How Much Do Ultrathin Polymers with Intrinsic Microporosity Swell in

Subscriber access provided by UNIV OF CALIFORNIA SAN DIEGO LIBRARIES

Article

How Much Do Ultra-Thin Polymers With Intrinsic Microporosity Swell In Liquids? Wojciech Ogieglo, Bader S Ghanem, Xiao-hua Ma, Ingo Pinnau, and Matthias Wessling J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.6b06807 • Publication Date (Web): 13 Sep 2016 Downloaded from http://pubs.acs.org on September 17, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry B is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

How Much Do Ultra-Thin Polymers With Intrinsic Microporosity Swell In Liquids? Wojciech Ogieglo,† Bader Ghanem,‡ Xiaohua Ma,‡ Ingo Pinnau,∗,‡ and Matthias Wessling∗,† †DWI - Leibniz Institute for Interactive Materials, Forckenbeckstr. 50, 52074 Aachen, Germany ‡King Abdullah University of Science and Technology (KAUST), Advanced Membranes and Porous Materials Center (AMPMC), Al-Jazri Building 4, Thuwal, 23955-6900, Kingdom of Saudi Arabia E-mail: [email protected]; [email protected] Phone: +966 (0) 12-808-2406; +49 241 80 95488

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract As synthetic membrane materials, polymers with intrinsic microporosity (PIMs) have demonstrated unprecedented permeation and molecular separation properties. Here, we report swelling characteristics of sub-micron thick supported films of spirobisindanebased PIMs, PIM-1 and PIM-6FDA-OH, for six organic solvents and water using insitu spectroscopic ellipsometry. Surprisingly, PIMs swell significantly in most organic solvents with swelling factors (SF = hswollen/hdry ) as high as 2.5. This leads to the loss of the ultra-rigid character of the polymer and produces equilibrated liquid-like swollen films. Filling of the excess frozen-in fractional free volume with liquid was discovered next to swelling-induced polymer matrix dilation. Water hardly swells the polymer matrix but penetrates into the intrinsic microporous structure. This study provides the first fundamental PIMs swelling data leading to better comprehension of their permeation properties. Such understanding is indispensable for applications such as solvent filtration, natural gas separation and ion retention in flow batteries.

Introduction Polymers with intrinsic microporosity (PIMs) are glassy polymers having remarkable amounts of frozen-in non-equilibrium excess free volume of up to 25 percent. PIMs have gained significant attention in recent years as new materials for molecular separations in membrane processes.1–4 PIMs are characterized by rigid backbones containing sterically hindered contortion sites, which frustrate molecular packing leading to extraordinarily high excess free volume fractions. Such properties are usually not found in typical solution-processable glassy polymers. As a result, PIMs possess unusually high gas permeabilities while retaining excellent selectivities, 5 far beyond the upper limits as defined by Robeson et.al.4

The vast majority of current membrane processes utilizes only a few of relatively low free volume polymers such as cellulose acetate, polysulfones or polyimides.6 In comparison to PIMs, these materials possess significantly lower permeabilities which limit process efficiencies. In ACS Paragon Plus Environment

Page 2 of 26

Page 3 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the rapidly growing expansion of the industrial membrane field to new areas, such as removal of condensable hydrocarbon vapors from natural gas streams,6,7 the potentially high permeabilities of PIMs could result in minimizing membrane area requirements. Very recently, PIM-1 − the most prominent and detailed studied material of the PIMs class, has shown promising properties as an efficient and stable material for organic solvent nanofiltration (OSN) enabling catalyst recovery in homogeneous catalysis8 or for removal of butanol from aqueous solutions by pervaporation in fermentation based butanol production.9 In addition, applications of PIMs are not limited to membrane technology. Recently, these polymers have also been used as sensors,10 gas adsorbers,11 and in catalysis. 12 Very surprisingly, we recently discovered that the hydrophobic PIM-1 transported protons through water-filled free volume while larger hydrated ions were retained.13 Comprehensive understanding of the swelling behaviour of such PIMs in different solvents is therefore indispensable.

In contact with liquids, polymers are known to swell depending on the interaction with sol-



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

vent. The swelling extent and its kinetics range from classical Fickian diffusion to swelling stress induced deviations therefrom.14,15 Membrane swelling is a direct consequence of component solubility which in turn, together with diffusivity, determines both membrane permeability and selectivity.6 While the interplay of solubility/swelling and diffusivity has been studied for at least four decades,14,16 the peculiarities of the swelling in PIMs are unexplored.

Swelling of PIMs is even more complex in comparison to the established glassy and rubbery materials because PIMs are non-equilibrium glassy materials with very high internal free volume. As a result ”inverse” transport behavior is often found in such high free volume polymers17–19 because they preferentially transport large molecules due the high solubility inside the free volume. In fact, molecular mixtures are essentially transported through the microporous polymer network rather than through the polymer matrix. Here, we investigated fundamentally to what extent PIMs, having exceptionally high excess free volume, swell in liquids and how the degree of swelling depends on the properties of the liquid.

Background Swelling factors of organic liquids in polymers (SF = hswollen/hdry ) of more than 2 are frequently reported. In fact, this renders the swollen membrane polymer a minor component in the mixture per volume. In the case of rubbery (or liquid) polymers, such as polydimethylsiloxane (PDMS), swelling can often be accurately described as ideal mixing of liquids. A useful thermodynamic description has been provided by the Flory-Huggins or Flory-Rehner theories (the former particularly for cross-linked materials such as PDMS).20,21 A key consequence is that for rubbery polymers volume additivity holds and thus the penetrant volume fraction can simply be calculated from the swelling factor as φpenetrant = 1-1/SF. For instance for a polymer swollen to SF = 2, the φpenetrant = 0.5. An important difference for


Page 4 of 26

Page 5 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


glassy polymers, including PIMs, is that during swelling the excess free volume may be partly occupied (for low SF ) or even fully relaxed during penetrant-induced glass transition if a resulting mixture produces equilibrium liquid (for high SF ). Therefore, simply calculating solvent volume fraction from SF always carries a significant error (underestimation) which causes a small value of partial molar volume of solvent in the polymer. Theoretical handling of the sorption in glassy polymers has been significantly developed in the recent years and other models beyond Flory-Huggins have been introduced.22–24

The errors are particularly severe for superglassy polymers with high free volume fractions, such as PIMs or PTMSP.25 For instance, if a glassy polymer possessing 25% microporosity (excess free volume typical for PIMs) is swollen to SF = 1.4, the actual penetrant volume fraction would be about 50% larger than calculated from the simple liquid-liquid mixing rule (as for rubbery polymers). This behavior corresponds to unusually low penetrant partial molar volumes in PIMs, in some cases approaching 0 if the polymer hardly swells (e.g. PIM-1 in water). The mismatch between the low penetrant partial molar volumes and the actual mixture composition may have significant consequences in understanding, predicting and modelling membrane processes involving PIMs as many mass transport rate equations require the quantification of the partial molar volume.26

For efficient separations, membranes are usually produced with thin (< 1 µm) defect-free selective layers. This may be achieved for instance by producing a composite architecture with the thin selective polymer film on top of a porous support substrate of different chemical structure. The lateral geometrical confinement of the thin film has significant consequences for its swelling behavior. Firstly, the penetrant-induced dilation of the material is only possible perpendicular to the substrate. This is different in the commonly used gravimetric methods where a bulk sample is able to swell in all 3 dimensions. In particular, supported thin layers need to dilate more to compensate for the lateral confinement which, especially



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 26

for large SF, produces a different free energy state in terms of strain induced on polymer chains.15,21

Secondly, it has been established that ultra-thin glassy polymer films may deviate from their thick counterparts in various important properties, such as glass transition temperature Tg or physical aging rate. These phenomena indeed become relevant for synthetic membranes having a selective layer thickness smaller than (< 1 µm). Therefore, it is also indispensable to obtain the physical properties of materials making up the separation layer of a practical membrane. This can be done by analyzing the ultra-thin substrate-confined layers, preferably in-situ in contact with the penetrant.27

Recently, we successfully demonstrated an in-situ study of thin glassy polymer films with spectroscopic ellipsometry.25,27,28 This non-invasive optical technique allows for an independent determination the film thickness as well as its refractive index. As a result it is possible to distinguish the effects of polymer dilation caused by the penetrant from the change in its density, which is often influenced by the occupation of accessible free volume sites. We hypothesize here that the latter effect is particularly prominent in PIMs.

The organic solvent-induced swelling was investigated for two structurally different spirobisindanebased PIMs: (a) the classical ladder-type polymer PIM-1 and (b) a polyimide PIM-6FDA-OH (structures given in Supporting Information). All films were prepared in supported geometry on silicon wafers and had thicknesses in a technologically relevant range of 50 - 100 nm. Both PIMs significantly swelled in most solvents and underwent penetrant-induced glass transition producing thermodynamically equilibrated liquid-like mixtures. Penetrant volume fractions were calculated including the solvent-induced relaxation of the excess free volumes. These values are significantly larger as compared with values simply calculated from φpenetrant = 1− 1/SF, where relaxation of free volume is not included. The generated data are anticipated


Page 7 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


to aid in the evaluation of PIMs for membrane processes involving organic solvents.

Results and discussion

Figure 1: Swelling factors of PIM-1 (a) and PIM-6FDA-OH (b) and the corresponding refractive indices (c and d) in 6 organic solvents and water. All films were in a range of 53-72 nm dry thickness and rejuvenated with n-hexane prior to swelling measurements. PIM-6FDA-OH dissolved in methyl ethyl ketone and acetone. Dry refractive indices are indicated as dashed horizontal lines. All PIMs films used for the solvent swelling experiments were solvent-rejuvenated (n-



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

hexane) to reset the aging. This procedure has been found crucial to assure adequate reproducibility of the results. The calculated swelling factors (SF = hswollen/hdry ) for PIM-1 and 6FDA-PIM-OH for six organic solvents and water using the spectra obtained from ellipsometry are shown in Figure 1 a and b . In almost all cases, apart from water and for solvents actually dissolving the polymers, thickness equilibration occured very quickly (within ≤ 10 min). As reported by other authors,29,30 aromatic toluene swelled PIM-1 more than aliphatic n-hexane. As discussed earlier, due to lateral confinement the entire polymer dilation is only possible in one direction: perpendicular to the substrate, as opposed to bulk, 3D swelling where the relative change of sample dimensions would be smaller. 15 For instance, it follows from simple geometrical considerations (ignoring effects of elastic contributions to free energy,21 because PIM-1 is not chemically cross-linked) that swelling of a 1D-confined thin film to SF = 2 would correspond to a dilation by a factor of 21/3 = 1.26 along each of the spacial dimensions of an equivalent 3D sample.

Previously, we demonstrated that at the same temperature as in this study (c.a. 22 ◦C) swelling factors of ∼1.2 were sufficient to induce glass transition in thin supported films of swollen polystyrene.14 Here, all organic solvents swelled PIMs in a range from 1.35 to more than 2.5. Based on the theory of Chow et al. on the depression of Tg in polymer-diluent systems31 for PIM-1 with an estimated Tg around 450 ◦C32 the measured penetrant volume fractions should result in the depression of the mixture Tg much below room temperature. This, together with an almost instantaneous equilibration, strongly suggests that in a mixture with liquid the PIMs are no longer glassy, but rubbery, i.e. liquid-like. Achieving a rubbery state in these PIMs in the absence of a solvent by simply heating the polymers to high temperatures is not possible because thermal decomposition occurs before sufficient mobility of the rigid polymer chains is reached. On the other hand, the limited swelling of the hydrophobic PIM-1 in water and the slow equilibration suggest, that water-exposed PIM-1 remains very rigid and does not plasticize enough to become rubbery. In contrast,


Page 8 of 26

Page 9 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


more hydrophilic PIM-6FDA-OH swelled significantly in water (SF = 1.25) and equilibrated rather quickly. In this case, however, a certain degree of matrix rigidity may be preserved, as the magnitude of swelling corresponds somewhat to a transition range between glassy and rubbery states. In fact, the more polar solvents (water, ethanol, isopropanol) swelled the PIM-6FDA-OH more than PIM-1.

Figure 1 c and d shows the kinetics of the refractive indices of the swollen films. Refractive indices of dry samples prior to swelling experiments (ndry ) are also indicated as vertical dashed lines. In consistency with the kinetics of the SF, the equilibrium of nswollen was typically reached on similar timescales. Interestingly, some of the polymer/solvent mixtures’ refractive indices increased above ndry while others decrease below ndry . We interpret this as a direct consequence of an interplay between matrix dilation and occupation of the accessible excess free volume. To understand why, the Bruggemann effective medium approximation (EMA)33 was used. For a two-component mixture (e.g. polymer and solvent) the refractive indices can be linked with volume fractions as:

2

2

2

2

φ1 n21 − mix + φ2 n22 − mix =0 2 2 + 2n + 2n 1 mix 2 mix

(1)

where φ1 = 1 - φ2. Equation 1 implies that a mixture should have a refractive index inbetween that of pure components. This holds well for mixtures of rubbery polymers and solvents (like PDMS and n-hexane14). Here, however, for lower swelling factors, e.g. water, n-hexane, ethanol for PIM-1 and n-hexane, toluene for PIM-6FDA-OH, the refractive indices rose above the refractive index of the dry polymers. This counter-intuitive effect, also observed by Rowe et al.,34 but to a much smaller extent for water vapor sorption in polysulfone and Matrimid, is a consequence of the existence of particularly large excess free volume fractions in the materials. A dry PIM sample can be viewed as a mixture of voids



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 26

(representing excess free volume with n = 1 and volume fraction of fv ) and polymer matrix (nmatrix). Using the well-known group contribution methods, according to Bondi35 for van der Waals molar volumes and Krevelen36 for molar refraction it is possible to obtain estimates for the nmatrix of 1.676 for PIM-1 and 1.602 for PIM-6FDA-OH (Supporting Information). These values are also shown in Figure 1c and d. As a consequence of the very large excess free volumes the nmatrix values for both PIM-1 and PIM-6FDA-OH are far higher than ndry . When nmatrix values are substituted into Equation 1 the results are fv = 0.259 ± 0.029 and 0.143 ± 0.007, respectively, calculated from 5 spots of 6 separate, dry, freshly rejuvenated PIM samples. The obtained fv values are well within the range expected for PIMs (for PIM-1 above 0.2), although more precise data is lacking in the literature due to a very strong history and treatment effects which all influence the ultra-glassy polymers’ state.

In cases where during moderate swelling the penetrant occupies the excess free volume of the polymer the refractive index of a mixture may rise above ndry approaching nmatrix. This is similar to the often invoked ”Langmuir-type” of sorption within free volume elements of polymer matrices, where the density of the polymer-penetrant mixture increases above the density of the pure polymer. The corresponding increases in the refractive index are seen in some cases in Figure 1 c and d. The swollen polymer now needs to be considered a mixture of a polymer matrix with solvent rather than dry polymer with solvent. In the cases where nswollen ≤ndry the effect of polymer matrix dilation, related to ”Henry-type” of penetrant dissolution within the matrix, is more prominent and the nswollen is able to drop below ndry . It is important to note, that for both cases reported here, that is, when nswollen ≥ndry and when nswollen ≤ndry , it can be implied that there is no remaining excess free volume in the swollen PIMs. In other words, the kinetically trapped excess free volume is fully relaxed by the solvent. Hence, swollen polymers comprise liquid-like equilibrium mixtures. The interplay between the magnitudes of SF and npenetrant determines if nswollen will be greater or smaller than ndry . From Figure 1 it is clear that for the lower SF typically nswollen ≥ndry


Page 11 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


and vice versa.

For water with PIM-1 the equilibration took much longer than in all other cases and, while the swelling factor reached rather low values of ∼6%, nswollen increased very significantly. This clearly indicates, that water did not dilate the rigid hydrophobic PIM-1 matrix significantly, but rather slowly diffused through accessible excess free volume and then filled it to a large extent. On the other hand, for the more hydrophilic PIM6FDA-OH significant swelling in water (SF = 1.25) is accompanied with nswollen staying virtually identical to ndry , indicating that both effects, matrix dilation and excess free volume filling, occur simultaneously. The slow, but persistent, dynamics of nswollen visible in Figure 1d hint towards only partial plasticization of PIM-6FDA-OH with water and in this case the polymer matrix remains relatively rigid, as noted earlier. In the case of PIM-1 with toluene a distinct pro- gressive reduction of SF is accompanied by relatively constant refractive index. This clearly indicates slow dissolution of the polymer into this solvent, because the film density (n) stays similar while less and less film remains (SF).

The relaxation of the excess free volume in a glassy polymer by the solvent has significant consequence on the accuracy of the calculated penetrant volume fraction. This is schematically shown in Figure 2a. For rubbery polymers volume additivity can usually be assumed. Thus, φpenetrant can be simply calculated as mentioned earlier:

φpenetrant =

hswollen − hdry = 1 − 1/SF hswollen

(2)

Because of the simplicity of the method it has frequently been used in the literature not only for rubbery polymers, but also for glasses.37–39 Its use in glassy polymers, however, inherently carries an error related to the contribution of the excess free volume, viewed alternatively



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 26

Figure 2: Scheme on calculation of penetrant volume fraction in initially rubbery and glassy polymers (a); for the glassy polymers it is assumed that the mixture with solvent is an equilibrium liquid (no longer glassy); (b) penetrant volume fractions in PIM-1 and PIM6FDA-OH for all used solvents, fv are 0.259 ± 0.029 and 0.143 ± 0.007, respectively, as calculated from a combination fo Bruggeman EMA (equation 1) and group contribution methods.35,36

as a reduced partial molar volume of the penetrant as compared to the volume additivity approach.40 As discussed earlier, for a glassy polymer with a certain fv , the volume fraction of the penetrant in equilibrium mixture will be larger than simply calculated as 1 - 1/SF. Here, it is helpful to consider a hypothetical equivalent of a glassy polymer with hmatrix representing the dense polymer matrix. Considering the swollen polymer, it follows:

φpenetrant =

hswollen − hdry (1 − fv) = 1 − 1/SF + fv/SF hswollen

(3)

The addition of a simple term fv/SF accounts for the excess free volume in swollen glassy polymers. The penetrant volume fractions are plotted versus SF in Figure 2b with and without the implementation of the correction for fv . For fv we used the estimated values of 0.259 ± 0.029 and 0.143 ± 0.007 for PIM-1 and PIM-6FDA-OH, respectively. As expected


Page 13 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


for PIMs due to large fv the corrected values are significantly larger. In particular, at low SF the impact of fv is especially pronounced. For instance, liquid water only dilated PIM-1 by about 6%; however, the water volume fraction inside the polymer film was as large as 30%. This agrees very well with a sharp increase of the polymer refractive index seen in Figure 1 c and d and indicates a large degree of free volume penetration by water in this hydrophobic polymer. As anticipated, the larger SF and the smaller the fv , the less impact relaxation of the excess free volume has on the value of penetrant volume fraction and the difference between both curves diminishes.

The significance of this analysis for membrane processes can be readily outlined. When used in moderately swelling solvents (like water, ethanol or n-hexane) the swelling of the membrane layer may be limited, however, the actual volume fraction of the solvent will be very significant. This will have consequences on the membrane performance where solubility effects play a crucial role.13

In principle, the Bruggeman EMA approach (equation 1) could be also directly used to calculate φpenetrant by mixing nsolvent, nmatrix and nswollen. In the case of PIMs, particularly PIM-1, this procedure was not effective because of inability to experimentally determine nmatrix. In our previous work a method for obtaining nmatrix by extrapolation of nmatrix = f(T) from above Tg yielded correct results.25 In the case of PIMs the polymers either fully or partly decompose while approaching Tg and thus such extrapolation is either invalid or carries a very large error. Thus in these cases the simple method based on equation 3 seems preferred given reasonable estimates for fv .

To overcome the need for an estimate for fv , one may want to combine the EMA equation 1 with equation 3. In this way, the information about the swollen mixture from the refractive index, eq. 1, and thickness increase, eq. 3, can be combined to yield an expression:



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(1 −

1 SF

+

fv SF

)

2 matrix

2 mi x

2

n2matrix + 2 nmix

1 + ( SF

2 2 fv solvent mix 2 =0 SF ) n 2 solvent + 2 nmix

Page 14 of 26

(4)

Because equation 4 has two unknowns, fv and nmatrix, by measuring swelling of the same polymer in two solvents of known nsolvent it would be possible to directly obtain information on the excess free volume fraction (fv ) as well as refractive index of polymer matrix (nmatrix). Unfortunately, this would require refractive index accuracy much greater, than the limit that can be achieved with ellipsometry (∼0.001). Considering equation 4 any small error in the index is amplified greatly by raising to the power 2, subtraction of two relatively small resulting numbers and finally division. Therefore, although valid theoretically we found this approach of limited practical applicability. To use the approach, we estimate, that the refractive index accuracy should be on the order of 10−5 or better.

Finally, Figure 3 shows the penetrant volume fraction versus the Hildebrand parameter for all used solvents and polymers. The data for PIM-1 display good agreement with literature values reported for bulk samples18 and show the characteristic increase from both sides of the region where either PIM-1 or PIM-6FDA-OH dissolve at around δ ≈ 18.5 to 18.7 and δ ≈ 18.5 to 19.7 MPa0.5, respectively. For solvents with δ below the dissolution region (hydrophobic n-hexane and toluene) large differences between PIM-1 and PIM-6FDA-OH are found. PIM-1 being more hydrophobic as compared to PIM-6FDA-OH possesses, as expected, much larger n-hexane and toluene volume fractions. The more hydrophilic the solvents are (δ above the dissolution region) the less difference in the penetrant volume fraction between the two polymers was found. We interpret this remarkable effect to be a consequence of an interplay between the swelling (matrix dilation) and occupation of the available excess free volume. For instance, PIM-1 hardly swells in water, but due to a large available excess free volume (fv = 0.259) water is able to penetrate into the rigid polymer matrix to a large extent. On the other hand, the more hydrophilic PIM-6FDA-OH, possessing only about half of the excess ACS Paragon Plus Environment

Page 15 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


free volume as compared to PIM-1 (fv = 0.143), was able to absorb similar volume fraction of water due to much more significant swelling related to dissolution within the polymer matrix occurring simultaneously with the excess free volume filling. Data shown in Figure 3 may be helpful in estimating the performance of the PIMs membranes for applications involving organic solvents. We hypothesise, that solvents with different δ values can be approximately located on the diagram by interpolation. Whether or not, this is true for solvent mixtures (incl. aqueous mixtures) is currently under investigation.

Figure 3: Penetrant volume fraction (calculated including the influence of excess free volume relaxation) versus Hildebrand solubility parameter for PIM-1 and PIM-6FDA-OH

Conclusions In conclusion, swelling and penetrant volume fraction of thin supported PIM-1 and PIM6FDA-OH were determined in six common organic solvents and water with film thicknesses below 100 nm. To limit the influence of physical aging all samples were solvent-rejuvenated prior to swelling experiments. It was found that PIMs swell very significantly in most stud-



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 26

ied solvents with swelling factors as high as 2.5 and reach equilibrium swelling within only 10 min. In most cases the rigid polymer matrices plasticized enough to produce fully liquid polymer-solvent mixtures. The applied methodology allowed distinguishing the effects of polymer matrix dilation and occupation of the excess free volume. The latter has been found particularly significant for moderate swelling degrees (≤ 1.7) where the index of the swollen polymer has risen above the index of a dry polymer. This counter-intuitive effect has been shown to directly reflect a balance between penetrant dissolution in the polymer matrix and occupation of the excess free volume. Consequently, if penetrant volume fraction in a swollen glassy after swelling with solvent is calculated simply using the dilation of the film, as frequently done in previous work, the values are significantly underestimated. This is of particular importance in high free volume PIMs. Our results are anticipated to aid in evaluation of PIMs for important membrane applications involving organic solvents.

Experimental details Materials and film preparation PIM-1 was synthesised as described earlier, by polycondensation of 5,51 ,6,61 -tetrahydroxy3,3,31 ,31 -tetramethyl-1,11 -spirobisindane and 1,4-dicyanotetrafluorobenzene under high-intensity mixing at 155 ◦ C. 41 The product was cyclic-free with molecular weight polydispersity of about 2. PIM-6FDA-OH is a hydroxyl-functionalized polyimide with intrinsic microporosity. It was synthesised by a reaction of 4,4-(hexafluoroisopropylidene)-diphtalic anhydride and 3,3,31 ,31 -tetramethyl-1,11 -spirobisindane-5,51 -diamino-6,61 -diol. The full synthetic route has been described earlier. 42

Solvents (n-hexane, toluene, ethanol, isopropanol, acetone, methyl ethyl ketone) were all obtained from Sigma-Aldrich and were used without further purification. Deionized water


Page 17 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


produced in-house was used. Thin films were made by spin-coating from 0.5 - 1% (by weight) solution in THF on clean silicon wafers with native oxide of about 2 nm. The spinning rate was always kept constant at 2000 rpm and the spin-coater was flushed with dry nitrogen to aid solvent vapor removal and prevent unwanted effects of humidity. Prior to all swelling experiments the samples were rejuvenated with solvent (n-hexane) and annealed on a hot plate at 150 ◦C for about 10 min. In most cases exactly the same sample film was used in swelling experiments with different solvents to reduce errors arising from sample preparation (spin-coating). The film was always solvent-rejuvenated and annealed before the next experiment. This procedure was found to result in solvent-free, fresh (not physically aged) samples with very reproducible refractive index (as measured with ellipsometry). Without solvent treatment the control over the sample properties due to physical aging, and thus reproducibility of measurements, was poor. The aging effects are known to be especially substantial in PIMs43,44 and would significantly influence the dry sample properties (thickness, density) which could result in errors in the apparent SF.

Swelling measurements with in-situ spectroscopic ellipsometry In spectroscopic ellipsometry a change in polarization state of light reflected from a thin film sample is measured. The data is represented for each measured light wavelength as a pair of psi (Ψ) and delta (∆) values related to the p (in-plane) and s (out-of-plane) reflection coefficients by:

ρ=

rp rs

= tan(Ψ) ei∆

(5)

The extraction of film properties, such as thickness and refractive index, is done with an



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 26

optical model assumed to represent optical features of the sample. The data generated by the model are then numerically fit to the measured data. More details on the technique and optical modeling can be found elsewhere.28,45 In this study, a simple uniform optical model was used representing the polymer film by Cauchy-type dispersion on a silicon wafer (including native silicon oxide). Optical constants of the substrate were taken from the literature.46 The modeling was done over a wavelength range of 550 - 1000 nm where the sample could be assumed transparent. As an exception, to extract the optical absorption of dry films in the near UV-VIS range (Supporting Information) a B-spline optical model was used.47 A RC2 variable angle spectroscopic ellipsometer (J. A. Woollam Co., Inc.) operating in a wavelength range 193 - 1000 nm was used in all measurements. For in-situ experiments involving liquid solvents a trapezoidal sample cell with quartz windows perpendicular to probing light was used. The incident angle was 70◦. The small ∆ offsets introduced by the windows were corrected for using a 25 nm SiO2/Si calibration wafer immersed in a liquid. The window offsets were found not to significantly depend on the type of solvent used. Additionally, a small angle offset, being a result of an imperfection of the cell symmetry was fit together with sample parameters. The angle offset was found to be almost identical in all experiments and improved fit quality to a large extent. In all swelling experiments the temperature was held constant at 22 ± 1 ◦C. Further details of the setup can be found elsewhere.14 Refractive indices of the solvents (if necessary converted to the values at 632.8 nm by using Cauchy dispersion) were taken from the literature.48–50 The used values were: nwater = 1.332, nethanol = 1.360, nisopropanol = 1.376, nhexane = 1.373, nmethylethylketone = 1.377, nacetone = 1.358 and ntoluene = 1.489.


Page 19 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Supporting Information Available Supporting information includes the chemical structures of both PIMs used in this study, the details of a method used to calculate excess fractional free volume in PIMs using group contribution methods, and ellipsometry-determined optical spectra (n and k) of both PIMs.

This material is available free of charge via the Internet at http://pubs.acs.org/.

Acknowledgement This publication is based upon work supported by the King Abdullah University of Science and Technology (KAUST) Office of Sponsored Research (OSR) under Award No. SEED Fund OSR-2015-SEED-2445-01.

References (1) Budd, P. M.; Ghanem, B. S.; Makhseed, S.; McKeown, N. B.; Msayib, K. J.; Tattershall, C. E. Polymers of Intrinsic Microporosity (PIMs): Robust, Solution-Processable, Organic Oanoporous Materials. Chem. Commun. 2004, 230–231. (2) Budd, P. M.; McKeown, N. B.; Fritsch, D. Free Volume and Intrinsic Microporosity in Polymers. J. Mater. Chem. 2005, 15, 1977–1986. (3) Li, P.; Chung, T. S.; Paul, D. R. Gas Sorption and Permeation in PIM-1. J. Membr. Sci. 2013, 432, 50–57. (4) Robeson, L. M. The upper Bound Revisited. J. Membr. Sci. 2008, 320, 390–400.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 26

(5) Swaidan, R.; Ghanem, B.; Pinnau, I. Fine-Tuned Intrinsically Ultramicroporous Polymers Redefine the Permeability/Selectivity Upper Bounds of Membrane-Based Air and Hydrogen Separations. ACS Macro Lett. 2015, 4, 947–951. (6) Baker, R. W. Future Directions of Membrane Gas Separation Technology. Ind. Eng. Chem. Res. 2002, 41, 1393–1411. (7) Schultz, J.; Peinemann, K.-V. Membranes for Separation of Higher Hydrocarbons from Methane. J. Membr. Sci. 1996, 110, 37–45. (8) Gorgojo, P.; Karan, S.; Wong, H. C.; Jimenez-Solomon, M. F.; Cabral, J. T.; Livingston, A. G. Ultrathin Polymer Films with Intrinsic Microporosity: Anomalous Solvent Permeation and High Flux Membranes. Adv. Funct. Mater. 2014, 24, 4729–4737. (9) Zˇa´k,M.; Klepic, M.;

Sˇtastna´, L. Cˇ.; Sedla´kova´, Z.; Vychodilova´, H.; Hovorka, Sˇ.;

Friess, K.; Randová, A.; Broˇzová, L.; Jansen, J. C. et al. Selective Removal of Butanol from Aqueous Solution by Pervaporation with a PIM-1 Membrane and Membrane Aging. Sep. Purif. Technol. 2015, 151, 108–114. (10) Rakow, N. A.; Wendland, M. S.; Trend, J. E.; Poirier, R. J.; Paolucci, D. M.; Maki, S. P.; Lyons, C. S.; Swierczek, M. J. Visual Indicator for Trace Organic Volatiles. Langmuir 2010, 26, 3767–3770. (11) Wood, C. D.; Tan, B.; Trewin, A.; Su, F.; Rosseinsky, M. J.; Bradshaw, D.; Sun, Y.; Zhou, L.; Cooper, A. I. Microporous Organic Polymers for Methane Storage. Adv. Mater. 2008, 20, 1916–1921. (12) McKeown, N. B.; Makhseed, S.; Budd, P. M. Phthalocyanine-Based Nanoporous Network Polymers. Chem. Commun. 2002, 2780–2781. (13) Chae, I. S.; Luo, T.; Moon, G. H.; Ogieglo, W.; Kang, Y. S.; Wessling, M. UltraHigh Proton/Vanadium Selectivity for Hydrophobic Polymer Membranes with


Page 21 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Intrinsic Nanopores for Redox Flow Battery. Adv. Energy Mater. 2016, DOI 10.1002/aenm.201600517. (14) Ogieglo, W.; Wormeester, H.; Wessling, M.; Benes, N. E. Temperature-Induced Transition of the Diffusion Mechanism of N-hexane in Ultra-Thin Polystyrene Films, Resolved by In-Situ Spectroscopic Ellipsometry. Polymer 2013, 54, 341–348. (15) Ogieglo, W.; van der Werf, H.; Tempelman, K.; Wormeester, H.; Wessling, M.; Nijmeijer, A.; Benes, N. E. N-Hexane Induced Swelling of Thin PDMS Films Under Non-Equilibrium Nanofiltration Permeation Conditions, Resolved by Spectroscopic Ellipsometry. J. Membr. Sci. 2013, 437, 313–323. (16) Crank, J.; Park, G. S. Diffusion in polymers; Academic Press London, 1968; Vol. 107. (17) Merkel, T. C.; Freeman, B. D.; Spontak, R. J.; He, Z.; Pinnau, I.; Meakin, P.; Hill, A. J. Ultrapermeable, Reverse-Selective Nanocomposite Membranes. Science 2002, 296, 519–522. (18) Thomas, S.; Pinnau, I.; Du, N.; Guiver, M. D. Pure- and Mixed-Gas Permeation Properties of a Microporous Spirobisindane-Based Ladder Polymer (PIM-1). J. Membr. Sci. 2009, 333, 125–131. (19) Thomas, S.; Pinnau, I.; Du, N.; Guiver, M. D. Hydrocarbon/Hydrogen Mixed-Gas Permeation Properties of PIM-1, an Amorphous Microporous Spirobisindane Polymer. J. Membr. Sci. 2009, 338, 1–4. (20) Flory, P. J.; Volkenstein, M. Statistical Mechanics of Chain Molecules. Biopolymers 1969, 8, 699–700. (21) Flory, P. J.; Rehner, J. J. Statistical Mechanics of Cross-Linked Polymer Networks II. Swelling. J. Chem. Phys. 1943, 11, 521–526.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 26

(22) Vrentas, J. S.; Vrentas, C. M. Sorption in Glassy Polymers. Macromolecules 1991, 24, 2404–2412. (23) Tsujita, Y. Gas Sorption and Permeation of Glassy Polymers with Microvoids. Prog. Polym. Sci. 2003, 28, 1377–1401. (24) Minelli, M.; Campagnoli, S.; De Angelis, M. G.; Doghieri, F.; Sarti, G. C. Predictive Model for the Solubility of Fluid Mixtures in Glassy Polymers. Macromolecules 2011, 44, 4852–4862. (25) Ogieglo, W.; Wormeester, H.; Wessling, M.; Benes, N. E. Effective Medium Approximations for Penetrant Sorption in Glassy Polymers Accounting for Excess Free Volume. Polymer 2014, 55, 1737–1744. (26) Wijmans, J. The Role of Permeant Molar Volume in the Solution-Diffusion Model Transport Equations. J. Membr. Sci. 2004, 237, 39–50. (27) Ogieglo, W.; Wormeester, H.; Wessling, M.; Benes, N. E. Probing the Surface Swelling in Ultra-Thin Supported Polystyrene Films During Case II Diffusion of N-Hexane. Macromol. Chem. Phys. 2013, 214, 2480–2488. (28) Ogieglo, W.; Wormeester, H.; Eichhorn, K.-J.; Wessling, M.; Benes, N. E. In Situ Ellipsometry Studies on Swelling of Thin Polymer Films: A Review. Prog. Polym. Sci. 2015, 42, 42–78. (29) Scholes, C. A.; Jin, J.; Stevens, G. W.; Kentish, S. E. Hydrocarbon Solubility, Permeability, and Competitive Sorption Effects in Polymer of Intrinsic Microporosity (PIM-1) Membranes. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 397–404. (30) Jue, M. L.; McKay, C. S.; McCool, B. A.; Finn, M. G.; Lively, R. P. Effect of Nonsolvent Treatments on the Microstructure of PIM-1. Macromolecules 2015, 48, 5780–5790.


Page 23 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(31) Chow, T. S. Molecular Interpretation of the Glass Transition Temperature of PolymerDiluent Systems. Macromolecules 1980, 13, 362–364. (32) Staiger, C. L.; Pas, S. J.; Hill, A. J.; Cornelius, C. J. Gas Separation, Free Volume Distribution, and Physical Aging of a Highly Microporous Spirobisindane Polymer. Chem. Mater. 2008, 20, 2606–2608. (33) Bruggeman, D. A. G. Calculation of Various Physics Constants in Heterogenous Substances I Dielectricity Constants and Conductivity of Mixed Bodies from Isotropic Substances. Ann. Phys 1935, 24, 636–664. (34) Rowe, B. W.; Freeman, B. D.; Paul, D. R. Effect of Sorbed Water and Temperature on the Optical Properties and Density of Thin Glassy Polymer Films on a Silicon Substrate. Macromolecules 2007, 40, 2806–2813. (35) Bondi, A. Van der Waals Volumes and Radii. J. Phys. Chem. 1964, 68, 441–451. (36) Krevelen, D. W. Properties of Polymers: Their Correlation with Chemical Stucture, Their Numerical Estimation and Prediction from Additive Group Contributions; Elsevier Amsterdam, 1990. (37) Stenbock-Fermor, A.; Knoll, A. W.; Boker, A.; Tsarkova, L.; Bo, A.; Tsarkova, L. Enhancing Ordering Dynamics in Solvent-Annealed Block Copolymer Films by Lithographic Hard Mask Supports. Macromolecules 2014, 47, 3059–3067. (38) Stenbock-Fermor, A.; Rudov, A. A.; Gumerov, R. A.; Tsarkova, L. A.; Böker, A.; Möller, M.; Potemkin, I. I. Morphology-Controlled Kinetics of Solvent Uptake by Block Copolymer Films in Nonselective Solvent Vapors. ACS Macro Lett. 2014, 3, 803–807. (39) Elbs, H.; Krausch, G. Ellipsometric Determination of Flory-Huggins Interaction Parameters in Solution. Polymer 2004, 45, 7935–7942.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 26

(40) Wijmans, J. G.; Baker, R. W. The Solution-Diffusion Model: A Review. J. Membr. Sci. 1995, 107, 1–21. (41) Du, N.; Song, J.; Robertson, G. P.; Pinnau, I.; Guiver, M. D. Linear High Molecular Weight Ladder Polymer via Fast Polycondensation of 5,5,6,6-Tetrahydroxy-3,3,3,3Tetramethylspirobisindane with 1,4-Dicyanotetrafluorobenzene. Macromol. Rapid Commun. 2008, 29, 783–788. (42) Ma, X.; Swaidan, R.; Belmabkhout, Y.; Zhu, Y.; Litwiller, E.; Jouiad, M.; Pinnau, I.; Han, Y. Synthesis and Gas transport Properties of Hydroxyl-Functionalized Polyimides with Intrinsic Microporosity. Macromolecules 2012, 45, 3841–3849. (43) Harms, S.; Ra¨tzke, K.; Faupel, F.; Chaukura, N.; Budd, P. M.; Egger, W.; Ravelli, L. Aging and Free Volume in a Polymer of Intrinsic Microporosity (PIM-1). J. Adhes. 2012, 88, 608–619. (44) McDermott, A. G.; Budd, P. M.; McKeown, N. B.; Colina, C. M.; Runt, J. Physical Aging of Polymers of Intrinsic Microporosity: a SAXS/WAXS Study. J. Mater. Chem. A 2014, 2, 11742–11752. (45) Jellison, G. Data Analysis for Spectroscopic Ellipsometry. Thin Solid Films 1993, 234, 416–422. (46) Jellison, Jr, G. Spectroscopic Ellipsometry Data Analysis: Measured Versus Calculated Quantities. Thin Solid Films 1998, 313-314, 33–39. (47) Johs, B.; Hale, J. S. Phys. Status Solidi A. Physica Status Solidi (a) 2008, 205, 715– 719. (48) Rheims, J.; Köser, J.; Wriedt, T. Refractive-Index Measurements in the Near-IR Using an Abbe Refractometer. Meas. Sci. Technol. 1997, 8, 601.


Page 25 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(49) Chu, K.-Y.; Thompson, a. R. Densities and Refractive Indices of Alcohol-Water Solutions of n-Propyl, Isopropyl, and Methyl Alcohols. J. Chem. Eng. Data 1962, 7, 358–360. (50) Wohlfarth, C. In Refractive Indices of Pure Liquids and Binary Liquid Mixtures (Supplement to III/38); Lechner, M. D., Ed.; Springer Berlin Heidelberg: Berlin, Heidelberg, 2008; Chapter Refractive, pp 108–115.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 26

TOC Graphic


How Much Do Ultrathin Polymers with Intrinsic Microporosity Swell in

Recommend Documents