Interplay of Segmental and Global Dynamics in Cross-Linking Networks

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This research is for a systematic study on one important type of complex systems, namely chemically cross-linked polymer networks. Information about the dynamics of these systems will be generated experimentally by broad-band dielectric relaxation spectroscopy and dynamic mechanical spectroscopy, and will be verified against the appropriate models. The proposed research adds a major new dimension to the earlier studies of dynamics in polymer networks in that the systems selected for study dimension to the earlier studies of dynamics in polymer networks in that the systems selected for study exhibit, in addition to the transverse dipole moment component, a persistent cumulative dipole moment along the chain contour that can be relaxed via the normal mode process. This part of the dipole moment of the polymer molecules is present when the repeat unit lacks a plane of symmetry perpendicular to the chain contour. For a sequence of n such units, the dipole vector must correlate with the displacement vector and thence from the studies of the normal processes one can determine not only the relaxation time but also the mean square end-to-end distance. Therefore, the network-forming materials chosen for study will have a unique characteristic in that their molecular architecture will be conducive to the detection of the entire hierarchy of molecular motions, ranging from local to segmental to global. It is anticipated that the controlled variation in the structure and topography will result in the ability to tailor-make networks with desired characteristics whose response can be varied by a selective engagement on the time scales shorter or longer than the characteristic time of each dynamic process. Ultimately, it will be possible to realize the macroscopic properties of the networks by molecular concepts.%%5The proposed research will have implications broader than the aforementioned goals. This is so because the interplay of dynamics of various length scales is crucial to the behavior of many other complex systems, such as self-assemblies and various nanostructures, as well as to the host of bio-related issues that range from gene therapy, protein dynamics and protein folding to the loops and cruciforms that affect the local and global dynamics of DNA's. The integration of research and education will be realized through lectures and laboratory courses for undergraduate and graduate students. The proposed work will also contribute to the enhancement of ethnic diversity by accommodating a number of under-represented minority students from New York City high schools under the aegis of the YES (Youth in Engineering and Science) summer program at Polytechnic.