FY2010 - Oak Ridge National Laboratory

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Director’s R&D Fund— Science for Extreme Environment: Advanced Materials and Interfacial Processes for Energy 05843 Theoretical Studies of Decoupling Phenomena in Dynamics of Soft Materials Alexei Sokolov and Vladimir Novikov Project Description Understanding the dynamics of soft materials is the key to understanding and controlling their unique properties. However, current knowledge of the dynamics in these materials is very limited and many phenomena are not yet understood even on a qualitative level. Among them is a decoupling of various processes from a structural relaxation. It includes (1) decoupling of chain relaxation from segmental relaxation in polymers; (2) decoupling of ionic conductivity from the structural relaxation; and (3) decoupling of protein’s biochemical activity from the solvent’s viscosity. The major goal of the research is to develop a solid theoretical foundation that can address and explain these decoupling phenomena. The work will be done mostly on an analytical level using theoretical models of polymer dynamics and the concept of dynamic heterogeneity in disordered materials. It will help in guiding the experiments and in explaining results accumulated using various techniques, including neutron scattering studies performed at SNS. This fundamental understanding is crucial for the development of new materials for energy applications (such as batteries, fuel cells, organic photovoltaic cells, carbon capture), for bio-related technologies (enzymatic activity, bio-inspired catalysis), and processing of lightweight materials (polymers). Mission Relevance Soft materials (e.g., polymers, colloids, biomaterials) attract the significant attention of researchers due to their potential application in many fields, from energy and lightweight materials to biotechnologies and biomedical applications. Molecular motions play the key role in most of the unique properties of soft materials. However, understanding and controlling the microscopic mechanisms of molecular motions still remain a great challenge. The project is focused on development of a fundamental understanding of decoupling phenomena in the dynamics of soft materials. It has direct connections to DOE missions because it addresses problems important for electrical energy storage (batteries), carbon capture, and fuel cells. Also, explanation of decoupling of segmental and chain relaxations in polymers and its dependence on polymer structure is necessary for a broad variety of applications, from polymer processing to biotechnologies. Results and Accomplishments We investigated the connection of decoupling phenomenon to the dynamical heterogeneity of polymeric, molecular, and inorganic glass formers. To this end we used the inelastic light scattering to analyze the universal feature of the dynamics in various glass-forming materials—the boson peak. This peak in the THz frequency range is associated with a characteristic length scale of the frozen-in dynamical heterogeneities. We estimated the (heterogeneity) length scale obtained from the boson peak spectra. It has been shown in our previous publications that the decoupling correlates with the steepness of the temperature dependence of structural relaxation (the so-called fragility). So, it has been expected that the decoupling might depend on the length scale of the dynamic heterogeneities. However, our analysis revealed that fragility does not correlate directly to the length scale of the dynamic heterogeneities. Only its density (pressure) dependence shows some correlations with this length scale. The presented results call for a revision of traditional view on the role of heterogeneity in structural relaxation of glass-forming systems and, respectively, in the decoupling phenomenon. In addition, our analysis of coherent neutron scattering data (literature data) suggests that the decoupling phenomena appears in the wave-vector 38
Director’s R&D Fund— Science for Extreme Environment: Advanced Materials and Interfacial Processes for Energy dependence of the characteristic relaxation time (Q) of the intermediate scattering function. If true, this opens a direct way for analysis of the microscopic mechanism of the decoupling in dynamics. Information Shared Hong, L., V. N. Novikov, and A. P. Sokolov. 2011. “Is there a connection between fragility of glass forming systems and dynamic heterogeneity/cooperativity?” J. Non-Cryst. Sol. 357, 351–356. 39
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FY2010 - Oak Ridge National Laboratory

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