of the magnetic properties of synthetic superlattices.

A study of single-arm relaxation in a polystyrene star polymer by neutron spin echo spectroscopy

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTA study of single-arm relaxation in a polystyrene star polymer by neutron spin echo spectroscopyD. Richter, B. Farago, J. S. Huang, L. J. Fetters, and B. EwenCite this: Macromolecules 1989, 22, 1, 468–472Publication Date (Print):January 1, 1989Publication History Published online1 May 2002Published inissue 1 January 1989https://doi.org/10.1021/ma00191a085RIGHTS & PERMISSIONSArticle Views211Altmetric-Citations17LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (623 KB) Get e-Alerts Get e-Alerts

Dynamics of Weakly Connected Solids: Sintering of Polymeric Aerogels

ABSTRACTA combination of small angle scattering (SAS) and neutron spin-echo (NSE) spectroscopy is used to characterize the structure and dynamics of polymeric silica aerogels during sintering. The SAS data indicate that densification at short length scales precedes the densification at longer scales (comparable to that of the pore structure). Interpreted within the fracton model, the NSE data are consistent with an initial decrease in connectivity during relatively early stages of densification.

Neutron spin-echo investigations on the dynamics of polymers

Neutron spin-echo spectroscopy allows the observation of long-range internal relaxation mechanisms of macromolecules simultaneously in space and time. Thereby, it facilitates a microscopic study of molecular models applied for the explanation of macroscopic viscoelastic properties of polymer materials. After a short outline of the method, this paper discusses experimental results on chain relaxation in dilute solution of linear as well as of branched polymer chains choosing the star polymer as a prototype. Thereafter, chain motion in dense polymer systems under varying topological constraints is addressed. Results on concentrated solutions, melts and chains trapped in networks with different mesh sizes are presented and compared with theoretical models. Finally aspects of the dynamics of polymer networks are discussed and spin-echo results on the motion of labelled cross links are presented.

Is the magnetic moment of the neutron a dipole or an Amperian current loop?

In classical electrodynamics the two model representations of magnetic moments, viz. the dipole and the Amperian current loop models, give equivalent results. In contrast, for particles with overlapping wave functions, this equivalence breaks down. In the case of the neutron a heated theoretical debate in the 1930’s on the choice of the correct model has been experimentally settled in the 1950’s in favour of the current loop model. The question became of down-to-earth technical relevance in neutron spin echo spectroscopy.