The high strength-to-weight ratio and robust mechanical properties of carbon-fiber-reinforced polymers (CFRPs) make them attractive for use in wind-power turbine blades, automobile components, and in airplanes and spacecraft. However, challenges — such as weak interfacial adhesion and poor recyclability — remain. A team of researchers at Oak Ridge National Laboratory (ORNL; Oak Ridge, Tenn.; www.ornl.gov) has synthesized a new type of CFRP aimed at overcoming these challenges.
The approach taken by the researchers was to incorporate a covalent adaptive network (CAN) into the polymer matrix in ways that mimic natural composite materials in marine mollusks. CANs contain dynamic covalent bonds that can undergo exchange reactions to rearrange the network structures. CANs are incorporated to make a dynamic cross-linked thermoset polymer, known as a vitrimer, which can change its topology upon heating, the researchers say.
The ORNL team developed a boronic-ester-functionalized triblock copolymer and dynamic multidiol cross-linker to prepare the vitrimer resin. They functionalized the carbon fiber surface using pinacol (a diol), which can form dynamic covalent bonds with the boronic ester group on the polymer matrices, according to ORNL. This exchangeable crosslinking improves the bonding between fiber and polymer.
“The [carbon] fiber and the polymer have a very strong interfacial adhesion due to the dynamic bonds,” says ORNL chemist Tomonori Saito, who led with study with ORNL’s Md Anisur Rahman. The interface locks materials together through covalent interactions and unlocks them on demand using heat or chemistry.
The dynamic crosslinking also allowed the researchers to recycle the CFRP without losing mechanical properties. Upon recycling, “we recover 100% of the starting materials — the crosslinker, the polymer, the fiber,” Rahman said. And “our composite’s strength is almost two times higher than a conventional epoxy composite,” he adds. The ORNL scientists found that the degree of dynamic crosslinking (around 5%) is important — excessive crosslinking embrittles the polymer.
The ORNL team is working on reducing costs of the material to optimize potential commercial applications.