These materials also can be transformed into new polymer structures to further bolster their strength by 50%—making them ultra strong and lightweight. This research was published in the journal Science, with collaborators including UC Berkeley, Eindhoven University of Technology and King Abdulaziz City for Science and Technology (KACST), Saudi Arabia.

Polymers are long chains of molecules connected through chemical bonds. Although now ubiquitous in myriad applications, today’s polymer materials are limited in some ways. In transportation and aerospace, structural components or composites are exposed to many environmental factors (de-icing of planes, exposure to fuels, cleaning products, etc.) and exhibit poor environmental stress crack resistance (i.e., catastrophic failure upon exposure to a solvent).

Also, these polymers are difficult to recycle because they cannot be remolded or reworked once cured or thermally decomposed by heating to high temperatures. As a result, these end up in the landfill together with toxins such as plasticizers, fillers, and color additives which are not biodegradable.

In a perspective on the IBM paper, also published in Science, Dr. Tim Long from Virginia Tech noted that:

Thermoplastics are polymers that become pliable or moldable at elevated temperatures, but return to a solid state when cooled. These polymers can thus be readily processed or reprocessed upon heating, and are therefore widely used in food and beverage packaging. In contrast, thermosets are chemically crosslinked polymers, networks, or gels with chemical bonds between chains that do not thermally dissociate, even at high temperature. They are ideal for high-temperature electronic or automotive applications, but cannot be reprocessed or recycled either by melting or by solution processing.

Many researchers are now challenging this classical definition with concepts of recyclable networks, reworkable encapsulants, reversible gels, self-healing polymeric coatings, and stimuli-responsive polymeric structures. The goal is to create reversible thermosets that combine the desirable thermal and chemical stability of conventional thermosets with recyclability and reprocessability.

IBM’s discovery of a new family of materials with a range of tunable and desirable properties provides a new opportunity for exploratory research and applications development to academia, materials manufacturers and end users of high performance materials.

The researchers discovered two new related classes of materials which possess a very distinctive range of properties that include high stiffness, solvent resistance, the ability to heal themselves once a crack is introduced and to be used as a resin for filled composite materials to further bolster their strength.

Also, the ability selectively to recycle a structural component would have significant impact in the semiconductor industry, advanced manufacturing or advanced composites for transportation, as one would be able to rework high-value but defective manufactured parts or chips instead of throwing them away. This could bolster fabrication yields, save money and significantly decrease microelectronic waste.

Although there has been significant work in high-performance materials, today’s engineered polymers still lack several fundamental attributes. New materials innovation is critical to addressing major global challenges, developing new products and emerging disruptive technologies. We’re now able to predict how molecules will respond to chemical reactions and build new polymer structures with significant guidance from computation that facilitates accelerated materials discovery. This is unique to IBM and allows us to address the complex needs of advanced materials for applications in transportation, microelectronic or advanced manufacturing.

—James Hedrick, Advanced Organic Materials Scientist, IBM Research

IBM’s computational chemistry hybrid approach lab experimentation with the use of high-performance computing to model new polymer forming reactions. The unconventional method is a departure from traditional techniques and led to the identification of several previously undiscovered classes of polymers in what was believed to be an established area of materials science researched extensively since the 1950s.

Ideally, scientists could insert a list of requirements into a computer to design a material that meets those exact conditions. However, the reality now is that materials are still primarily discovered only by experimenting in the lab based on the scientist’s knowledge, experience and educated guesses. IBM Research’s computational chemistry efforts can take out a lot of this guesswork and accelerate a whole new range of potential applications from developing a disease-specific drugs or cheap, light, tough and completely recyclable panels on a car.

These polymers are formed from the same inexpensive starting material through a condensation reaction, were created in an operationally simple procedure, and are incredibly tunable.

At high temperatures (250 ˚C) the polymer becomes incredibly strong due to a rearrangement of covalent bonds and loss of the solvent that is trapped in the polymer (now stronger than bone and fiberboard), but as a consequence is more brittle (similar to how glass shatters).

This polymer remains intact when it is exposed to basic water (high pH), but selectively decomposes when exposed to very acidic water (very low pH). This means that under the right conditions, this polymer can be reverted back to its starting materials, which enables it for reuse for other polymers.

New ultra-strong polymer reinforced with carbon nanotubes. Click to enlarge.

The material can also be manufactured to have even higher strength if carbon nanotubes or other reinforcing fillers are mixed into the polymer and are heated to high temperatures. This process enables polymers to have properties similar to metals, which is why these “composite blends” are used for manufacturing in airplane and cars. An advantage to using polymers in this case over metals is that they are more lightweight, which in the transportation industry translates to savings in fuel costs.

At low temperatures (just over room temperature), another type of polymer can be formed into elastic gels that are still stronger than most polymers, but still maintains its flexibility because of solvent that is trapped within the network, stretching like a rubber band.

Probably the most unexpected and remarkable characteristic of these gels is that if they are severed and the pieces are placed back in proximity so they physically touch, the chemical bonds are reformed between the pieces making it a single unit again within seconds.

This type of polymer is called a “self healing” polymer because of its ability to do this and is made possible here due to hydrogen-bonding interactions in the hemiaminal polymer network. One could envision using these types of materials as adhesives or mixing in with other polymers to induce self-healing properties in the polymer mixture. Furthermore, these polymers are reversible constructs which means that can be recycled in neutral water, and that they might find use in applications that require reversible assemblies, such as drug cargo delivery.

Resources

• Jeannette M. García, Gavin O. Jones, Kumar Virwani, Bryan D. McCloskey, Dylan J. Boday, Gijs M. ter Huurne, Hans W. Horn, Daniel J. Coady, Abdulmalik M. Bintaleb, Abdullah M. S. Alabdulrahman, Fares Alsewailem, Hamid A. A. Almegren, and James L. Hedrick (2014) “Recyclable, Strong Thermosets and Organogels via Paraformaldehyde Condensation with Diamines,” Science doi: 10.1126/science.1251484

• Timothy E. Long (2014) “Toward Recyclable Thermosets,” Science doi: 10.1126/science.1254401