This award supports theoretical research and education in statistical and dynamical properties of polymers. The work is supported by the Division of Chemistry and the Division of Materials Research. The theory enhances the basic ability to predict macroscopic properties, as measured experimentally or described in computer simulations, based on the microscopic molecular structure and physical parameters. The research uses traditional methods of equilibrium (integral equations) and non-equilibrium statistical mechanics. Guenza and coworkers employ an original approach to describe the cooperative dynamics of a group of interacting macromolecules in dynamically heterogeneous environments, i.e. the Cooperative Dynamics Generalized Langevin Equation (CDGLE). This is a microscopic, site-specific, mean-field theory, which has successfully explained the experimentally observed center-of-mass anomalous diffusion in polymer melts, and the anomalous relaxation of their normal modes, on the basis of intermolecular time-dependent correlation, originated from the dynamically heterogeneous nature of polymer liquids.
The research develops along three main lines or subprojects. The first focuses on testing CDGLE and extending its range of applicability to liquid of polymers with increasing degree of polymerization, i.e. across the unentangled-to-entangled transition. In the second line, CDGLE is modified to describe polymer dynamics in dilute solutions, including polymers of biological significance such as proteins. Intramolecular potentials are derived from molecular dynamics computer simulations. This research will build on past success where the theory has been able to make predictions in good quantitative agreement with experimental data of NMR relaxation, X-ray Debye?Waller temperature factors and NMR order-parameters for the test protein CheY. Further testing against different proteins and other experimental data is undertaken to ensure the generality of the approach. Finally, the third subproject extends the coarse-graining procedure, developed by Guenza and coworkers, which maps polymers into collections of interacting soft-colloidal particles. This procedure provides the effective intermolecular potential entering CDGLE. The approach is extended to include a refined intramolecular coarse-graining and the development of a procedure for multiscale modeling of polymer liquids.
The impact of the project extends beyond the research. The development of this project produces computer codes which predict these polymer properties. Once tested, the codes will be available to the scientific community through a user-friendly website. The project is an opportunity for the PI to continue her efforts to engage women and minorities as she has done in past research projects. Because the work is design to have realistic consequences, there are close collaborations with experimental groups which further enhances the value of the professional experience for the students in the project. Students are also involved in outreach activities to publicize their research.
NONTECHNICAL SUMMARY: This award supports theoretical research and education in statistical and dynamical properties of polymers. The work is supported by the Division of Chemistry and the Division of Materials Research. The theory enhances the basic ability to predict material properties, as measured experimentally or described in computer simulations, based on the molecular structure and physical parameters of constituent polymer molecules. The use of a unified approach to polymer dynamics aids in developing a comprehensive understanding of polymer motion. The latter has been a long-standing goal of both practical and fundamental interest in polymer physics, since all polymeric materials (fibers, plastics, coating materials, etc.) are processed in their liquid state. The goal is to provide a theoretical tool that formally connects the effect of chemical parameters (e.g., polymer type, weight, concentration, and temperature) to the global properties (e.g., ease of flow, hardening temperature). The tools developed will be useful in designing custom-tailored polymeric materials of synthetic or biological significance.
The impact of the project extends beyond the research. The development of this project produces computer codes which predict these polymer properties. Once tested, the codes will be made available to the scientific community through a user-friendly website. The project is an opportunity for the PI to continue her efforts to engage women and minorities as she has done in past research projects. Because the work is design to have realistic consequences, there are close collaborations with experimental groups which further enhances the value of the professional experience for the students in the project. Students are also involved in outreach activities to publicize their research.
The properties of polymeric materials of biological or synthetic origin such as nucleic acids, proteins, block copolymers and polymer mixtures, depend on structure and dynamics occurring on variety of lengthscales and associated timescales. From the microscopic bond length (tenth of nanometers) to the macroscopic properties of phase separation (the size of the container), the important lengthscales cover many orders of magnitude. Similar behavior is observed in the associated timescales, which bridges from the bond vibrations (tenth of picoseconds) to the time of polymer diffusion and viscoelasticity (minutes and longer). Because of the large range of lengthscales and timescales, and given that computer simulations are limited to specific and restricted windows of timescales, it is difficult to develop approaches that can cover the physical properties of interest across many scales. The development of multiscale modeling approaches to treat structure and dynamics of macromolecules across multiple length and timescales have been the essence of the research developed in the Guenza group during this NSF supported grant. The work has produced fifteen papers and one invited review article. Intellectual merit: The Guenza group derived an original approach for the cooperative dynamics of unentangled polymer melts, the Cooperative Dynamics Generalized Langevin Equation (CDGLE). The theory, compared with theoretical and experimental data, successfully describes the interplay between polymer dynamics and dynamical heterogeneities, which are present in unentangled and entangled polymer melts, as they exhibit inter-converting regions of slow and fast dynamics, typical of frustrated systems. The theory predicts correctly subdiffusive dynamics, normal mode relaxation, and agrees quantitatively with measurements of Neutron Spin Echo spectroscopy of polyethylene melts performed by Richter and coworkers. Guenza also proposed an original method, based on integral equation theory, to coarse-grain homopolymer melts, polymer mixtures, and block-copolymers. A multiscale modeling procedure has been designed, which speeds up simulations for homopolymer melts and blends. An analytical solution of the multi-blocks potential showed that the formalism ensures not only structural but also thermodynamic consistency between the atomistic and the coarse-grained descriptions. Theoretical predictions of the structural and thermodynamic properties, i.e. pair distribution functions, directly measured from the mesoscopic simulations of the coarse-grained polymer liquids were found to be in quantitative agreement with united atom simulations. An original approach was proposed to rescale the dynamics of mesoscale simulations of coarse-grained systems, which predict with quantitative accuracy their dynamics, as measured in simulations and experiments. Broader Impact: As discussed above coarse-grained models and multiscale modeling procedures are extremely important as the next step in the development of novel computational formalisms to simulate macromolecular systems across many lengthscales. The model developed by Guenza and coworkers has advantages over competing approaches. It is generally applicable, free from the usual problems of thermodynamic inconsistency and lack of transferability of the potential, which plague numerical coarse-grained models. It is analytically solvable for a class of systems, comprehensive of a large number of experimental conditions. For this reason the potentials developed in the Guenza group are expected to be useful for the community at large. The potential is easily integrated in popular simulation packages. Outreach activities developed during the course of this grant include training of undergraduate, and graduate students, with minority and female students being part of the research group. Guenza has spearheaded with colleagues at the University of Oregon a novel program for improving science literacy of undergraduate non-science majors, and the creation of a 4-credit hour advanced placement chemistry course for high-school students, that she teaches during furlough days. Guenza has participated to the organization of a number of national and international meetings, such as the ACS National Meeting and the 6th and 7th DMRCS.
