The highly flammable short-chain hydrocarbons (typically 5-14 carbon atoms) that constitute gasoline, kerosene, and other liquid transportation fuels readily form a cloud of fuel droplets during impact; these droplets can ignite, producing a fireball.

Longer hydrocarbon chains can readily interact with these fuel molecules, forming larger droplets that don’t hang as long in the air, thereby reducing the probability of a catastrophic explosion.

In the wake of the 1977 Tenerife airport disaster (583 killed in a post-crash fireball), researchers looked into the possibility of using ultralong, associative polymers as a fuel additive to increase the drop diameter in the post-impact mist, thereby limiting the event to a relatively cool, short-lived fire. These polymers interfered with engine operation, however, and their ultralong backbones degraded upon pumping.

Ultralong polymers (weight-average molecular weight M_w ≥ 5000 kg/mol) exhibit striking effects on fluid dynamics even at low concentration; for example, polymer concentrations of 100 parts per million (ppm) or less can enable mist control and drag reduction. The key to both mist control and drag reduction is the ability of polymers to store energy as they stretch, such that the fluid as a whole resists elongation. The high potency of ultralong linear polymers is due to the onset of chain stretching at low elongation rates and to the chains’ high ultimate conformational elongation. For example, increasing Mw from 50 kg/mol to 5000 kg/mol decreases the critical elongation rate by more than three orders of magnitude and increases the ultimate molecular elongation by two orders of magnitude.

Unfortunately, ultralong backbones undergo chain scission during routine handling because hydrodynamic tension builds up along the backbone to a level that breaks covalent bonds; this “shear degradation” continues until the chains shorten to a point that their valuable effects are lost (M_w < 1000 kg/mol). Self-assembly of end-associative polymers creates supramolecules that can potentially break and reassociate reversibly, but formation of megasupramolecules (M_w ≥ 5000 kg/mol) at low concentration has never been realized for two reasons: (i) End-to-end association at low concentration predominantly leads to rings of a small number of chains (5), and (ii) the size of the building blocks is limited because end association is disfavored when they are larger than 100 kg/mol.

… Our goal is to create megasupramolecules at low concentration that behave like ultralong polymers, exhibiting expanded (“self-avoiding”) conformation at rest and capable of high elongation under flow. This is in contrast to the collapsed, inextensible supramolecules formed by long chains with associative groups distributed along their backbone. To mimic ultralong polymers, association must occur at chain ends and be predominantly pairwise.

—Wei et al.

The new additive, created in the laboratory of Julia Kornfield, professor of chemical engineering, consists of long telechelic polymers (LTPs) capped at each end by units that act like Velcro. (Telechelic polymers are capable of entering into further polymerization or other reactions through their reactive end-groups.) The individual polymers spontaneously link into the ultralong chain megasupramolecules.

Megasupramolecules, Kornfield says, have an unprecedented combination of properties that allows them to control fuel misting, improve the flow of fuel through pipelines, and reduce soot formation. Megasupramolecules inhibit misting under crash conditions and permit misting during fuel injection in the engine.

Although supramolecules also detach into smaller parts as they pass through a pump, the process is reversible. The Velcro-like units at the ends of the individual chains simply reconnect when they meet, effectively healing the megasupramolecules.

When added to fuel, megasupramolecules significantly affect the flow behavior even when the polymer concentration is too low to influence other properties of the liquid. For example, the additive does not change the energy content, surface tension, or density of the fuel. In addition, the power and efficiency of engines that use fuel with the additive is unchanged—at least in the diesel engines that have been tested so far.

The supramolecules spend most of their time coiled up in a compact conformation. When there is a sudden elongation of the fluid, however, as in an impact, the polymer molecules stretch out and resist further elongation. This stretching allows them to inhibit the breakup of droplets under impact conditions—thus reducing the size of explosions—as well as to reduce turbulence in pipelines.

The concept of using ultralong polymers as fuel additives was tested in a full-scale crash test of an airplane in 1984. The plane was briefly engulfed in a fireball, generating negative headlines and causing ultralong polymers to quickly fall out of favor, said research scientist and co–first author Ming-Hsin “Jeremy” Wei.

In 2002, Virendra Sarohia at JPL sought to revive research on mist control in hopes of preventing another attack like that of 9-11, and contacted Kornfield, the corresponding author on the new paper. The first breakthrough came in 2006 with the theoretical prediction of megasupramolecules by Ameri David, then a graduate student in her lab. David designed individual chains that are small enough to eliminate prior problems and that dynamically associate together into megasupramolecules, even at low concentrations. He suggested that these assemblies might provide the benefits of ultralong polymers, with the new feature that they could pass through pumps and filters unharmed.

When Wei joined the project in 2007, he set out to create these theoretical molecules. Producing polymers of the desired length with sufficiently strong “molecular Velcro” on both ends proved to be a challenge. With the help of a catalyst developed by Robert Grubbs, the Victor and Elizabeth Atkins Professor of Chemistry and winner of the 2005 Nobel Prize in Chemistry, Wei developed a method to precisely control the structure of the molecular Velcro and put it in the right place on the polymer chains.

Simon Jones, an industrial chemist now at JPL, helped Wei develop practical methods to produce longer and longer chains with the Velcro-like end groups. Co–first author and Caltech graduate student Boyu Li helped Wei explore the physics behind the exciting behavior of these new polymers.

Joel Schmitigal, a scientist at the US Army Tank Automotive Research Development and Engineering Center (TARDEC) in Warren, Michigan, performed essential tests that put the polymer on the path toward approval as a new fuel additive.

Looking to the future, if you want to use this additive in thousands of gallons of jet fuel, diesel, or oil, you need a process to mass-produce it. That is why my goal is to develop a reactor that will continuously produce the polymer—and I plan to achieve it less than a year from now.

—Ming-Hsin Wei

The work was funded by TARDEC, the Federal Aviation Administration, the Schlumberger Foundation, and the Gates Grubstake Fund.

Resources

• Ming-Hsin Wei, Boyu Li, R. L. Ameri David, Simon C. Jones, Virendra Sarohia, Joel A. Schmitigal, and Julia A. Kornfield (2015) “Megasupramolecules for safer, cleaner fuel by end association of long telechelic polymers” Science 350 (6256), 72-75 doi: 10.1126/science.aab0642