Structure and dynamics of polymer rings by neutron scattering: breakdown of the Rouse model Ana R. Br as, * a Rossana Pasquino, c Thanasis Koukoulas, ef Georgia Tsolou, ef Olaf Holderer, b Aurel Radulescu, b J€ urgen Allgaier, a Vlasis G. Mavrantzas, ef Wim Pyckhout-Hintzen, a Andreas Wischnewski, a Dimitris Vlassopoulos cd and Dieter Richter a Received 4th July 2011, Accepted 8th September 2011 DOI: 10.1039/c1sm06257c We present a static and quasi-elastic neutron scattering study on both the structure and dynamics of a ring polymer in a ring and linear polymer melt, respectively. In the first case, the ring structure proved to be significantly more compact compared to the linear chain with the same molecular weight. In the mixture, the ring molecules swell as was confirmed by small angle neutron scattering (SANS) in accordance with both theory and simulation work. The dynamical behavior of both systems, which for the first time has been explored by neutron spin echo spectroscopy (NSE), shows a surprisingly fast center of mass diffusion as compared to the linear polymer. These results agree qualitatively with the presented atomistic MD simulations. The fast diffusion turned out to be an explicit violation of the Rouse model. 1 Introduction The application and industrial processing of many soft condensed-matter systems strongly depend on their rheological properties that are determined by the interactions and motions of the constituent structural units such as chain molecules, aggre- gates, colloidal particles, and surfactants. Their understanding is one of the great challenges of basic soft condensed matter science and would facilitate the molecular design of new materials. Among these materials polymers stand out. They display very rich dynamics that is currently described in terms of entropically driven Rouse dynamics and the reptation model involving snakelike motions along a tube formed by the mutually inter- penetrating chains. 1 In terms of this model, a theory of visco- elasticity has been developed that describes the main features of polymer melt rheology. Reptation including its limiting processes, such as fluctuations of the chain ends (contour length fluctuations, CLF), as well as constraint release (CR) where tube building chains move away laterally, always involve the chain ends in a decisive way. Having no ends, ring polymers are topologically different and must lack most of the dynamical processes that govern the dynamics of linear and, in particular, branched polymers and indeed recent rheological results display dramatic differences to those from linear chains. 2–4 In this work we present a new microscopic approach by a combination of SANS and NSE. Before further investigation of large rings, 5 this study focuses on the Rouse regime of rings, smaller than the linear entanglement molecular weight. In fact, all former experimental work on the dynamics of rings concen- trated on macroscopic properties. 4,6–17 Therefore, a microscopic understanding of the underlying dynamical processes, which determine the mechanical and rheological properties, is still missing. NSE allows us to directly access the segmental dynamics of a polymer ring. The present work is the first NSE approach to the relaxation mechanisms in a melt of low molecular weight poly (ethylene oxide) (PEO) rings, sustained by a SANS structural characterization. Very recently pulsed-field-gradient (PFG) nuclear magnetic resonance spectroscopy (NMR) results were published on similar systems focusing on the long range diffu- sion. 16,17 On the contrary, NSE allows investigation of the dynamics on the sample specific length and time scales. Based on NSE spectroscopy in conjunction with SANS, viscosity measurements and MD simulation, we demonstrate a clear violation of the Rouse model with respect to the ring center of mass diffusion. Additionally, it has been found that no internal modes contribute to the dynamics in our NSE window. This is in a J€ ulich Centre for Neutron Science JCNS (JCNS-1) & Institute for Complex Systems (ICS), Forschungszentrum J € ulich GmbH, 52425 J€ ulich, Germany. E-mail: a.bras@fz-juelich.de; Fax: +49-2461-61-2610; Tel: +49-2461-61-5093 b J€ ulich Centre for Neutron Science JCNS, Forschungszentrum J € ulich GmbH, Outstation at FRM II, Lichtenbergstraße 1, 85747 Garching, Germany c FORTH, Institute of Electronic Structure and Laser, Heraklion, Crete, 71110, Greece d University of Crete, Department of Materials Science and Technology, Heraklion, 71003, Greece e University of Patras, Department of Chemical Engineering, Patras, 26504, Greece f FORTH-ICE/HT, Institute of Chemical Engineering and High- Temperature Chemical Processes, Patras, 26504, Greece This journal is ª The Royal Society of Chemistry 2011 Soft Matter , 2011, 7, 11169–11176 | 11169 Dynamic Article Links C < Soft Matter Cite this: Soft Matter , 2011, 7, 11169 www.rsc.org/softmatter PAPER Downloaded by Forschungszentrum Julich Gmbh on 13/05/2013 07:49:17. 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