Abstract
Mixed-matrix membranes (MMMs) consisting of an ortho-hydroxy polyamide (HPA) matrix, and variable loads of a porous polymer network (PPN) were thermally treated to induce the transformation of HPA to polybenzoxazole (β-TR-PBO). Two different HPAs were synthesized to be used as a matrix, 6FCl-APAF and tBTpCl-APAF, while the PPN used as a filler was prepared by reacting triptycene and trifluoroacetophenone. The permeability of He, H2, N2, O2, CH4 and CO2 gases through these MMMs are analyzed as a function of the fraction of free volume (FFV) of the membrane and the kinetic diameter of the gas, allowing for the evaluation of the free volume. Thermal rearrangement entails an increase in the FFV. Both before and after thermal rearrangement, the free volume increases with the PPN content very similarly for both polymeric matrices. It is shown that there is a portion of free volume that is inaccessible to permeation (occluded volume), probably due to it being trapped within the filler. In fact, permeability and selectivity change below what could be expected according to densities, when the fraction of occluded volume increases. A higher filler load increases the percentage of inaccessible or trapped free volume, probably due to the increasing agglomeration of the filler. On the other hand, the phenomenon is slightly affected by thermal rearrangement. The fraction of trapped free volume seems to be lower for membranes in which the tBTpCl-APAF is used as a matrix than for those with a 6FCl-APAF matrix, possibly because tBTpCl-APAF could approach the PPN better. The application of an effective medium theory for permeability allowed us to extrapolate for a 100% filler, giving the same value for both thermally rearranged and non-rearranged MMMs. The pure filler could also be extrapolated by assuming the same tendency as in the Robeson’s plots for MMMs with low filler content.
Highlights
Mixed-matrix membranes (MMMs) have emerged as promising materials for gas separation in membrane technology
MMMs benefit from the potential synergy between the polymeric m2 aoftr14ix and the fillers, which enhances the properties of MMMs compared to those of the pure polymer [5] and exhibits a superior performance in terms of gas permeability and/or selpechtaisveit[y1–[64]
The results for the pure filler would not depend on whether data corresponding to thermally rearranged or non-thermally rearranged MMMs were used, as far as we can assume that the porous polymer network (PPN) is not substantially affected by the thermal treatment
Summary
Mixed-matrix membranes (MMMs) have emerged as promising materials for gas separation in membrane technology. -Ma-ocoargee can be aantdtrKibourotesd[10to] otbhseerdveed-wtheatttisnoglveonft tehveappooraltyiomne, rthicercmhaalienfsfeoctns atnhde thexetreersnualtlinsgusrtfraecsesesof the paratticthleesp[o7l]y.mMero–ofirlelear nindteKrfoacreosca[u1s0e] doebfseecrtsvseudchthaastinsotelrvfeacnetveoviadpfoorrmataitoionn, .thTehremfoarlmeaftfieocnts and theofrethsueslteindgefesctrtsesaslleoswasttthheegpasoelsytmo epra–sfsilalnerd,inhteenrcfea,cdeectearuiosreatdeesftehcetsapsupachrenatssienletectrifvaictye void foraomnf dathtiienocpnro.elaTyshmeesetrfhoicermmpeaarttmriioxenaabnoildfitttyhhoeefsfieMlldeMer,Mfgeiscv.tisTnhaglelrsoieswefastcottohleresakgcayansiengstievtreofaapcneasisn.scaonmdp,lehteendceeta, cdhemteerniot rates the apparent selectivity and increases the permeability of MMMs. forAmAalotiwloonwaodafhdnehosenis-oisonenlebbcetetitvwweeeveeonnidtthsheienddtihisspepeienrsrteesderfdaacnaidanldtrheteghiceoonnc.otOinnttuhioneuruscoapuuhssaepssehcsaocsnoetusrilbcdouluteianlddg ltteooattdhheeto the forfmoramtiaotnionofonf ionnte-srfealceicatlivvoeidvsoaidrestihnetahlteerinattieornfaocfitahlerpeogliyomn.erOptahcekrincgauinsethsecvoinctinriibtyuotifnthgeto the fordmisapteiorsnedopf ainrttieclrefsa,ctihael rvepouidlssivaerefotrhceebaelttwereaentiothne otwf othpehapsoelsyamnderthpeadcikffienrgenitncotehfeficvieicnitnsity of theofdtihsepremrsael dexppaanrstioclnes[8, ].thMeoreopvuelrs,iivneterffoarccieal bveotidwseoernsitehveest-win-oa-pcahgaesceasnabnedattrhiebudteifdferent coetfofitchieendets-woeftttihnegromf athleepxoplaynmseiorinc c[h8a]i.nMs oonrethoevexr,teirnntaelrsfuacrfiaaclevoofitdhse oprarsticelveses[7-i]n. The manufacture of highperformance MMMs depends on the appropriate filler selection to prevent the formation of non-selective voids caused by the low polymer–filler affinity [3,16] In this sense, metal– organic frameworks (MOFs) with high surface area and porosity [17], covalent organic frameworks (COFs) or porous aromatic frameworks (PAFs) with large surface areas and thermal stability [18,19], and hypercrosslinked polymers (HCPs) with significant potential for CO2 adsorption [20], have been successfully used as fillers in MMMs for gas separation. This work analyzes these correlations and proposes a simple model for permeability in terms of free volume fraction with two hydroxy polyamides (HPAs) and their thermally rearranged β-TR counterparts
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