專題演講

2018/01/23(Tue)     14:00 -15:00    1st Floor(auditorium)

Title

Megasupramolecules

Speaker

Prof. Julia A. Kornfield

Department of Chemical Engineering, California Institute of Technology

Abstract

Ultralong polymers (weight-average molecular weight Mw ≥ 5000 kg/mol) exhibit striking effects on fluid dynamics even at low concentration. For example, at 100 ppm, they can control mist and reduce turbulent drag, thanks to their ability to store energy as they stretch, such that the fluid as a whole resists elongation. 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 their valuable effects are lost (Mw < 1000 kg/mol). After 9/11, we took a fresh look at polymers for mist control in the hope of discovering a polymer that might make it impossible to use a jumbo jet to bring down a skyscraper. For use in fuel, the polymer would need to be effective at very low concentration. Prior literature showed that self-assembly of end-associative polymers creates supramolecules that break and reassociate reversibly, but formation of ultra-long supramolecules at low concentration had never been achieved. Statistical mechanics showed us how to design polymers that self-assemble into “megasupramolecules” (≥5000 kg/mol) at low concentration (≤0.3%wt). Theoretical treatment of the distribution of individual subunits—end-functional polymers—among cyclic and linear supramolecules predicts that megasupramolecules can form at low total polymer concentration if, and only if, the backbones are long (>400 kg/mol) and end-association strength is optimal. Viscometry and scattering measurements of long telechelic polymers having polycyclooctadiene backbones and acid or amine end groups verify the formation of megasupramolecules. They inhibit misting (the cause of fuel fireballs) and reduce drag in the same manner as ultralong covalent polymers. With individual building blocks short enough to avoid hydrodynamic chain scission (weight-average molecular weights of 400 to 1000 kg/mol) and reversible linkages that protect covalent bonds, overcoming the obstacles of shear degradation and engine incompatibility. Megasupramolecules confer a variety of benefits beyond mist control and are currently being evaluated for commercialization. (paper)