Colorless, non-fluorescent microcapsules use a type of fluorescence called aggregation-induced emission (AIE), which becomes brighter as the indicator solidifies from solution and is visible under ultraviolet (UV) light. (Credit: Autonomous Materials Systems Group, Beckman Institute for Advanced Science and Technology, University of Illinois)
Pipes can burst for many reasons, such as subzero temperatures or catastrophic events, or continuous use and time can wear the material, leaving dents and dings that become evident only when it is too late.
A new remedy to identify the small and often microscopic damages in composite materials and polymers before they can cause irreparable failure has been developed by the Autonomous Materials Systems (AMS) Group at the Beckman Institute for Advanced Science and Technology.
Autonomous indication of small cracks has exciting potential to make structures safer and more reliable by giving time to intervene and repair or replace the damaged region prior to catastrophic failure.
Nancy Sottos. Professor, Beckman Institute
She is also one of the authors of the study titled ‘A Robust Damage-Reporting Strategy for Polymeric Materials Enabled by Aggression-Induced Emission,’ which has been recently reported in ACS Central Science. The research project of which the paper is a part is a finalist for the Institution of Chemical Engineers (IChemE) Global Awards 2016.
Fluids containing microcapsules filled with turn-on fluorescence indicators were isolated by researchers and later introduced into polymeric materials.
We've developed microcapsules that are colorless and non-fluorescent when intact. We can embed them into materials, and when damage occurs, the microcapsules will release their payload and become fluorescent, indicating that repair is needed.
Maxwell Robb, Postdoctoral Fellow, Beckman Institute
Research conducted in the past by the study’s co-first author and postdoctoral research associate Wenle Li, had examined another form of indicator contained in microcapsules. This indicator, when released, underwent a chemical reaction to result in a color change. However, the characteristic of the chemical reaction restricted the system to a small range of materials.
The method developed in the recent study makes use of a form of fluorescence known as aggregation-induced emission (AIE). This fluorescence turns brighter when the indicator solution solidifies and is observable under UV light. The special indication mechanism, which depends on change of physical state rather than on a chemical reaction, allows for visualizing various types of damages and great performance in a wide range of materials.
"The elegance of this system lies in its versatility as well as its sensitivity," said Li. "We can easily visualize a fluorescence signal resulting from mechanical damage as small as two microns."
BP, a company interested in coating of gas and oil pipelines using polymer coating to identify damage, has funded the endeavor. The aim is to detect damage at its beginning stage to avoid further damage, improve reliability and safety, and to reduce life cycle expenses caused due to regular inspection and maintenance.
The researchers were able to explore the coatings and microcapsules of different materials, create images of them, and then match 3D models of damaged coatings to fluorescence signals. The team used the equipment in Beckman's Microscopy Suite.
"This is incredibly interdisciplinary work," said Robb. "Having knowledge about the aggregation-induced emission effect, and being able to design the chemistry of the microcapsule system was the starting point. Then there is the actual application of this technology into materials and coatings, which relies heavily on the expertise within materials science and engineering."
In addition to Sottos, Jeffrey Moore, professor of chemistry, and Scott White, professor of aerospace engineering, who also co-authored the study are part of the AMS Group. The team’s efforts have paved way for new innovations in self-healing and self-detecting materials.
To impact the coatings industry, materials with self-reporting capability must meet a few criteria: they must be simple, not change the way the materials are traditionally applied, and perform just as well. Our approach hits this target - the new self-reporting function is realized by just one simple additive.
Jeffrey Moore, Professor of Chemistry, Beckman Institute
The next steps of the study involve integrating self-healing materials with damage indication.
"If you could couple this technology that lets you know that damage has occurred with a self-healing material that tells you when the damage has been healed, it could be really powerful," said Robb.
"We have developed both turn-on fluorescence and color-changing indication systems. Our vision is to combine these multi-channel strategies to enable materials that monitor their mechanical integrity throughout the entire polymer lifecycle," said Li.