Jeffrey Gilman, NIST

Dr. Jeffrey W. Gilman
Research Polymer Chemist, NIST

While metal structures will show dents or dings after impact, a composite structure may show no visible signs of damage, despite containing mild breakage or cracks within.

Current research at the National Institute of Standards and Technology is creating a process that uses fluorescence to detect both damage and water in composites, a first for composites. The first system utilizes Förster resonance energy transfer (FRET), a method frequently used in molecular biology research to probe the interaction between proteins and other biopolymers. The second approach uses a mechanophore, a molecule that changes color in response to mechanical forces.

Jeffrey Gilman spoke with Composites Manufacturing Interviews on behalf of his research team at NIST about how these fluorescent dyes detect damage, and how far new research can take the industry.

Can you describe more about the damage-detecting process and how it works?

Our team has developed two damage-sensor approaches that both use fluorescent dyes. The first system utilizes Förster resonance energy transfer (FRET), which is a method frequently used in molecular biology research to probe the interaction on the 1 nm to 20 nm scale, between proteins and other biopolymers. Fluorescent proteins (FP) are commonly used in these biological studies, and they have recently been applied to detect damage in composites by Doug Clark (University of California Berkley) and Nico Bruns (University of Fribourg, Switzerland). However, because of the large size of FPs, we prefer to use small organic dyes in our application of FRET to study of the interface in composites.

NIST FRET efficiency map

FRET efficiency map, false-color image (normalized efficiency from 0 to 100 percent), of a polyethylene (PE)/nanofibrillated cellulose (NFC) composite where the cellulose fiber is labeled with 5-(4,6-dichlorotriazinyl)-aminofluorescein, and the PE is doped with Coumarin 30. (Image by permission from ACS.)

One example of the use of FRET, in a polyethylene (PE)/nanofibrillated cellulose (NFC) composite, is shown in the figure on the right, where FRET imaging enables visualization of the interface between the fiber and the matrix. The FRET efficiency map, which is color-coded in the figure, is a semi-quantitative measure of the quality of the interface. The higher the FRET efficiency, the more integrated the interface. These data also reveal other useful information about the composite, namely that there is no PE matrix, and no NFC-PE interface, within the interior of fiber agglomerates. Furthermore, the FRET imaging can be used to monitor interface damage on the nanoscale; when the composite is freeze-shocked in liquid nitrogen, the FRET signal at the interface is significantly reduced due to interface debonding.

The other damage-sensing approach uses a mechanophore (a molecule which changes color in response to mechanical forces). This is a simpler system than the FRET approach and it complements the FRET approach. It is utilized by chemically incorporating only a single dye-precursor (a mechanophore) into the fiber-matrix interface using a very straightforward procedure. First, a commercially available dye-precursor is reacted with the surface of the fiber by heating it in ethanol. Next, the functionalized-fiber is immersed in the resin and cured under normal conditions.  Initially, the molecule is colorless. Once damage occurs, and a bond is broken in the mechanophore, it becomes fluorescent, in the visible region, and the sample appears colored. Because the mechanophore is chemically bonded to the interface, it reports on damages to the interface when the composite is sufficiently deformed.

This research is still in the very early stages and much work remains to be done to determine how one can quantify the amount of damage and relate it to the increase in fluorescence.  All of the mechanophore work so far has been in epoxy composites and has been developed for silk-fibers with amine functionality. We are working on applying this same method to amine-sized glass and carbon fibers. We also have initial results that show that this damage-sensing molecule can detect the presence of water in a composite, if the composite is first exposed to UV light. This aquaphore work is also at the preliminary stages and like the mechanophore system, it needs to be compared to other methods to evaluate it completely.