Chemomechanics: a bridge across the formidable gap

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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.