Physical Chemistry Student Seminar Series Crossover Behavior of Gas Transport in Polymer Melts: A Molecular Simula<on Approach to the Robeson Plot Abstract Dr. Kai Zhang (Kumar Group) Tuesday, 3/7/17 at 5:30 pm Miller Seminar Room, 328 Havemeyer Abstract: Efficient and selec-ve transport of small gas molecules using lightweight and easily processable polymeric membranes is crucial in many clean energy technologies. For a pair of wanted (A) and unwanted (B) gases, the performance of membrane separa-on is characterized by the permeability of A (P A ) and the selec-vity of A with respect to B (α AB =P A /P B ). Empirically, the so-called Robeson plot, i.e. the scaHer plot of α AB versus P A , exhibits a tradeoff rela-on, whose upper bound sets the op-mal membrane separa-on limit. Simple thermodynamic and kine-c argument like the Freeman theory predicts that the slope magnitude (λ) of the upper bound is purely determined by the molecular size ra-o as λ=(d B /d A ) 2 -1. However, systema-c studies of the Robeson plot on the molecular level and a quan-ta-ve understanding of the microscopic mechanisms that control the separa-on ability are s-ll missing. We perform molecular dynamics simula-on to calculate the permeability of model gas molecules in coarse-grained polymer melts, from which the corresponding Robeson plot is constructed. We observe a crossover behavior of gas transport as the gas size is tuned from below to above approximately twice the monomer size. The Freeman predic-on is only valid in the large gas size limit. Further inves-ga-ons on the gas dynamics show that its diffusion behavior changes from a geometric obstruc-on mechanism to an ac-vated trap-and-jump process. Plenty of pizza will be provided.

Crossover Behavior of Gas Transport in Polymer …...Polymer Melts: A Molecular Simula

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Page 1: 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.