Chemomechanics: a bridge across the formidable gap

The overarching objective of this proposal is to validate experimentally and bring to a new level of utility a conceptual framework for understanding and exploiting the chemical response of polymeric materials to mechanical loads.

The enormous technological importance of polymeric materials is due largely to the remarkable range of their mechanical properties, i.e., their responses to mechanical loads. At the macroscopic scale such loads (stresses) change bulk shapes of objects, but the material response extends across many orders of magnitude in length and time. Almost as soon as the nature of polymers had been recognized certain simple manipulations of polymer solids, melts or solutions were shown to result in fragmentation of polymer backbones without the high temperatures that are normally required for strong covalent bonds to break at detectable rates. The effect is often called mechanochemistry. Mechanochemistry is thought to be important in controlling (1) crack propagation and catastrophic materials failure, (2) stability of surface-anchored polymers in microfluidic diagnostics and high-performance chromatography and (3) behavior of desalination membranes, impact-resistant materials (e.g., bulletproof vests) and tires; and in affecting technological processes as diverse as (4) jet injection (e.g., during inkjet material deposition in organic electronics), (5) polymer melt processing, (6) high-performance lubrication, (7) enhanced oil recovery (e.g., polymer flooding), (8) turbulence drag reduction (e.g., in pipelines, fire fighting, irrigation). Exploiting coupling between localized reactivity and mechanical loads could both advance these technologies and yield fundamentally new materials and processes, including polymer photoactuation (i.e., direct conversion of light into motion to power autonomous nanomechanical devices, control information flow in optical computing, position mirrors or photovoltaic cells in solar capture schemes), efficient capture of waste mechanical energy, materials capable of autonomous reporting of internal stresses and self-healing and tools to study polymer dynamics at sub-nm scales.

To realize this remarkable potential fully the materials science community needs a set of theoretical, computational, synthetic and physicochemical tools and models to guide our effort to identify chemical compositions and molecular structures of monomers and polymer architectures that yield bulk materials with desired stress-responsive characteristics and to enable molecular studies of polymer dynamics particularly at the 5-100 nm lengthscale (the so called "formidable gap"). Achieving this goal requires a general, quantitative understanding of the relationship between the macroscopic parameters that define mechanical loads (e.g., stress or strain tensors) and the molecular properties that govern the changes in chemical reactivity (e.g., energies of activation). EPSRC funding will enable us to develop such understanding with a program that integrates (macro)molecular design and synthesis, physical measurements (using a variety of modern spectroscopic techniques, including single-molecule force spectroscopy and high-resolution X-ray photoelectron spectroscopy), instrument design, quantum-chemical computations, statistical-mechanics and finite-element modeling and theory.

To accomplish this overall objective we will use a series of reactive monomers specifically designed for efficient and accurate kinetic measurements of localized reactivity and molecular interpretation of the results across the whole range of physical systems whose behavior is governed by dynamics of stretched macromolecules. These systems range from individual isolated stretched polymer chains all the way to bulk amorphous polymers under load.

I suggest that in the short-term (2-4 years) the main beneficiaries will be academic scientists who design, synthesize and study stress-responsive polymers. Over mid-term (4-7 years) broader materials science community will benefit from increased awareness of the diverse chemistry that contributes to the behavior of polymeric materials under loads, a wider adoption of the conceptual framework and experimental tools to study polymer behavior under load that we'll develop and validate, and broad recognition of the opportunities that exploiting coupling between local reactivity and load creates for designing new materials and processes. The most direct societal impact of the proposed program will likely result from it enabling the development of new materials and processes. First such processes and materials may become marketable within ~10 years. Over the next ~15 years the idea of mechanical load as a variable in chemical kinetics could become sufficiently well known to be incorporated in chemical intuition with the potential impact on chemical research similar to that of the broad adoption of the transition state theory by the synthetic community.

A fundamental challenge in the modern materials science is the development of conceptual frameworks, models, and experimental and computational tools to enable engineering bulk materials properties at the molecular level. Our project will contribute to meeting this challenge by developing comprehensive understanding of how localized chemical reactions control the response of polymers to loads. Modern materials are constantly subject to mechanical loads, from production through disposal or recycling, and their response to such loads often determines their technological value. Considerable empirical evidence exists of mechanical loads dramatically altering the kinetic stability of individual covalent bonds making up the material. Such load-induced highly-localized chemistry has been shown, or is thought, to control (1) crack propagation that contributes to catastrophic materials failure, (2) stability of surface-anchored polymers in microfluidic diagnostics and high-performance chromatography and (3) behavior of desalination membranes, impact-resistant materials (e.g., bulletproof vests) and tires; and to affect technological processes as diverse as (4) jet injection (e.g., during inkjet polymer deposition in organic electronics), (5) polymer melt processing, (6) high-performance lubrication, (7) enhanced oil recovery (e.g., polymer flooding), (8) turbulence drag reduction (e.g., in pipelines, fire fighting, irrigation). Exploiting coupling between localized reactivity and mechanical loads could both advance the above technologies and yield fundamentally new materials and processes, including polymer photoactuation (i.e., direct conversion of light into motion, to power autonomous nanomechanical devices, control information flow in optical computing, adjust positions of mirrors or photovoltaic cells in solar capture approaches and refresh electronic Braille displays), efficient capture of waste mechanical energy, materials capable of autonomous reporting of internal stresses and self-healing and tools to study polymer rheology and dynamics at sub-nm scales.

Despite considerable progress made through empirical approaches, these benefits are unlikely to be fully unrealized until our fundamental understanding of coupling between localized reactivity and mechanical loads becomes far more sophisticated. Such coupling occurs across the "formidable gap", the 5 - 100 nm lengthscale, where neither continuum mechanics nor chemical kinetics alone offer adequate description of dynamics and experimental tools to study the dynamics are particularly limited. Whereas much effort has been devoted to scaling this gap from the continuum-mechanics limit, we'll bridge it using chemical tools, positioning chemistry at the center of solving one of the "Grand Challenges" on the 21st century.