crossover behavior of gas transport in polymer …...polymer melts: a molecular simula

Physical Chemistry Student Seminar Series

CrossoverBehaviorofGasTransportinPolymerMelts:AMolecularSimula<onApproachtotheRobesonPlot

Abstract Dr. Kai Zhang

(Kumar Group)

Tuesday, 3/7/17 at 5:30 pm

Miller Seminar

Room, 328 Havemeyer

Abstract: Efficientandselec-vetransportofsmallgasmoleculesusinglightweightandeasilyprocessablepolymericmembranesiscrucialinmanycleanenergytechnologies.Forapairofwanted(A)andunwanted(B)gases,theperformanceofmembranesepara-onischaracterizedbythepermeabilityofA(PA)andtheselec-vityofAwithrespecttoB(αAB=PA/PB).Empirically,theso-calledRobesonplot,i.e.thescaHerplotofαABversusPA,exhibitsatradeoffrela-on,whoseupperboundsetstheop-malmembranesepara-onlimit.Simplethermodynamicandkine-cargumentliketheFreemantheorypredictsthattheslopemagnitude(λ)oftheupperboundispurelydeterminedbythemolecularsizera-oasλ=(dB/dA)2-1.However,systema-cstudiesoftheRobesonplotonthemolecularlevelandaquan-ta-veunderstandingofthemicroscopicmechanismsthatcontrolthesepara-onabilityares-llmissing.Weperformmoleculardynamicssimula-ontocalculatethepermeabilityofmodelgasmoleculesincoarse-grainedpolymermelts,fromwhichthecorrespondingRobesonplotisconstructed.Weobserveacrossoverbehaviorofgastransportasthegassizeistunedfrombelowtoaboveapproximatelytwicethemonomersize.TheFreemanpredic-onisonlyvalidinthelargegassizelimit.Furtherinves-ga-onsonthegasdynamicsshowthatitsdiffusionbehaviorchangesfromageometricobstruc-onmechanismtoanac-vatedtrap-and-jumpprocess. Plenty of pizza will be provided.