Surface damage in coatings, plastic components and reinforced plastic composites is often difficult to detect and challenging to repair in service. This collaborative research project has resulted in new experimental and computational tools to support the development of sustainable plastics that self-report the presence of damage and potentially initiate self-repair wherever and whenever it occurs. Damage-reporting plastics were created by linking force-activated molecules, known as mechanophores, directly into polymer chains or by anchoring the mechanophores at an interface between a rigid reinforcement (such as silica) and a polymer matrix. The application of force or initiation of damage in the plastic transforms the mechanophore into a new chemical species for signaling or for initiating productive changes in materials properties. A critical challenge for this project work was to impart self-reporting properties to high stiffness, high durability plastics, typical of materials used for coatings or other structural applications.

Our first accomplishment was the discovery of a new color-changing mechanophore, naphthopyran (NP), thus adding to the library of force sensitive molecules. We systematically studied the effects of how the molecular attachment of the mechanophores to the polymer chains in the plastic influenced activation.  The NP mechanohores were incorporated in bulk polymer and characterized under tensile force. Importantly for the development of self-reporting plastics, the NP mechanophore changed to a bright yellow color upon application of force (Figure 1). The mechanophore response was also analyzed using CoGEF molecular level calculations.   We found that although CoGEF is a promising tool for rational design of mechanophores, there are additional effects such as electronic structure that also play an important role in mechanophore activation and design.

A second key accomplishment was the development of force-focusing strategies to give better control over mechanophore activation in plastics, composites and coatings.  We hypothesized that attaching the mechanophores to a rigid interface will lead to activation at lower forces, providing an important tool for controlling mechanophore activity. New experimental and computational tools were developed for probing mechanophore activation at an interface.  We successfully demonstrated the activation of maleimide-anthracene (MA) mechanophores at the interface between a patterened epoxy film and a fused silica subrate (Figure 2) as well as the activation of mechanophores at the interface of polymers and nanoparticles (Figure 3).  As hypothesized, we found that mechanophores linked to an interface require less strain and less stress for mechanical activation than in bulk linear polymers. This result opens promising design routes for self-reporting composite materials.

A new method of mechanochemical activation by mechanical scanning probe lithography (m-SPL) was also demonstrated. MA mechanophores anchored at the interface between a polymer brush and a silicon substrate were exposed to highly localized shear forces from an AFM tip. This localized activation may lead to possible application of mechanophore functionalized interfaces for surface lithography.

More broadly, the academic research team worked in partnership with PPG scientists to translate scientific advances to commercially viable polymeric coatings and educate engineering students as well as the general public on how materials can impact sustainability. An interactive exhibit entitled “Self-Healing and Sustainability” was created for the University of Illinois Engineering Open House (EOH) Event in 2015 and 2017 (Figures 4-6).   With over 250 unique exhibits and attractions, EOH brings more than 20,000 visitors, primarily school children, teachers and parents, to campus.  In addition, laboratory kits on light propagation and damage detection were created for 6-8th grade and 9-12th grade students.  Additionally, four graduate students and two postdoctoral researchers received education, training and mentoring as part of this program.

Last Modified: 10/31/2017
Modified by: Nancy R Sottos