(51a) Mega-Supramolecules for Safer, Cleaner Fuel By End-Association of Long Telechelic Polymers
After the Tenerife airport disaster in 1977, many millions of dollars were spent on research to find a polymer to control fuel misting in crashes. Although the field had been essentially dormant since 1986, the events of 9/11/2001 motivated us to renew the search for polymers that could prevent it from happening again. Until now, polymers for mist control and drag reduction were exclusively ultra-long (and, consequently, ultra fragile) polymers. We envisioned “mega-supramolecules” having molecular weight similar to ultra-long polymers but with reversible physical links along the backbone. The reversible links were designed to be much stronger than thermal energy to provide rheological properties like those of ultra-long polymers and to be much weaker than a covalent bond, so that they could relieve tension before chain scission occurs. Contrary to widely held expectations, we show that, despite being 10-times longer than conventional telechelics, "long telechelic polymers" (LTPs) are still able to associate. The statistical mechanics of ring-chain equilibrium indicated that assembly of mega-supramolecules is feasible—if the individual polymeric units are very long telechelic polymers.
The present results show that a solution of 500,000 g/mol chains at low concentration can control post-impact mist and reduce turbulent drag—without degrading in the intense flow conditions that fuels routinely experience (megasupramolecules reassemble after passing through pumps, filters and turbulent flow). Even at low concentration (0.3% or less), they form multimillion molecular weight flexible supramolecules—“mega-supramolecules." In liquid fuels (particularly diesel and jet fuel), megasupramolecules confer properties that are relevant to security, transportation safety, energy conservation and air quality. In addition to extensive fundamental results regarding the synthesis and thermodynamics of very long end-associative chains (including neutron scattering and rheological evidence for the formation of mega-supramolecules), we present engine test and post-impact flame propagation results. In contrast to prior mist-control polymers, these mega supramolecules do not adversely affect engine power or efficienty: The ability of the supramolecules to temporarily fall apart in intense flow conditions apparently allows the to fuel to atomize graciously in the engine. The most significant effect of LTP in diesel engines was a 15% reduction in soot formation. Megasupramolecules also provide drag reduction that lasts: the polymers survive flow through pumps and turbulent pipelines.
Thus, the end-to-end associations cohere well enough to confer benefits typically associated with ultra-long polymers—including mist control and drag reduction, and reversibly dissociate under flow conditions that would break covalent bonds, allowing the individual LTPs to survive pumping and filtering—and allowing treated fuel to burn cleanly and efficiently in unmodified diesel engines. From this we infer that, inhibition of post-crash drop breakup and pipeline turbulence can be achieved at hydrodynamic forces at which the physical junctions remain intact; whereas, atomization in the engine occurs with forces that pull apart the reversible physical links in the mega-supramolecules. From Macosko's "two-step ring-opening metathesis" route to telechelic polymers, the synthesis of LTPs is amenable to scale-up (projected cost per gallon of fuel treated ca. 5-10¢/gal). After a 30-year hiatus in polymer research to improve fuel safety and stewardship, LTPs inspire renewed interest because they represent a class of polymers that reduce pumping costs and improve transportation safety and are compatible with unmodified engines and routine fuel handling.