This award supports theoretical and computational research and education on reversible polymer networks. These polymer networks show intriguing nonlinear behavior when subjected to shear or stress. For instance strong shear thinning has been observed when a uniform stress is applied to a model telechelic polymer. The system yields and forms two or more shear bands. In addition, it is found that rheochaotical fluctuations in the stress profile exhibit self-organized critical behavior. Under mechanical stress, slow aging, creep, dramatic failure, and rupture are observed, dependent on the stress level. After cessation of stress, the system slowly heals. These polymer systems consist of hydrophilic polymers with hydrophobic end-groups. In aqueous solution they form network structures since the hydrophobic ends self-assemble into micelles. It is believed that stress induces a novel microscopic network structure that might have spatial as well as temporal organization. Using a novel Molecular Dynamics/Monte Carlo hybrid model, the PI will study those microscopic topological changes and determine how they connect to the macroscopic nonlinear response. The topology of the network is characterized by the properties of its connectivity matrix, for example spectral density or degree distribution. In reversible networks micelles break apart and new ones form over time. The transition rates for these processes will be obtained from simulations and employed to construct a master equation that describes the network kinetics and can be used for further theoretical investigation. The results will be compared with experiments.
The PI will also perform simulations and compare the simulation data to experiments on physically associating triblock copolymers that show rheological behavior very similar to that observed in biopolymer networks, for example a cross-linked actin network in a human cell or a cellulose network in a plant cell. In particular, all exhibit strain stiffening at relative low values of strain. Biological systems perform their function under applied mechanical stress. The PI will investigate the role of changes in network topology and dynamics.
This project will provide valuable training opportunities for graduate and undergraduate students. It is integrated in an interdisciplinary educational program in math/physics and biology at San Diego State University. As part of this program the P.I. teaches a course on synthetic and biopolymers, in which results of her research group are discussed. San Diego State University enrolls a large number of minority students, many of whom take this course, participate in this program, or are part of the PI's research group.
NONTECHNICAL SUMMARY This award supports theoretical and computational research on long chain-like molecules, polymers, that can form junctions between their ends. The ends of the polymers prefer not to be in contact with water. In water, these molecules form network structures. Experiments show interesting behavior when these networks are subjected to mechanical stress. For instance, when slowly deformed the system suddenly yields. At higher deformations bands appear in which the molecules move at different speeds. After stopping the deformation, the system remembers it for a long time, but eventually slowly returns back to its original state. It is believed that stress induces a novel microscopic network structure. The PI will use simulation to study this behavior and to characterize the way the molecules are organized in the network. The research will advance understanding of experiments.
The PI will also study polymers that behave similarly to networks of polymers that occur in living systems like the scaffolding in animal and plant cells. Biological systems perform their function under applied mechanical stress and the PI will investigate the role of changes in the way the polymers are organized in the network.
Polymer networks in which bonds between molecules constantly break and recombine abound in nature. Examples are the cellulose network in plant cells and the fibril network in a human cell. The special mechanical properties of these polymer networks, for example their ability to relax back to their original structure and restructure after cessation of stress, enable these cells to perform their biological functions. These self-healing properties of transient networks are instrumental to the design of self-repairing smart nanomaterials and sensors.
This project will provide valuable training opportunities for graduate and undergraduate students. It is integrated in an interdisciplinary educational program in math/physics and biology at San Diego State University. As part of this program the P.I. teaches a course on synthetic and biopolymers, in which results of her research group are discussed. San Diego State University enrolls a large number of minority students, many of whom take this course, participate in this program, or are part of the PI's research group.
