Research

Transformational technologies ranging from vaccines that remain viable at room temperature to flexible and stable solar cells and electronics await the development of new polymeric materials, soft materials made from long molecular chains with repeating molecular units, that push the limits of material performance. Many of the most promising materials for these new technologies derive their potential from two shared features: they solidify without forming a crystal, through a process known as the glass transition; and their molecules possess strong interactions. At the same time that these features are key to the potential of these materials, they also present a challenge to rational materials design. A fundamental understanding of the physics of the glass transition, the central determinant of the properties of these materials, is still lacking. This problem is especially acute in strongly interacting polymers, because strong interactions are resistant to standard theoretical approaches and are difficult to efficiently capture in computer simulations. As a result, it has not been possible to study molecular behavior at sufficiently long time scales and in sufficiently diverse chemistries to both unravel the fundamental physics of these materials and guide their design.

This effort area is aimed at overcoming these challenges. The Simmons group is combining new theoretical approaches and an improved strategy for simulating glass-forming materials to establish fundamental insights and design guidelines for strongly interacting glass-forming polymers. This strategy will enable access to very long time scales and tens of thousands of chemistries to identify common aspects of the molecular physics of these materials and translate them into predictive theories for their properties. This theoretical understanding, in turn, will guide selection of molecular structures yielding unique, targeted material properties. Success of this project will contribute to accelerating the development of materials with the potential to improve human health, enable a cleaner domestic energy economy, enhance the lightness and durability of auto and aircraft components, and broaden the versatility of electronics and solar cells. 

Over the last several decades, enormous scientific and public attention have been focused on the promise of nanotechnology to provide better materials needed to enable major technological advances. One major application of nanotechnology is the development of polymeric nanocomposites. For example, elastomeric nanocomposites - soft polymers reinforced by introduction of nanoparticles - can exhibit a remarkable combination of mechanical toughness with high stretchability. For this reason, these materials a predominant choice for a range of applications spanning from energy systems to car tires. Moreover, nanocomposite elastomers represent the canonical example of a larger class of materials - polymeric hybrid solids - that integrate coexisting soft and rigid domains on the nanoscale to realize unique combinations of properties such as high permeability with high toughness. Despite nearly 100 years of use of elastomeric nanocomposites in the context of filled rubber, the molecular mechanisms by which introducing nanoparticles can lead to these dramatic properties  remain incompletely understood. A better understanding of these materials could open to door to major technological advances driven by improved material properties.

In new work supported by the Department of Energy - Basic Energy Sciences program, the Simmons group is employing simulation to probe the underlying molecular mechanisms by which nanoparticles enable polymers to exhibit extraordinary mechanical resilience under high deformation and load.  This incredible 'toughening' effect is known to emerge from nanoparticles' ability to augment the 'dissipation' (conversion to heat) of deformation energy - rendering it unable to contribute to failure. However, the microscopic mechanisms by which energy is dissipated in nanocomposites remain poorly understood and a matter of considerable debate despite decades of research. A resolution to this question could allow us to tailor nanoparticles and nanocomposites to yield targeted mechanical properties, opening the door to new tough, lightweight multifunctional materials.

In prior work supported by a Rubber Industry Consortium (CenTire), the Simmons group employed molecular simulations to obtain new insight into the mechanism by which nanoparticles stiffen  arguably the very first synthetic ‘nanocomposite’ material developed by science – filled rubber. Rubber used in tires and other mechanically demanding applications almost universally incorporate a nanoparticulate filler – most commonly carbon black – in order to improve mechanical strength and durability. These fillers can yield extraordinary improvements in rubber properties, increasing strength by nearly a factor of 10. Findings from that work resulted in new simulation methods for filled rubber and in new understanding of how nanoparticles increases the stiffness of rubber under stretching.

Overview. The push towards materials and devices with nanoscale features demands a fundamental understanding of how material properties are altered at these length scales. Many next-generation advanced material applications, ranging from polymer-based solar energy to purification of water and air, employ glass-forming polymers with internal and external structures on nanoscale dimensions. The presence of structure and interfaces on these length-scales can dramatically alter the glass transition, mechanical  properties, and transport properties of polymers and other soft materials. Our group employs molecular dynamics simulations of nanostructured materials to provide new insights into the variables that control these effects, in order to enable the rational control of engineering properties in next-generation nanostructured materials. Materials under study include nanolayered polymers, block copolymers, ionomers, and polymeric nanoparticles.  

Recent synthetic advances open the door to a new century of polymers replicating the fine control of chemical sequence that gives biological polymers such as proteins and DNA their extraordinary functionalities. Unlocking this potential will require entirely new strategies for design of polymer sequence, and entirely new theories for the physics of sequence-controlled polymers, equal to the problem of recapitulating nature's 4 billion years of evolutionary sequence design on the time scale of human lives.

In this effort, the Simmons group is combining molecular simulation, polymer theory, machine learning, and evolutionary algorithms to guide design of sequence-controlled polymers with properties exceeding those accessible with classical polymers. Early work has identified strategies for design of sequence-controlled copolymer compatibilizers - molecules that can enable control over interfacial properties in applications ranging from films to emulsions.