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DOI: 10.1002/ijch.201200081

Tuning Gas-Diffusion through Dynameric Membranes:Toward Rubbery Organic Frameworks (ROFs)

Gihane Nasr,[a] Arnauld Gilles,[a] Thomas Macron,[a] Christophe Charmette,[a] Jose Sanchez,[a] andMihail Barboiu*[a]

Among the industrial methods used for capturing CO2

(absorption, distillation, etc.),[1] membrane technolo-gy,[2–11] which offers advantages such energy saving,simple design and scale-up, is becoming continually moreprevalent. High permeability combined with reasonableselectivity is the most important goal in developing mem-branes for gas separation. This goal is usually achievedthrough the use of polymeric membranes, through whichgas transport is controlled by gas-diffusivity in glassypolymers and by gas-solubility in rubbery polymers. De

novo design of synthetic membrane materials like block-co-polymers,[2–5] polymeric composites,[6,7] mixed-matrixhybrids[8] and pseudo-microporous polymers[9–10] metal or-

ganic frameworks- MOFs

[1,11]

has been identified as anemerging area of interest. The combination or replace-ment of classical glassy polymers with crystalline MOFs,ZIFs, or zeolites provides molecularly controlled permea-bility and selectivity. However, attempts to obtain me-chanically stable and homogeneous layers on various sup-ports have been met with difficulty.

Taking advantage of the high permeabilities and flexi-ble casting properties observed for rubbery polymers, wedecided to build ROFs as new membrane separation sys-tems for gases. ROFs may provide premises for more finestructural interaction of diffusing gas molecules with mo-

lecular addressable domains. Minimizing the size of ultra-

dense addressable transporting domains[12–15] would makeit possible to improve the limits of interaction of gas mol-ecules with percolated conductive domains with high dif-fusional behaviors.[16–19] Such an improvement is specifi-cally of interest to membrane scientists (Figure 1).

Furthermore, the size of addressable elementary do-mains for the diffusion of gas molecules is reminiscent of the situation where pixel size determines the quality of resulting images in LCD devices. Within this context, wepreviously showed that dynamic covalent polymers,[15] ordynamers,[20–23] generated from reversibly interactingmonomers, offer the possibility to address these

issues.

[24,25]

In dynamers, the components are reversiblyconnected, and they self-assemble in such a fashion thattheir overall morphology overrides defects during the for-

Abstract: Obtaining high permeability whilst keeping a rea-sonable selectivity is the most important challenge in thedevelopment of membrane systems for gas separation. Sat-isfactory performance is usually obtained with polymericmembranes through which gas transport is controlled bygas-diffusivity in glassy polymers and by gas-solubility inrubbery polymers. During the last decade, important advan-ces in this field have been made possible by molecular con-

trol of gas separation properties. The combination or re-placement of classical glassy polymers with metal-organiccrystalline frameworks (crystalline MOFs), such as zeoliticimidazolate frameworks (ZIFs) or other zeolites, providesreasonable permeability through the porous networksformed, and high selectivity, due to so-called ‘selectivity cen-ters’, which interact specifically with the gas molecules. De-spite impressive progress, difficulties in obtaining homoge-neous, mechanically stable, thin layer MOFs on various sup-

ports have been encountered. Given the observed high per-meabilities of rubbery polymers and their flexible castingproperties, it should be very interesting to build rubbery or-ganic frameworks (ROFs), as alternative materials for gasmembrane separation systems. Here we use low macromo-lecular constituents and dialdehyde core connectors, inorder to constitutionally generate ROFs. Distinct from rub-bery polymeric membranes, the performance of the ROFs

depends univocally on diffusional behaviors of gas mole-cules through the network. For all gases, a precise molecularcomposition of linear and star-type macromonomers gener-ates an optimal free volume for a maximal diffusion throughthe matrix. These results should initiate new interdisciplina-ry discussions about highly competitive systems for gas sep-aration, which are constitutionally controlled on a molecularscale.

Keywords: constitutional materials · dynamers · gas transport · membranes · self-assembly

[a] G. Nasr, A. Gilles, T. Macron, C. Charmette, J. Sanchez,M. BarboiuInstitut Europen des Membranes – ENSCM-UMII-CNRS5635 Place Eugne Bataillon CC 047F-34095 Montpellier, Cedex 5(France)e-mail: [email protected]

Supporting information for this article is available on the WWWunder http://dx.doi.org/10.1002/ijch.201200081.

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mation and orientation of conductive domains, under thepressure of internal structural stabilization. Such behaviorcould mediate the self-assembly of low-size addressabledomains, in which diffusional percolation pathways mightbe formed (Figure 1).

For all these reasons, in this study, nanometric macro-monomers and dialdehyde core connectors have beenused to conceive dense ROFs for the preparation of dy-nameric membranes for selective gas transport.[22–25] Theisophthalaldehyde (1), the bis(3-aminopropyl)-polytetra-hydrofuran (Mn~1100 gmol1, polyTHF, 2), and theglyceroltris[poly(propylene glycol), amine terminated]ether, (Mn~3000 gmol1, polyMePEG, 3) building blocksare the precursors of the dynamers 11-x · 2 · 3x, used to caststretchy membrane films (Figure 2).

Compounds 11-x · 2 · 3x (Figure 2a) generate dynamericmembranes based on three structural features: 1) The hy-drophobic linear polyTHF (1) has been used to generate

rigid crystalline phases considered to have low-permeabil-ity for gas transport. 2) The hydrophilic star-type polyMe-PEG (3) allows for high solubility of CO2, and thus con-tributes to the cross-linking of the dynameric network.The relative amount of 3 controls the free volume, andthus the permeability, of the membrane films. The pres-ence of methyl (Me) groups induces a lack of tacticity inthe macromonomeric chains, suppressing their crystalliza-tion. The network might be considered to contain combi-nations of linear and cross-linked arrays, resulting fromthe variable relative amounts of the linear polyTHF (1)and the star-type polyMePEG (3), which are intercon-

nected with the core connectors via reversible iminebonds. 3) The macromonomers might have a protectiveeffect against the hydrolysis of the imine bonds, favoringimine exchange. This would contribute to the implemen-tation of dynamic adaptive reversible rearrangements of the components, leading, during membrane preparation,to a high level of correlativity of the nanodomains. [19,23]

1H NMR analysis in CDCl3 allows easy identification of the peaks corresponding to total conversion to imino-compounds 11-x · 2 · 3x (Figure 1S). The elastomeric behav-ior of compounds 11-x · 2 · 3x is confirmed by their glasstransition temperature, Tg ~69 to 628. Interestingly,

the Tg values show a minimum at 33% of 3 (Figure 3d),indicating a high free volume of the matrix around thiscomposition, distinct from the compact matrix of linear1 and the highly cross-linked matrix of 3. The increasingTg values for high content of 3 follow the same pattern asthe crystallization temperature, Tc, which cannot be de-

tected for highly cross-linked polymers 11-x · 2 · 3x, x=

0.7or 1.0 (Figure 2S, Table 1S).The pure gases permeabilities for the 11-x · 2 · 3x rubbery

membranes as a function of % 3 mol/mol content areshown in Figure 3a. As a general trend for all gases mea-sured (He, O2, N2, and CO2), the permeability reachesa maximum around 33% of 3. The rubbery blends allowhigh permeabilities for the CO2 and interesting CO2/lightgas permselectivities (Figure 3a,b). Generally, PEG-typematerials exhibit very low permeability (~12 Barrers) dueto their high degree of crystallinity,[26] which can be dis-rupted in the presence of non-PEG rubbery polymers, re-sulting in the formation of solubility-driven selective

transport.To elaborate on this very sharp control of the permse-

lectivity of such rubbery dynameric membranes, a detailedoverview of their solubility/diffusivity selectivity is pre-sented in Figure 3c,d. First of all, a sorption analysis of CO2 was performed. The obtained sorption coefficients,1.26 to 1.87102 cm3(STP)cm3cmHg1, are in the samerange as previously reported.[2–8] Amazingly, over a rangeof 6.5–50% 3 the experimental sorption coefficientsremain practically constant (Figure 3c). Then, as the con-tent of 3 increases, these values strongly decrease towardvalues at the method detection limit.

These experiments confirm that the CO2 sorption is re-

lated to two opposite effects: with increasing concentra-tion of 3 in the polymeric blend, the rivalry between anincrease in the solubility of quadrupolar CO2 relative tothe polar polyMePEG chains of 3, on the one hand, andthe cross-linking behaviors of 3, on the other hand, keepsthe sorption coefficients nearly constant. The values of diffusion coefficients D, obtained from permeability ex-periments, illustrate that the diffusivity of CO2, whichreaches a maximum at ~33% of 3, is correlated with thefree volume of the dynameric network (Figure 3d). Theincrease of the free volume of the dynamer is most likelycaused by incorporation of 3, which causes the otherwise

compact and low-diffusive matrix of linear polyTHF tobecome less compact (Figure 3 e, right). This structuralbehavior is certainly related to a critical amount of thecross-linker 3, which generates the optimal spacing of thelinear polyTHF chains and allows gas molecules to dif-fuse through the film. More interestingly, the optimal per-formances are observed at 33% molar ratio of 3. Thismeans a molar ratio (1)/(3) of 2/1 mol/mol, for which theprobable pseudo-porous geometry of a highly diffusivematrix is responsible for the observed maximum values(Figure 2 b, middle). The free volume generated corre-lates with Tg values (Figure 3d), and is related to a strong

Figure 1. Converging the structural behaviors of block co-poly-mers toward ultradense rubbery organic frameworks-block co-dy-

namers, controlled at the molecular level (see text for details).

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increase in permeability, which can thus be mainly attrib-uted to optimal gas diffusivity and not to an increase insolubility, which remains almost constant over a largeconcentration domain (Figure 3c). Further addition of 3

generates a critical cross-linking of the matrix and prohib-

its diffusion/sorption of CO2 through the membrane. Itmay be concluded that CO2 permeation and selectivityare controlled by gas-diffusivity through rubbery dyna-meric membranes, which can be finely tuned at the nano-metric level.

Figure 2. a) Synthesis and b) schematic representation of chain packing of dynameric membranes combining polyTHF ( 1, red sticks) and

polyMePEG, (3, green ‘Y’ shapes), connected via isophthaldimine cores (2, blue circles). Structural diversity of matrices can be obtained:

(left) linear compact (high content of 2), (center) free volume matrix (maximum value of diffusivity at % 3=33%), and (right) highly cross-

linked (high content of 3). c) Images of self-standing dynameric membrane films with elastomeric behavior.

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Figure 3. Experimental profiles of a) pure gas permeabilities, b) pure CO2/N2 selectivities at 298 K and 1.0105 Pa, c) solubilities of CO2,determined by absorption and d) diffusivity of CO2, calculated by using the time-lag method, as a function of the molar ratio % 3= (1)/(3),

mol/mol, of components. e) Mechanistic illustration of diffusive controlled transport of CO2 through membranes of variable composition:

(left) linear compact (low content of 3), (center) mixed free volume composite (maximum value of diffusivity at % 3=33%, and (right)

highly cross-linked matrix (high content of 3).

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In conclusion, ROFs for dynameric membrane filmscan be rationally designed and synthesized for selectiveseparation of CO2. This example shows that gas transportthrough ROF-based membrane films can be controlled bydiffusion and mutual constitutional interactions betweenthe gas molecules and the dynameric network, at molecu-

lar level. For all gases, the molar ratio (1)/(3) of 2/1 mol/mol (33%) generates the optimal free volume matrix formaximum diffusion. The cross-linking component 3

pushes back the macromonomeric chains of 1 and ensuresa total free volume increase at 33% mol/mol of 3 in thepolymeric matrix, and thus the highest diffusion for allgases at this ratio. Moreover, due to structural behaviorsand an increased CO2-philic character of 3, the transportof CO2 is strongly favored and is controlled by both in-creasing CO2 sorption (solubility) and diffusivity.

Based on this discovery, one could imagine a fundamen-tal transition from macromolecular design toward consti-

tutional approaches,[27–29] which might push the limits and

achieve the molecular limit of gas permeable membranes.Finally, the rubbery dynameric membranes presentedhere allow a maximum permeability of 180 Barrers forCO2 and an interesting PCO2

/PN2=16 permeselectivity. For

industrial applications, higher CO2 permeability is mosteffective at reducing costs. Relying on higher selectivity,instead, would require large membrane areas. Membraneswith high CO2 permeance (1000 Barrers) are required,while a selectivity of 20–40 is enough for industrial pur-poses.[30]

Within this context, the ROF-based membranes reportedhere show a strong potential. Prospects for the future in-clude the development of these ROFs as an alternative to

high performing MOFs in the design of novel dynamic sys-tems presenting a greater degree of structural complexity.These results should initiate new interdisciplinary discus-sions about highly competitive systems for gas separation,which are constitutionally controlled at the molecular scale.

Acknowledgements

This work was conducted as part of DYNANO, PITN-GA-2011-289033 (www.dynano.eu) and ANR 2010BLAN 717 2.

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Received: October 31, 2012Accepted: November 21, 2012

Published online: February 7, 2013

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