Motional broadening of the quasi-elastic peak in neutrons scattered from polymeric materials

Allen, G. C.; Brier, P.N.; Goodyear, Grant; Higgins, J. S.

doi:10.1039/fs9720600169

Cited by 22 publications

(18 citation statements)

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“…Our experiments and calculations based on siloxanes show changes with topology that are not only in agreement with previous QENS data 54 but also mimic the trend identified by both bulk viscosity measurements [10][11][12] as well as self-diffusion and spin-spin relaxation measurements 15 according to which rings have slower dynamics (higher viscosities or smaller diffusion coefficients) compared to linear chains at low molar mass but show opposite behaviour in the high molar mass range, above the entanglement molecular weight, M e . Similar observations have been made by Ozisik et al 75 and Hur et al 25,26 in their computer simulations of cyclic and linear polyethylenes.…”

Section: Temperature Dependence Of Segmental Motion For Linear and Cysupporting

confidence: 92%

“…Following previous studies, including our own work [53][54][55][56] , here we present a systematic investigation of the influence of topology on local chain motion by comparing QENS data of linear and cyclic PDMS. The local dynamics of PDMS has been investigated extensively by neutron scattering [55][56][57] and other experimental techniques such as dielectric spectroscopy [58][59][60][61] as well as simulations 62 .…”

Section: Introductionmentioning

confidence: 99%

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Effect of Chain Length and Topological Constraints on Segmental Relaxation in Cyclic PDMS

et al. 2018

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We present a detailed investigation of local dynamics of linear and cyclic poly(dimethylsiloxane) (PDMS) covering a wide range of molar masses. To aid interpretation of the experimental data, QENS measurements in the time scale from 2 to 200 ps and at Q = 0.3 to 1.8 Å–1 are complemented by theoretical calculations. These make use of a methodology developed by us elsewhere applicable to both simple chain models and real chains and applied here, for the first time, to cyclic PDMS. Analysis of the incoherent dynamic structure factor at T < T m shows that the rotational motion of the methyl groups is unaffected by polymer topology. At higher temperatures, the QENS data are described by a model that consists of two dynamic contributions: methyl group rotation and segmental motion, the latter described by a stretched exponential function. Relaxation times of both linear and cyclic PDMS increase with increasing molar mass. Several features predicted by theory are also reproduced by the experimental data. We show, unambiguously, that rings have higher relaxation times for the segmental motion compared to linear chains of the same number of monomer units. Theoretical calculations support the idea that such slowing down of local dynamics is due to the topological constraint imposed by the ring closure, a constraint which becomes negligible for very large molar masses. Our calculations suggest that due to its albeit small conformational rigidity, cyclic PDMS undergoes an additional constraint which further increases the relaxation time, producing a shallow maximum for N ≈ 50 repeat units. A similar feature is also observed in the experimental QENS data. Values of activation energy, E a, are derived from analysis of the temperature dependence of the quasi-elastic broadening and are found to be in agreement with viscosity measurements reported in the literature. Although the pronounced molar mass dependence of E a for linear PDMS is certainly linked to the presence of mobile chain ends, for the cyclic polymers the behavior appears to be more complex than anticipated.

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Section: Temperature Dependence Of Segmental Motion For Linear and Cysupporting

confidence: 92%

Section: Introductionmentioning

confidence: 99%

Effect of Chain Length and Topological Constraints on Segmental Relaxation in Cyclic PDMS

et al. 2018

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“…To date, neutron scattering investigations of ring dynamics are very limited [ 44 , 54 , 88 , 104 , 105 ]. In the following sections, NSE and QENS measurements will be reviewed.…”

Section: Dynamics Of Cyclic Polymersmentioning

confidence: 99%

“…QENS has been extensively used over many years to study local dynamics in polymers. However, there is little in the literature on the comparison between cyclics and linear chains, except for QENS measurements on PDMS [ 104 , 105 , 113 ]. The local dynamics of linear PDMS has been investigated extensively by neutron scattering [ 114 , 115 , 116 ], dielectric spectroscopy [ 117 , 118 , 119 , 120 ], NMR [ 17 ], as well as simulations [ 121 ], and these studies provide a starting point for a detailed comparison with ring dynamics.…”

Section: Dynamics Of Cyclic Polymersmentioning

confidence: 99%

Local Effects of Ring Topology Observed in Polymer Conformation and Dynamics by Neutron Scattering—A Review

Arrighi

Higgins

2020

Polymers

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The physical properties of polymers depend on a range of both structural and chemical parameters, and in particular, on molecular topology. Apparently simple changes such as joining chains at a point to form stars or simply joining the two ends to form a ring can profoundly alter molecular conformation and dynamics, and hence properties. Cyclic polymers, as they do not have free ends, represent the simplest model system where reptation is completely suppressed. As a consequence, there exists a considerable literature and several reviews focused on high molecular weight cyclics where long range dynamics described by the reptation model comes into play. However, this is only one area of interest. Consideration of the conformation and dynamics of rings and chains, and of their mixtures, over molecular weights ranging from tens of repeat units up to and beyond the onset of entanglements and in both solution and melts has provided a rich literature for theory and simulation. Experimental work, particularly neutron scattering, has been limited by the difficulty of synthesizing well-characterized ring samples, and deuterated analogues. Here in the context of the broader literature we review investigations of local conformation and dynamics of linear and cyclic polymers, concentrating on poly(dimethyl siloxane) (PDMS) and covering a wide range of generally less high molar masses. Experimental data from small angle neutron scattering (SANS) and quasi-elastic neutron scattering (QENS), including Neutron Spin Echo (NSE), are compared to theory and computational predictions.

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“…As I subsequently realized, this was because I was observing the polymer molecules moving over only very short distances, and in this spatial range, the motion is like a liquid. Effectively, the neutron was picking up the high-frequency wriggling of small segments of the molecule (7). I spent several years following up this observation, as I describe later, but I think it is time to consider the neutron scattering technique in a little more detail.…”

Section: Introductionmentioning

confidence: 99%

Neutron Scattering from Polymers: Five Decades of Developing Possibilities

Higgins

2016

Annu. Rev. Chem. Biomol. Eng.

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The first three decades of my research career closely map the development of neutron scattering techniques for the study of molecular behavior. At the same time, the theoretical understanding of organization and motion of polymer molecules, especially in the bulk state, was developing rapidly and providing many predictions crying out for experimental verification. Neutron scattering is an ideal technique for providing the necessary evidence. This autobiographical essay describes the applications by my research group and other collaborators of increasingly sophisticated neutron scattering techniques to observe and understand molecular behavior in polymeric materials. It has been a stimulating and rewarding journey.

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Motional broadening of the quasi-elastic peak in neutrons scattered from polymeric materials

Cited by 22 publications

References 0 publications

Effect of Chain Length and Topological Constraints on Segmental Relaxation in Cyclic PDMS

Effect of Chain Length and Topological Constraints on Segmental Relaxation in Cyclic PDMS

Local Effects of Ring Topology Observed in Polymer Conformation and Dynamics by Neutron Scattering—A Review

Neutron Scattering from Polymers: Five Decades of Developing Possibilities

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