UV Light Exposure Causes Composite Material to Grow Stronger On Demand

A technique has been developed that causes a composite material to become stiffer and stronger when exposed to UV light. The technique can be controlled, allowing the strength of the material to be adapted to the requirements of the application.

To develop the technique, engineers attached UV-light-reactive molecules to reinforcing agents (in experiments, carbon nanotubes were used). The reinforcing agents were then embedded in a polymer. Researchers found that when the polymer was exposed to UV light, interaction between the reinforcing agents and the polymer increased, causing the material to become stiffer and stronger.

Synthetic process used by researchers to create photoresponsive reinforcing agents. A carbon nanotube (CNT) is treated with a photoreactive molecule called benzophenone (BP). After exposure to UV light, the molecules bond to the surrounding polymer chain. This creates a covalent link between the nanotube and the polymer. Courtesy of U.S. Army.

Researchers from the U.S. Army Research Laboratory (ARL) and the University of Maryland used polydimethylsiloxane (PDMS) with carbon nanotube (CNT) fillers functionalized with the photoreactive molecule benzophenone (BP). In experiments, the stiffness of the material was found to increase by as much as 93 percent and the strength by as much as 35 percent after a 5-min exposure to UV light. Varying radiation time provided a high degree of spatial control, needed to engineer heterogeneous materials with a desired behavior.

The technique developed in this work was demonstrated on a CNT/PDMS system, but researchers believe the chemistry used in the technique could be applied to a variety of reinforcement/polymer combinations, thereby expanding use of this control method to a wide range of material systems.

ARL research engineer Frank Gardea said the goal of the research was to have molecules interact in such a way that changes at the nanoscale could lead to observed changes at the macroscale.

“This research shows that it is possible to control the overall material property of these nanocomposites through molecular engineering at the interface between the composite components. This is not only important for fundamental science but also for the optimization of structural component response,” said postdoc research fellow Zhongjie Huang of the University of Maryland.

U.S. Army researchers imagine a rotorcraft concept, which represents reactive reinforcements that when exposed to UV light will increase the mechanical behavior on demand. The engineers said control of mechanical behavior could potentially lead to increased aerodynamic stability in rotorcraft structures. (CNT = carbon nanotube; BP = benzophenone.) Courtesy of U.S. Army.

ARL researcher Bryan Glaz said that one of the catalysts driving the work was the ability to engineer new structures, starting from the nanoscale, to enable advanced rotorcraft with significantly reduced maintenance requirements.

“The enhanced mechanical properties with potentially low weight penalties, enabled by the new technique, could lead to nanocomposite-based structures that would enable rotorcraft concepts that we cannot build today,” said Glaz.

“This is especially important for the envisioned future [military] operating environment, which will require extended periods of operation without the opportunity to return to stationary bases for maintenance,” he said.

Frank Gardea, a research engineer with the U.S. Army Research Laboratory's Vehicle Technology Directorate, collaborated with the University of Maryland to develop a technique that causes a composite material to become stiffer and stronger on demand when exposed to UV light. Courtesy of U.S. Army/David McNally.

This technique is amenable to layer-by-layer construction and shows potential as an additive manufacturing technique for engineering materials on demand with necessary heterogeneous makeups.

Future structures based on the research could lead to new composites with controlled structural damping and low weight. Controllable mechanical response could allow for the development of adaptive aerospace structures that could potentially accommodate mechanical loading conditions.

The research was published in Advanced Materials Interfaces (doi:10.1002/admi.201800038).