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Force sensitive polymers to play a vital role in future warning systems |
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New polymers engineered to respond to mechanical stress will help huge structural bridges of the future be built in a way that would offer a pre-damage warning. In the time to come, such applications will surely become a reality much to the credit of force-sensitive polymers developed by researchers at the University of Illinois at Urbana-Champaign (USA). The research team has developed solid polymers which respond to mechanical stress by changing its color. The colour change takes place due to presence of ring-shaped molecules called �mechanophores� which are incorporated into the polymer backbone. The rings of these mechanoresponive molecules break and turn red once the materials are subjected to excessive mechanical stress before breaking into two. As a result, the team found that such mechanophore-linked polymeric materials gave an �early warning� sign before completely damaging.
The researchers feel that parachute cords, climbing ropes, and smart coatings for bridges that change color when overstressed are several possible uses for force-sensitive polymers. Apart from acting as mere color sensor devices, these smart materials also offer promise to trigger self-repairing or self-strengthening chemical reactions under increased mechanical force. In highly important material systems, such as those used in aircraft components, self-sensing and self-reinforcing capabilities could be used slow the spread of damage by extending a material's lifetime, or even repair damage in early stages to avoid disastrous failure by inducing polymerization or cross-linking reactions under stress. This finding represents a major development in an attempt to build force sensitive functionalities into materials similar to the working of mechanoreceptor cells in our biological systems.
Prior to this work, the team demonstrated the use mechanical force to induce a reaction in mechanophore-linked polymers that were in solution. The researchers performed a similar feat using a solid polymeric material. In their work, the team used molecules called spiropyrans, a promising class of molecular probes that serve as color-generating mechanophores, capable of vivid color changes when they undergo mechanochemical change. Under normal conditions, these closed-ring shaped molecules are colorless; however, the spiropyran undergoes a ring-opening reaction to turn red or purple when exposed to certain levels of mechanical stress. The fact that this mechanically-induced chemical reaction is reversible makes it even more interesting as the opening and closing of the ring can be repeated.
The researchers prepared two different mechanophore-linked polymers and subjected them to different levels of mechanical stress. When a soft, stretchy elastomer was stretched until it broke in two, the color changed to red just before it snapped. The second polymer was formed into glasslike and brittle rigid beads several hundred microns in diameter. When the beads were squeezed, they changed from colorless to purple. Though the research group demonstrated spiropyran's ring-opening reaction enabled by mechanical force, the molecule also has tendency to change color under light. This would prove to be a shortcoming since it may misguide structural engineers to think in the wrong direction. As a first important step in modelling smart polymers, the team is aiming to instill self-healing functionality into polymer materials which could repair themselves under mechanical force.
Other recent promising work in field of force sensitive self-healing of polymers came from a research team at Eindhoven University of Technology, in the Netherlands. The research team engineered homogenous, latent polymerization catalysts which get activated once subjected to mechanical force or increased stress. The catalyst developed consists of a metal (Silver or Ruthenium) core complexed with a pair of organic ligands, which in turn, are attached to the polymer chains. The two ligands also play a role of catalyst in absence of metal. The material, thus formed, was dissolved by the team in a solution. The mechanical force supplied by ultrasound pulses through the liquid rips apart the link (weakest point in the chain) between the metal complex and one of the organic ligands attached to the polymer chains. As a result, the core metal center is released which acts as a catalysts for polymerization. However, under normal circumstances (when mechanical stress is not applied), these novel latent polymerization catalysts remain absolutely dormant and inactive. However, the team is yet to demonstrate the catalytic activity of solid stress-responsive metal-ligand bond when in a crosslinked network.
Basically, the new materials are able to sense the force that is being applied to them, and to optically alert engineers if it's within normal parameters, or if their assigned strain is exceeded. Parachute cords, climbing ropes, and smart coatings for bridges that change color when overstressed are several possible uses for force-sensitive polymers being developed.
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