Visual Indication of Mechanical Damage Using Core-Shell Microcapsules Susan A. Odom, †,‡ Aaron C. Jackson, †,§ Alex M. Prokup, †,‡ Sarut Chayanupatkul, †,§ Nancy R. Sottos, †,§ Scott R. White, †,⊥ and Jeffrey S. Moore* ,†,‡ † Beckman Institute for Advanced Science & Technology, ‡ Department of Chemistry, § Department of Materials Science and Engineering, and ⊥ Department of Aerospace Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, United States * S Supporting Information ABSTRACT: We report a new core-shell microcapsule system for the visual detection of mechanical damage. The core material, 1,3,5,7-cyclooctatetraene, is a conjugated cyclic olefin and a precursor to intensely colored polyacetylene. A combination of poly(urea-formaldehyde) and polyurethane is required to effectively encapsulate the volatile core material. Increasing the outer shell wall thickness and including a core- side prepolymer improves the thermal stability and free- flowing nature of these capsules, which tend to leach and rupture with thinner shell walls. Capsules ruptured in the presence of the Grubbs-Love ruthenium catalyst show immediate color change from nearly colorless to red-orange and dark purple over time, and color change in thin films resulted from scratch damage. KEYWORDS: damage detection, color change, ring-opening metathesis polymerization, microcapsule, encapsulation 1. INTRODUCTION Indicators in polymers and composite materials can reveal mechanical damage and the need for repair before damage becomes catastrophic. Toward this goal, a variety of research groups have introduced methods of detecting mechanical damage through color change. Mechanically active small molecules incorporated into polymer backbones or as cross- links between polymer chains have shown changes in color and/or fluorescence upon fracture, 1 tension, 2,3 and shear. 4 The force-induced dissociation of dye aggregates has been used to change photoluminescent character in stretched polymers. 5-7 Changes in pH have been used to detect compression in polymer brushes, 8 and microcapsules containing a pH-sensitive dye solution have also been utilized in carbonless copy paper, in which color change occurs upon a change in pH when ruptured microcapsule cores react with acidic clay or resin coatings. 9 Fluorescent dyes have been incorporated into filled hollow fibers to enhance damage visibility in the structure of the reinforced plastic. 10 None of these systems forms a colored product in tandem - or that is necessarily compatible - with a structural healing process. Here we selected a catalyst and ROMP-based colorimetric assay to be compatible with our previously demonstrated self-healing chemistry. Such a system could enable the same embedded catalyst to detect damage and heal a structural polymer. We previously reported the preparation of core-shell microcapsules containing the liquid monomer dicyclopenta- diene (DCPD), 11 which undergoes ring-opening metathesis polymerization (ROMP) when reacted with certain Grubbs’ ruthenium catalysts. 12 When both the monomer-filled capsules and catalyst are embedded in polymer composites, mechan- ically induced crack propagation was healed or arrested due to formation of polyDCPD at the crack interface. 13 Similarly, we sought to utilize ROMP of the cyclic monomer 1,3,5,7- cyclooctatetraene (COT) to produce polyacetylene (Figure 1a), an intensely colored conjugated polymer. This reaction has been reported with tungsten, 14-16 molybdenum, 17 and Received: August 5, 2011 Accepted: November 24, 2011 Published: November 24, 2011 Figure 1. (A) Ring-opening metathesis polymerization of 1,3,5,7- cyclooctatetraene. (B) Grubbs-Love catalyst. Letter www.acsami.org © 2011 American Chemical Society 4547 dx.doi.org/10.1021/am201048a | ACS Appl. Mater. Interfaces 2011, 3, 4547-4551