Wind turbines, aircraft, and automobiles could become more tough and durable thanks to research being conducted at the University of Houston Cullen College of Engineering. All of these use composites, materials that contain two or more constituents that have distinct properties. In these examples, the composites are composed of a polymer resin and reinforcing fibers such as glass or carbon.
Typically, objects made of polymer composites contain multiple components that are held together with polymer-based adhesives. But often the properties of these adhesives fall short, leading to failure at the connection between the components.
While it's obvious that better polymer-based adhesives are needed, creating them is a challenge. Often steps taken to increase the strength and stiffness of the polymer also result in decreased ductility and toughness (essentially the ability to absorb energy without breaking).
Megan Robertson and Mina Dawood, assistant professor of chemical and biomolecular engineering and civil and environmental engineering, respectively, have recently received a $300,000 grant from the National Science Foundation to develop new polymers for adhesives that do not make such a tradeoff. Though the execution of their idea is challenging, the approach itself is a simple one: mix two polymers together to get the best qualities of both.
Specifically, Robertson and Dawood will combine traditional epoxy resin polymers, which provide strength and stiffness, with a high-ductility polymer known as polydicyclopentadiene. While the grant is directed toward developing new polymer-based adhesives for joining structural composite materials, the findings will also support the creation of new polymer resins that can be incorporated into the composites themselves, based on the same polymer combination.
To create the polymers, researchers start with a liquid solution comprised of the small molecules that are the precursors to the polymer. The solution is then hardened through a curing reaction, causing the small molecules to chemically bond and form the polymer. This curing reaction is typically carried out at an optimal temperature or in the presence of a catalyst to speed up the reaction.
If all of the precursors to the epoxy resin and polydicyclopentadiene are mixed and then cured simultaneously, Robertson said, they will ideally form two large polymer networks that are completely intertwined but not chemically bonded together. The end product is known as an interpenetrating network. The fact that both networks extend throughout the entire specimen ideally will provide the best properties of each individual network, resulting in a material that is strong, stiff, ductile, and tough.
Finding two compatible curing processes for the two different polymers is one of the biggest challenges of this project, said Robertson. “We have to come up with methods for the curing reactions that are compatible with each other. The temperatures may be different. The catalyst for one could poison the other. The reaction rates may need to be optimized. There’s a lot of work to do to get the two reactions to occur simultaneously.”
Another hurdle, Robertson said, will be simply keeping the polymer solutions thoroughly mixed together. The materials will naturally tend to separate from one another as the polymers are formed. If the two aren’t fully mixed, the final product won’t be uniform and the polymer properties will suffer.
The initial efforts of this project will fall to Robertson, whose expertise lies in polymer development. Dawood will characterize the function of these interpenetrating networks as adhesives for polymer composites. Their findings could lead to new applications for these polymer composites.
“If you want to use these materials for more applications, you have to improve their properties or else they won’t hold up. That’s why we’re trying to make them less brittle. If they can absorb more energy before they break, they can survive in harsher environments,” Robertson said.