Stress-strengthening Synthetic Polymers by Covalent Mechanochemistry
Abstract
The proposed effort seeks to design, synthesize, and characterize a family of synthetic polymers that undergo activated remodeling via mechanochemistry (ARM) to self-strengthen/self-repair in direct response to mechanical force. In prior ARO-funded work, the PI developed a family of synthetic copolymers in which the same forces responsible for the destructive processes of bond scission and chain disentanglement are channeled into mechanophores that respond with constructive bond-forming reactions. This proof-of-concept demonstration lays the foundation for the proposed effort which seeks to extend the activated remodeling via mechanochemistry (ARM) concept to render synthetic materials with stress-responsive behavior analogous to that observed in biological systems. Specifically, the PI aims to: 1) Design a family of multi-state colorimetric mechanophores that combine the ease of activation and colorimetric response observed in spiropyrans with subsequent cross-linking reactivity that s comparable to or better than the proof-of-concept ARM system. These mechanophores will be incorporated into a range of polymers and the interplay between the mechanophore activation and subsequent cross-linking reactions will be explored to determine the rate determining steps and how they may be optimized. UV-vis will be employed to quantify activation. Additionally, a number of structure-activity studies are planned to examine the ARM response as a function of mechanophore concentration, polymer scaffold, stress/strain, time, and multiple iterations within a cyclic loading environment. The PI will also explore the impact of mechanophore activation and cross-linking on mechanical properties. 2) Create new cross-linking mechanophores that undergo reversible strengthening/stiffening under critical load and reverses in periods of rest. Effective crosslinking will require an extreme shift in equilibrium. Because this shift can be facilitated by large extensions in mechanophores, uncoiling of "auto-inhibiting" metal-ligand macrocycles to linear isomers has been proposed as the design principle as macrocylic mechanophores based on metal-ligand coordination will enable tunable kinetics based on choice of metal and inhibitory ligand and/or crosslinking ligand. These novel mechanophores will be incorporated into polymers via ROMP and tested for their activity using a number of techniques including gel permeation chromatography, UV-Vis spectroscopy, and 1H NMR . The kinetics of crosslinking and de-crossliking will be characterized by monitoring the force-triggered reaction and subsequent relaxation in periods of reset via 1H NMR and UV-Vis spectroscopy. The PI will also study how the kinetics can be tuned by changing the metal, linker lengths between ligands within the mechanophore, crosslinking ligand, and inhibitory ligand. Using these established kinetics, the PI intends to demonstrate mechanically triggered reversible cross-linking in both solution as well as in gels and bulk polymers and subsequently investigate responsiveness as a function of polymer scaffold, stress/strain state, time, mechanophore concentration, and cross-linking agent. 3) Establish signal amplification schemes to produce localized changes in chemical structure or materials properties. The PI intends to leverage the dihalo-functionality unveiled by the mechanically-induced ring opening of gem-dibromocyclopropane and gem-dichlorocyclopropane to initiate atom transfer radical polymerizations. Thus, mechanophore activation will lead to the growth of long, branched side chains which will effectively add mass and dramatically change the polymer s physical and mechanical properties. Once synthetic protocols are established in solution, the PI will leverage this chemistry to study signal amplification on surfaces and in bulk materials.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Jan 12, 2017
- Source ID
- W911NF1510143
Entities
People
- Stephen L Craig
Organizations
- Army Contracting Command
- Duke University
- United States Army