Associative polymers are giant molecules that contain "sticky" functional groups, binding them together with reversible physical bonds. Some of the most widely used polymers are associative polymers, including those used to make golf balls, the absorbing gels in diapers, flow modifiers for shampoos or conditioners, and new materials being developed for biomedical uses and enhanced oil recovery. The use of these existing materials and the development of new systems relies on understanding how the molecules move: how they are processed into the shape of a final object and how they deform under mechanical force. Ultimately, these properties are governed by the way that the molecules move on microscopic scales. Recently researchers have discovered that molecular motion in associative polymers shows unexpected relationships between how fast a molecule moves and the distance that it moves, prompting a reconsideration of some of the scientific understanding behind the design of the polymer properties. This project will use specialized X-ray and laser light techniques to study associative polymer motion at the micro and nanoscale, allowing us to further understand the mechanisms by which they move. Theory and simulation of these systems will be applied to relate molecular properties to the chemical design of new polymers, providing fundamental insight that allows faster discovery of improved polymeric materials. A diverse group of graduate and undergraduate researchers will apprentice through this project, developing scientific expertise that will contribute to the U.S. economy. The discoveries in this project will be shared with the public through YouTube and scientific publications and presentations and will also be developed in collaboration with high-school teachers into new materials to inspire the creativity and inventiveness of young students.
The study of associative polymers has led to a solid theoretical understanding based on transient network theory for telechelic polymers, which models the dynamics of the networks based upon kinetic equations that model bond association and dissociation during deformation. These concepts have also led to the development of sticky Rouse and sticky reptation theories, grounded in the same conceptual framework but applicable to polymers with associating groups spaced along the backbone. Rheological measurements on a huge variety of associative polymer systems have established the validity of these approaches, at minimum in a qualitative sense. However, these theories have not been similarly tested using experimental measurements of diffusion dynamics. Recent diffusion measurements performed using forced Rayleigh scattering (FRS) show something that is not anticipated by any of the theories: an apparent super-diffusive regime on length scales 10-1000 times greater than Rg, even though the gels show no signs of structure at this scale. Similar behavior has now been observed in four separate systems, suggesting it is common across many polymers. It is hypothesized that this super-diffusive regime is due to molecular "jumping" or "hopping" as the dominant diffusion mechanism at long length scales, something that is not anticipated in existing theories. Preliminary Brownian dynamics simulations performed on a simplified associative polymer model for center-of-mass diffusion show that this hypothesis produces diffusion results that qualitatively match the FRS measurements. Throughout the proposal, effects of molecular design will be systematically explored using FRS and X-ray photon correlation spectroscopy (XPCS), both in experiments and using simplified coarse-grained models. Because of the ubiquitous nature of associative polymers in applications and the key importance of dynamics in these materials for their end use, the potential for fundamental scientific insight provided by the proposed work will be large across many different industries and technology areas, ranging from oil recovery to lubricants to medicine to food. To share with the broader community excitement about the interesting physics of these everyday materials, a series of educational videos will be made for YouTube that explain different aspects of the experiments and theory as they develop. In the final year of the project, a collaboration will start with a high-school teacher to develop an educational model that combines the videos with simple lab rheology experiments to introduce students to the concepts of associative polymers.
