Highly Selective Vertically Aligned Nanopores in Sustainably Derived

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Highly Selective Vertically Aligned Nanopores in Sustainably Derived Polymer Membranes by Molecular Templating Xunda Feng, Kohsuke Kawabata, Gilad Kaufman, Menachem Elimelech, and Chinedum O. Osuji* Department of Chemical and Environmental Engineering, Yale University, New Haven, Connecticut 06511, United States S Supporting Information *

ABSTRACT: We describe a combination of molecular templating and directed self-assembly to realize highly selective vertically aligned nanopores in polymer membranes using sustainably derived materials. The approach exploits a structure-directing molecule to template the assembly of plant-derived fatty acids into highly ordered columnar mesophases. Directed self-assembly using physical confinement and magnetic fields provides vertical alignment of the columnar nanostructures in large area (several cm2) thin films. Chemically cross-linking the mesophase with added conventional vinyl comonomers and removing the molecular template results in a mechanically robust polymer film with vertically aligned 1.2−1.5 nm diameter nanopores with a large specific surface area of ∼670 m2/g. The nanoporous polymer films display exceptional size and charge selectivity as demonstrated by adsorption experiments using model penetrant molecules. These materials have significant potential to function as high-performance nanofiltration membranes and as nanoporous thin films for high-density lithographic pattern transfer. The scalability of the fabrication process suggests that practical applications can be reasonably anticipated. KEYWORDS: polymer membranes, vertically aligned nanopores, directed self-assembly, liquid crystals, sustainable polymers

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of high-density bit-patterned media. Considerable effort has focused on self-assembled mesophases created by precisely tailored block copolymers (BCPs)15,17−22 and amphiphilic small molecules23−27 as a potential route to achieving the desired aligned nanoporous morphology. Another approach involves the use of structure-directing agents to template the assembly of liquid crystalline (LC) mesophases by hydrogen bonding with appropriate monomers,28−31 including relatively simple or plant-derived species.32 Cross-linking of such supramolecular LCs followed by template removal can yield polymers with nanopores replicating the size of the molecular templates. Regardless of the type of mesophase considered, the system must be well-ordered and amenable both to facile microstructure alignment and selective removal of the minority component to generate nanopores in a suitably rigid polymer matrix. This design approach represents a highly challenging proposition in general, even for petroleum-derived materials, and, to date, the full combination of such assembly strategies

he production of useful polymers from renewable or sustainably derived materials represents an increasingly important societal concern.1 Synthetic routes have been developed for the polymerization of a broad range of sustainably derived monomers, including vegetable oils and fatty acids, terpenes, lactic acid, and saccharides.2−8 The appeal of sustainable polymers from environmental and economic perspectives has traditionally been tempered by inferior properties, particularly mechanical properties, relative to petroleum-derived materials. This narrative is changing in several contexts, however, for example in thermoplastic elastomers9−12 and packaging materials.13,14 In the context of nanoporous polymers, there is intense interest in producing polymers with vertically aligned (i.e., perpendicular to the film surface) monodisperse nanopores. This interest is driven by the potential of such materials to be used as filtration membranes that overcome longstanding performance bottlenecks due to the inherent broad size distribution and tortuosity of nanopores generated by conventional membrane fabrication approaches.15,16 These aligned nanoporous polymers are useful as selective adsorbents and selective adsorbent membranes with large specific surface areas, fast transport, and good solvent accessibility. Moreover, they can be utilized in pattern transfer applications, for the creation © 2017 American Chemical Society

Received: January 14, 2017 Accepted: March 16, 2017 Published: March 16, 2017 3911

DOI: 10.1021/acsnano.7b00304 ACS Nano 2017, 11, 3911−3921

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Figure 1. Schematic illustration for fabrication of ordered nanostructured polymers and nanoporous polymers from renewable natural fatty acids. (a) Molecular structures of polymerizable conjugated linoleic acid (CLA) isomers: cis- and trans-9,11- and -10,12-octadecadienoic acids. (b) Molecular structure of the template molecule 1,3,5-tris(1H-benzo[d]imidazol-2-yl)benzene (TBIB). (c) Supramolecular discotic complex TBIB/(CLA)3 with a C3 symmetry formed by fatty acids and the template molecule and its self-assembled Colh mesophase. (d) Ordered polymeric structure by radical cross-linking of the Colh mesophase. (e) Nanoporous polymer obtained after removing the template TBIB molecules. The TBIB can be recycled and reused for subsequent fabrication.

conjugated form of linoleic acid aids free-radical-initiated crosslinking as the reactivity of conjugated dienes is significantly higher than their nonconjugated counterparts. We show that TBIB/(CLA) 3 self-assembles into a thermotropic Col h mesophase that can be vertically aligned with high fidelity using a simple surface-confinement method. This alignment method can be optionally coupled with magnetic fields if desired. Cross-linking of the aligned TBIB/(CLA)3 mesophase followed by chemical removal of TBIB results in thin films with vertically aligned nanopores of ∼1.0−1.5 nm diameter. These polymer films display sharp selectivity for molecular solutes with different sizes and charges, as demonstrated by the size and charge selective adsorption of model penetrant molecules in aqueous solutions.

has not been extended to, nor effectively exploited with, sustainably derived materials. Here, we demonstrate an effective self-assembly approach for creating vertically aligned nanopores in polymer films using sustainably derived materials. Our “core-templated” strategy (Figure 1) is predicated on the use of a carefully tailored molecular species as a central element to template the assembly of a comparatively simple, sustainably derived monomer into a hexagonally packed columnar LC mesophase (Colh). Alignment of the mesophase followed by monomer cross-linking and removal of the template molecules results in the desired aligned nanoporous polymer. The template species can be recovered and reused in subsequent fabrications. The concept is realized here using 1,3,5-tris(1H-benzo[d]imidazol-2-yl)benzene (TBIB) as the templating core species and conjugated linoleic acid (CLA) as the renewable monomer. The template TBIB can form columnar LCs with a series of fatty acids, as recently reported by the Sijbesma group.32 There are two key features. The carboxylic acid headgroup of CLA can form hydrogen bonds with the basic benzoimidazole ring on TBIB to yield a supramolecular discotic complex composed of one TBIB and three CLA molecules (i.e., TBIB/(CLA)3) that then undergoes LC self-assembly. Additionally, the use of a

RESULTS AND DISCUSSION The C3 symmetric TBIB/(CLA)3 complex (Figure 1c) displays a Colh mesophase at room temperature. The formation of such thermotropic LC mesophases in systems with rigid aromatic cores and flexible aliphatic peripheries is well-documented in literature.29−34 The supramolecular mesophase exhibited a transition to an isotropic state (Colh-Iso) at 88.7 °C on heating and at 85.6 °C upon cooling, as determined by differential 3912


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Figure 2. (a) One-dimensional SAXS data of the supramolecular mesophase formed by TBIB/(CLA)3. The characteristic diffraction peak location ratio of 1:√3 confirms the Colh morphology. The intercolumnar distance a is calculated as 2.60 nm. (b) One-dimensional WAXS data showing two peaks at 4.7 and 3.5 Å corresponding to the π−π stacking of the template molecules and the aliphatic packing of the CLA, respectively. (c) Fan-like texture of the Colh mesophase observed by polarized optical microscopy (POM).

cross-linked skin layer that limits the diffusion of oxygen into the bulk of the material. The slow cross-linking kinetics and the need to ensure oxygen transport into the film impose practical constraints on the approaches that can be used to control the alignment of the mesophase. Photoinitiated radical crosslinking was therefore investigated as an alternative. Photoinitiated free-radical polymerization and cross-linking have been employed effectively to lock in the structure of polymerizable LC assemblies.38,39 CLA is amenable to polymerization and cross-linking using a range of chemistries including diene metathesis by Grubbs catalysts, controlled radical polymerizations, and thiol−ene click chemistry.4,6 For expediency, we pursued a simple UV-initiated free-radical polymerization route. The neat TBIB/CLA system containing a small amount of free-radical photoinitiator (ca. 1 wt %) 2methoxy-2-phenylacetophenone was polymerized for 24 h by exposure to a 365 nm UV lamp. The resulting films, however, displayed a relatively poor mechanical integrity and solvent resistance by comparison with air-exposed samples. Upon heating, the UV-polymerized Colh mesophase underwent an isotropic transition starting at 94 and terminating at 102 °C, that is, only ∼13 °C above the original clearing point of the nonpolymerized sample (88.7 °C). Furthermore, the polymerized sample could be dissolved in chloroform, suggesting that a well cross-linked network was not produced and that there was low conversion during the polymerization (Figure S8, Supporting Information). We speculate that the low conversion may be due to a combination of steric hindrance and an excessive concentration of radical scavenging species, for the modest reactivity of the conjugated fatty acid. This result necessitated the use of a more reactive comonomer to cross-link the system due to the limited freeradical reactivity of CLA. Divinylbenzene (DVB) and a butyl acrylate (BA) and 1,6-hexanediol diacrylate (HDA) mixture were selected as comonomer systems based on the prior success of these systems for the copolymerization of vegetable oils (triglycerides) and their derivative fatty acids with acrylate and styrene monomers.5,35 We observed formation of stable, homogeneous Colh mesophases with addition of small quantities of either a mixture of BA and HDA, or DVB (Supporting Information, Figures S9 and S10). Increasing the comonomer content resulted in increased d spacings for both the acrylate mixture and DVB (Supporting Information, Figure S10). The miscibility of the comonomers with CLA and immiscibility with TBIB indicate that the increased d spacing is most likely

scanning calorimetry (DSC) and polarized optical microscopy (POM) (Figure S5 in Supporting Information). The hexagonal Colh morphology of the TBIB/(CLA)3 system at room temperature was confirmed by 1-D small-angle X-ray scattering (SAXS) data that show the Bragg spacing d100 of 2.25 nm and d110 of 1.30 nm, as well as the characteristic diffraction peak location ratio of 1:√3 (Figure 2a). The intercolumnar distance a is 2.6 nm, as calculated by a = 2d110. The 1-D wide-angle Xray scattering (WAXS) data show evidence of π−π stacking interactions between the template TBIB molecules with a periodicity of 3.5 Å and liquid-like packing of the aliphatic chains at 4.7 Å (Figure 2b). The system displays the typical fanlike optical texture of Colh mesophases in POM (Figure 2c). The diameter, D, and volume fraction, φ, of columns in the system are related to the d spacing for hexagonal packing as shown in eq 1. For TBIB/(CLA)3, the weight fraction of TBIB is 0.34. We estimate the column diameter in the mesophase to be between 1.19 and 1.42 nm, based on estimates for the volume fraction of packed CLA in the system using the relevant mass densities of CLA and TBIB domains (Supporting Information). ⎛ 8 ⎞1/2 D = d ⎜φ ⎟ ⎝ 3π ⎠

(1)

CLA derived from plant oils is composed of an ill-specified mixture of various C18 fatty acids.35 We investigated the LC formation behavior of TBIB with single-component C18 fatty acids such as linolenic acid and oleic acid, as well as their mixtures to determine whether mesophase formation was sensitive to the feedstock composition. We found that stable Colh mesophases can also be formed at the 3:1 molar ratio of fatty acid to TBIB for both of these species and mixtures thereof (Figure S6, Supporting Information). Drying oils contain mixtures of unsaturated fatty acids and are used to create tough thin coatings or varnishes upon exposure to air. The “drying” or hardening of the material is due to oxygen-induced oxidative cross-linking of the fatty acids rather than any evaporation of volatile species.36,37 Likewise, exposure of the TBIB/(CLA)3 Colh mesophase to air under ambient conditions resulted in the formation of a dense crosslinked sample from an initially gel-like LC (Figure S7, Supporting Information). Full densification of the sample required in excess of 3 weeks of air exposure, however, which is an impractically long period of time for the present purposes. The slow kinetics are likely due to the creation of a densely 3913


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Figure 3. (a) Photographs of cross-linked films and their POM images after photoinitiated radical polymerization of mesophases containing DVB or acrylates. (b) Temperature-dependent 1D-SAXS data of the sample containing 20 wt % DVB showing the lock-in of the Colh morphology after polymerization. (c) FT-IR spectra of the sample with 20 wt % DVB before and after polymerization. Green panel is the zoomed-in view of the spectrum in the range of 1000−900 cm−1. (d) TEM micrograph of a sectioned polymer sample showing the hexagonal morphology. Inset: Fourier transform image.

due to swelling of the CLA surrounding the TBIB cores. Eventually, the system formed an isotropic phase at room temperature for comonomer weight fractions in excess of 0.31 and 0.27 for the BA/HDA and DVB comonomers, respectively (Supporting Information, Figure S10). Comonomer-containing mesophases of TBIB/(CLA)3 with added photoinitiator was photopolymerized by exposure to 365 nm UV light at room temperature for 24 h, resulting in the formation of rigid polymer films (Figure 3a). We systematically characterized two samples containing 20 wt % acrylates (BA/ HDA mixture) or DVB. The polymer films retained the fan-like optical textures of the Colh mesophase after photopolymerization as observed by POM, indicating that the LC assemblies remained stable during polymerization and did not undergo any polymerization-induced phase separation. The 1-D SAXS data showing a characteristic diffraction peak location ratio of 1:√3 indicates the retention of the Colh morphology after polymerization. The thermal stability of the cross-linked mesophase was confirmed by temperature-dependent SAXS and POM measurements. SAXS measurements on the cross-linked sample upon heating display stable characteristic 1-D diffraction peaks of hexagonal morphologies up to 240 °C (Figure 3b). POM micrographs show that the pattern of the LC texture was effectively invariant upon heating for the cross-linked sample, with only a slight and gradual alteration of the birefringent color, possibly due to the thermal disordering of the stacked TBIB molecules or simple temperature-induced change of the optical density of the sample (Figure S11). In contrast, the

nonpolymerized counterpart underwent an expected isotropic transition and then a sudden phase separation at elevated temperatures induced by the dissociation of the hydrogen bonds between CLA and TBIB and the crystallization of TBIB. The participation of the CLAs in the copolymerization was verified by FT-IR analysis of thin films of the Colh mesophases before and after polymerization. Figure 3c shows the FT-IR spectra of the DVB-containing mesophase before and after UV exposure. The cis- and trans-dienes of the CLAs have characteristic C−H vibration bands at 948 and 982 cm−1, respectively.40 It is clear that after UV exposure the IR band at 948 cm−1 completely disappeared and the absorbance intensity of the 982 cm−1 band significantly decreased, indicative of a high conversion of the CLAs into the polymer network. Although TBIB shows UV sensitivity, detailed assignment of the peaks in the spectra demonstrates that this molecule was stable upon UV exposure (Figure S12). Figure 3d shows a TEM micrograph of a cross-linked sample containing acrylate comonomer. The aromatic TBIB molecules located at the core of each column appear dark due to chemical staining by RuO4. A highly ordered hexagonal array of the supramolecular columns with a d100 spacing of ∼2.50 nm is observed, consistent with the SAXS characterization. The corresponding Fourier transform of this micrograph exhibits sharp 6-fold symmetric contributions (inset). For DVBcontaining samples, the selectivity of the chemical staining is reduced due to the uptake of RuO4 by the aromatic phenyl rings of the DVB molecules mixed in with CLA at the periphery 3914


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Figure 4. (a) Schematic illustration of face-on or edge-on orientation of the supramolecular columns depending on different surface properties of the substrates resulting in ionic or hydrophobic interactions between the surface and the columns. (b) POM image of homeotropically aligned Colh phase of the neat TBIB/(CLA)3 in a 15 μm thick film showing rectilinear defects. Inset: Dendritic growth of homeotropic (faceon, vertical) domains from the isotropic phase. Only the analyzer was inserted for the observation. (c) Vertical alignment of DVB-containing Colh phase in a 5 μm thick film confined by poly(sodium 4-styrenesulfonate) (PSS)-coated surfaces as confirmed by the through-thickness 2-D SAXS pattern exhibiting a 6-fold symmetry. (d) Magnetic field aligned thick film (thickness >5 μm) of DVB-containing Colh phase as illustrated by the schematic. Photo of 30 μm thick polymer films with randomly or vertically oriented supramolecular columns shows the different transparences. Optical extinction shown in the POM image confirmed the vertical alignment.

molecular species consisting of a rigid core with partial ionic character due to strong hydrogen bonding by acid−base proton sharing and an oleophilic periphery due to the aliphatic CLA and comononer chains. We speculated that a surface exhibiting strong affinity for the cores would favor face-on (i.e., homeotropic, vertical) alignment of the supramolecular columns, and that a surface with ionic character would provide the required affinity. Conversely, we speculated that an oleophilic or hydrophobic surface would favor edge-on orientation of the columnar structure or a planar anchoring. To this end, we modified the surface of glass slides by depositing a layer of polyelectrolyte poly(sodium 4-styrenesulfonate) (PSS) by spin coating. We used POM to characterize optical textures in samples sandwiched between PSS-coated glass slides during slow cooling (0.2 °C/min) from the isotropic phase. We observed the growth of dendritic morphologies (inset of Figure 4b) and finally optical extinction with the presence of rectilinear defects in a TBIB/(CLA)3 thin film (∼15 μm thick). These observations are consistent with homeotropic alignment of the Colh mesophase.45−47 In contrast, when the glass slides were treated by octadecyltrimethoxysilane (OTMS), the columnar superstructures adopted degenerate planar (i.e., edge-on) anchoring in the TBIB/(CLA)3 thin film. The columnar director is arbitrarily

of TBIB molecules, resulting in poor TEM contrast (Figure S13). Accounting for the presence of the comonomers, the column diameter is estimated to lie between 1.27 and 1.52 nm (Supporting Information). On the basis of the observed d100 spacings for the acrylate and DVB-containing samples (2.52 and 2.69 nm), the areal densities of the columns are calculated as 1.4 × 1013 and 1.2 × 1013 cm−2, respectively. Such large areal densities are compelling in the context of high specific surface area nanoporous materials and high-performance nanofiltration membranes. Effective utilization of the cross-linked nanostructured materials derived here requires alignment of the columnar structures and, in particular, vertical alignment in thin films. Surface anchoring of many discotic mesogens with aromatic cores is insensitive to surface chemistry and roughness, such that homeotropic alignment of their Col mesophases in thin films can be obtained simply by cooling isotropic melts confined by two solid surfaces.41−43 However, some discotic mesogens display homeotropic or planar alignment depending on the surface chemistry of the confining surfaces, possibly due to the distinct affinities of the discotic core versus the discotic periphery toward the surfaces in question.39,44 We examined whether surface anchoring and film confinement can serve to align the Colh mesophases. The hydrogenbonded TBIB/(CLA)3 is effectively an amphiphilic supra3915


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ACS Nano distributed in the plane of the film as evidenced by the fan-like texture observed by POM imaging (Figure S14).48,49 Surface-induced alignment using treated glass slides was also successful in the case of comonomer-containing samples, albeit with a greater sensitivity to the film thickness. Data are shown here for DVB-containing samples. We observed an increase in the population of rectilinear and other LC defects on increasing comonomer content at a fixed film thickness of 15 μm (Figure S15). This observation suggests that the preferred homeotropic anchoring at the PSS-coated surfaces was not extended effectively through the whole film.50 A reduction in thickness to about 5 μm resulted in uniform vertical columnar orientation in the mesophase as revealed by the through-thickness 2-D SAXS pattern showing a 6-fold symmetry (Figure 4c). For films with thickness larger than 5 μm, we employed a 6 T magnetic field to assist the alignment of the mesophases and annihilate LC defects. In analogy to Colh mesophases formed by discotic mesogens bearing aromatic cores, the TBIB/(CLA)3 mesophases possess negative magnetic anisotropy and therefore the columnar axes degenerately align perpendicular to the field direction.51 That is, the easy axis for magnetic alignment is in the plane of the TBIB, and the hard axis is along the columns. Degeneracy due to negative magnetic anisotropy can be broken by rotation of the sample around an axis perpendicular to the field, resulting in alignment of the hard axis parallel to the axis of rotation.52,53 Uniform and nondegenerate vertical alignment was successfully achieved in this manner, by continuous rotation of the sample around an axis normal to the field and normal to the film surface (Figure 4d). Samples were sandwiched between PSS-modified glass slides to ensure homeotropic alignment in the near-surface regions of the film and cooled slowly at 0.2 °C/min from the isotropic phase to room temperature. In this manner, we could readily align samples, including comonomer-containing materials. While we only explored thicknesses up to 30 μm here, the combination of space-pervasive magnetic field dictated bulk alignment and PSSsurface-dictated alignment can be leveraged to produce substantially thicker aligned samples if so desired. Visually, the polymer films produced from magnetically aligned mesophases were markedly more transparent than those from nonaligned samples, which displayed cloudiness due to visible light scattering by the randomly oriented Colh domains (Figure 4d). Optical extinction with the elimination of rectilinear defects observed by POM confirmed the formation of a uniform vertically oriented Colh structure. Removal of the TBIB core molecules in the aligned polymer to create ordered nanopores was carried out by immersion of the magnetically aligned, cross-linked films (30 μm thick) into a 0.1 wt % NaOH solution in DMSO (with 1 wt % methanol) for 48 h, followed by rinsing in deionized water. The efficacy of TBIB removal was verified by FT-IR spectroscopy (Figure 5a). Aromatic amines often show C−N stretching bands in the 1360−1250 cm−1 range.54 In addition, the absorption band at 3251 cm−1 is a characteristic of N−H stretching vibration in the TBIB molecules. The three absorption bands at 1311, 1281, and 3251 cm−1 found in the FT-IR spectrum of the cross-linked sample containing the TBIB template vanished after soaking in DMSO, indicating the complete removal of TBIB. It is noteworthy that soaking in the NaOH/DMSO solution also resulted in formation of nanopores decorated by sodium carboxylate groups. This reaction is suggested by the shift of the absorption band for CO stretch from 1691 to 1562 cm−1 in the FT-IR spectrum.

Figure 5. Characterization data for the nanoporous polymers produced by TBIB removal. (a) FT-IR spectra of the cross-linked samples before and after removing TBIB. The disappearance of the three absorption bands at 1281 and 1311 cm−1 from aromatic C−H stretch and at 3251 cm−1 from N−H stretch found in the IR spectrum of the cross-linked sample indicates the removal of the TBIB template from the polymer. The absorption band for CO stretch in the carbonyl groups of CLA shifted from 1691 to 1562 cm−1, due to the replacement of TBIB by Na+. (b) Twodimensional SAXS data and schematic models showing retention of the nanoporous structure and the alignment with water impregnation and collapse of the nanopores after drying completely. The collapse of the ordered nanopores is irreversible. The X-ray beam was incident perpendicular to the columns, i.e., parallel to the film plane film plane as indicated in the associated schematics. (c) One-dimensional SAXS data as integrated from the 2-D SAXS patterns in (b).

To quantify the release of the TBIB molecules from the cross-linked polymer, we performed UV−vis spectroscopic measurements on the NaOH/DMSO solution used for removal of TBIB (Figure S16). To this end, we immersed a 2 mg polymer film with aligned supramolecular columns into a 50 g NaOH/DMSO solution and measured the time-dependent UV−vis absorption at 316 nm of the solution. Based on the absorbance value obtained after immersion for 48 h, we estimated that ∼97% TBIB was extracted from the polymer film into the DMSO solution. Two-dimensional SAXS characterization was utilized to verify the retention of both the columnar nanopores and 3916


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Figure 6. (a) Molecular structures and space-filling models of methylene blue (MB), Rhodamine 6G (R6G), basic yellow 1 (BY1), Alcian blue 8G (AB8G), and methyl orange (MO). (b) Selective adsorption of MB into nanopores (∼1.2−1.5 nm diameter) from a solution of MB and R6G as confirmed by UV−vis spectra and a color change of the solution. (c) Selective adsorption of BY1 into nanopores from a solution of BY1 and AB8G as confirmed by UV−vis spectra and a color change of the solution. (d) Selective adsorption of MB into nanopores from a solution of MB and MO as confirmed by UV−vis spectra and a color change of the solution.

due to the large Laplace pressures associated with nanometerscale pores.17,55 Such collapse is not uncommon in nanoporous polymers, and we anticipate that its occurrence can be preempted if required by increasing the bulk modulus of the polymer by increasing the cross-link density of the material. The enhanced scattering intensity in the nondried films is consistent with the expected increase in electron density contrast on replacing TBIB by water and confirms that wellaligned solvent-accessible pores were successfully produced by TBIB removal. We observed an increase of 0.14 nm in the d100 spacing of the water impregnated nanoporous material after TBIB removal relative to the pristine sample (2.83 vs 2.69 nm). The marginal nature of the change in d spacing indicates that the material does not swell appreciably in water. This suggests that the dimensions of the pores as set by TBIB are well preserved in the final nanoporous material. We investigated the selectivity of the nanopores by characterizing the adsorptive uptake of molecular species in water with different sizes and charges. Figure 6a shows the molecular structures of five dyes employed in this study: methylene blue (MB), basic yellow 1 (BY1), Alcian blue 8G (AB8G), rhodamine 6G (R6G), and methyl orange (MO). MB, BY1, AB8G, and R6G are positively charged, while MO is negatively charged. The effective van der Waals sizes of these

their alignment in the samples after TBIB removal. Scattering was performed with X-rays incident perpendicular to the film thickness as schematically illustrated by the 3-D cartoons in Figure 5b, providing a cross-sectional view. The equatorial scattering in the 2-D SAXS pattern of the pristine cross-linked polymer confirms the vertical alignment of the supramolecular columns (top panel of Figure 5b). SAXS characterization of samples after TBIB removal showed no evidence of nanostructure if the sample was completely dried after the deionized water rinsing step (bottom panel of Figure 5b). Moreover, TEM visualization of thin sectioned specimens of the completely dried polymer sample without TBIB showed no nanoporous feature (see Figure S17). The collapse of the nanopores was found irreversible even if the dried sample was swelled again by water. Conversely, if the sample was not deliberately dried, SAXS data showed a complete retention of the well-ordered and aligned hexagonally packed nanostructure, as confirmed by the anisotropic 2-D SAXS pattern (middle panel of Figure 5b). In addition, as seen from the 1-D integrated data in Figure 5c, the scattering intensity, compared to that of the pristine polymer (before TBIB removal), was enhanced. The absence of scattered intensity in the dried samples is a clear indication that the pores collapsed during drying, likely 3917


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Using Sv = 670 m2·g−1 as a representative value for specific surface area, for the 2 mg of nanoporous polymer utilized, the available area is 1.34 m2. We neglect the contribution from the external film surface area (3 × 10−4 m2). The test solution contained 4.4 × 10−7 moles of MB (Supporting Information). Assuming a projected molecular area for MB59,60 of 1.35 nm2 and a maximum packing fraction of 0.547 for random sequential adsorption,61 complete MB uptake would require approximately 0.65 m2. Complete uptake would produce an MB areal density of 0.4 nm−2, well away from the ∼4 nm−2 that would be required for charge inversion of the nanopore surface based on our estimate of the areal density of sodium carboxylate groups. Notwithstanding any uncertainties in the relevant experimental measurements and the projected molecular area, the proximity of the required area (0.65 m2) and estimated available area (1.34 m2) suggests that a large fraction of the pore surface is in fact solvent-accessible. The issue of accessibility was further considered in experiments which examined the relative adsorption kinetics of aligned and nonaligned material. These experiments also highlight the critical role of pore alignment in determining the transport properties and performance of nanoporous membranes. Both polymer films possessing the same thickness of 15 μm and weight of 2 mg were immersed into two 2.8 g MB water solutions with an MB weight fraction of 3.6 × 10−6. We measured the time-dependent UV−vis absorbance of the MB solutions in the presence of nanoporous polymer films (Figure 7). As expected, the polymer film with vertical nanopores

species are 1.4, 1.4, 2.8, 1.8, and 1.2 nm. The data for MB, AB8G, R6G, and MO are from the literature,56−58 and the size of BY1 was calculated by the MM2 force field method using the Chem3D software package. The nanopore surfaces are expected to be negatively charged due to the presence of sodium carboxylate groups formed by the action of NaOH on the carboxylic acid headgroups of CLA during the TBIB leaching step. Figure 6b shows the result of the simultaneous adsorption of MB and R6G. The strong absorption bands in the UV−vis spectrum centered at 526 and 664 nm belong to R6G and MB, respectively, as shown for an aqueous solution of a mixture of the dyes before contact with the nanoporous polymer. The aligned nanoporous polymer was introduced to the solution and allowed to equilibrate for 2 days. The 664 nm absorption band is completely absent in the equilibrated sample, indicating a complete depletion of MB from the solution by adsorption onto the nanopore surfaces. By contrast the intensity of the R6G band at 526 nm displays only a slight decrease, ∼10%. The net result is a change from an initially violet colored solution to a light red or pink solution with a completely blue polymer film at the bottom of the vial. We surmise that R6G uptake in the 1.2−1.5 nm pores is significantly precluded by size exclusion of the larger sized R6G (1.8 nm) relative to MB (1.4 nm). This observed selectivity is remarkable given the small difference in size between the two dye molecules. The high selectivity of the nanopores due to size exclusion is further demonstrated by adsorption of BY1 and AB8G from aqueous solutions (Figure 6c). After equilibration for 2 days, the color of the solution changed from green to blue and the UV−vis absorption band of BY1 centered at 407 nm vanished, indicating that the smaller sized BY1 molecules were completely adsorbed into the nanopores. The effectively unchanged UV−vis absorbance at 618 nm reflects the complete or 100% rejection of AB8G by the nanopores. The observed higher rejection of AB8G relative to R6G can be ascribed to the ∼1 nm difference between the molecular size of AB8G and the expected diameter of the nanopores. Figure 6d shows the result of a simultaneous uptake experiment using a mixture of MB and MO in water. The UV−vis spectrum shows a stable absorption band centered at 465 nm for MO but complete loss of the 664 nm MB absorption band after solution equilibration with the nanoporous polymer. The color of the solution changed from green to yellow/orange after equilibration. The results point to a strong selectivity in the adsorption of MB over MO into the nanopores. Given that MO is slightly smaller than MB, we attribute this selectivity to the difference in the charge of the species and conclude that MO is rejected from the pores due to Donnan exclusion. The absence of a meaningful decrease in the 465 nm MO band suggests that adsorption on the exterior surface of the polymer film does not occur to a substantive degree. The ability of the polymer film to completely adsorb MB from solution is due to its large specific surface area and, presumably, the accessibility of that area. We can estimate the specific surface area and areal density of sodium carboxylate groups at the pore wall based on the structural data of the nanoporous polymers (Supporting Information). We estimate the specific surface area Sv is roughly between 470 and 870 m2/ g. The areal density σ of sodium carboxylate groups on the pore wall is ∼4 nm−2.

Figure 7. UV−vis absorbance of MB solutions as a function of time during adsorption into aligned and nonaligned nanoporous polymers. The experimental data were fitted using an exponential decay function as shown in the graph.

displays significantly faster adsorption kinetics than that with randomly aligned nanopores. This can be seen more quantitatively by fitting the experimental data of UV−vis absorbance A versus time t using an exponential decay function, A = A0e−t/t0, where A0 designates initial absorbance and t0 is an exponential time constant that characterizes the rate of adsorption. The value of t0 from the nonaligned nanoporous polymer is 5.5 times larger than that of the aligned system, which demonstrates pore orientation plays a critical role in determining the adsorption kinetics. The importance of alignment in providing rapid access of species to interior spaces for adsorption increases as the dimensions of the sample increase. Moreover, nanopore alignment is known to have a 3918


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assist dissolution of TBIB and formation of the supramolecular complex. The resulting solution was then allowed to stand for 30 min under ambient conditions before solvent evaporation at room temperature under nitrogen atmosphere. The obtained supramolecular discotic complex, TBIB/(CLA)3, was then dried in vacuum overnight. NMR spectra of the TBIB/(CLA)3 complex are displayed in Supporting Information, Figures S3 and S4. 1H NMR (CDCl3/ CD3OD (99:1), 500 MHz): δ (ppm) 9.05 (s, 3H), 7.69 (m, 6H), 7.32 (m, 6H), 6.29 (t, 3H, J = 12.5 Hz), 5.94 (t, 3H, J = 11.0 Hz), 5.65 (m, 3H), 5.29 (t, 3H), 4.01 (s, 6H), 2.42 (t, 6H, J = 7.5 Hz), 2.17−2.06 (m, 12H), 1.72 (quintet, 6H, J = 7.5 Hz), 1.39−1.28 (m, 48H), 0.90− 0.86 (m, 9H). 13C NMR (CDCl3, 125 MHz): δ (ppm) 177.75, 177.73, 150.31, 138.53, 134.81, 134.62, 131.11, 130.17, 129.93, 128.70, 128.59, 126.19, 125.67, 125.58, 123.40, 115.18, 34.29, 32.91, 32.89, 31.76, 31.50, 29.71, 29.44, 29.41, 29.37, 29.32, 29.26, 29.21, 29.20, 29.16, 29.12, 28.94, 27.68, 25.00, 22.64, 22.58, 14.10, 14.07. Preparation of Mesophases Containing TBIB/(CLA)3 and Cross-Linker. Homogeneous mesophases were prepared by addition of various quantities of acrylate or vinyl comonomers to the supramolecular discotic TBIB/(CLA)3 complex. The comonomers aided cross-linking and also served as a means to modify the phase behavior of the system. A mixture of BA and HDA with a fixed weight ratio of 4:1 was chosen for forming acrylate comonomer-containing mesophases and DVB for vinyl comonomer-containing mesophases. To ensure homogeneous mixing, after the addition of a desired amount of comonomer/cross-linker, the mesophases were vortexed and centrifuged for a minimum of 10 cycles. Mesophase Polymerization/Cross-Linking. Mesophases containing a small amount of radical photoinitiator (∼1 wt %) 2-methoxy2-phenylacetophenone were polymerized by exposure to 365 nm UV light using a focused spot UV beam (100 W Sunspot SM spot curing system at a distance of ∼2 cm) for 1 h followed by a benchtop lamp (8 W UVL-18 EL lamp at a distance of ∼2 cm) for 24 h. Removal of TBIB Template Molecules. Polymerized mesophase samples were immersed into a DMSO solution of NaOH (0.1 wt %) for 48 h at room temperature (∼21 °C). To prepare the 0.1 wt % NaOH in DMSO solution, a 10 wt % NaOH in methanol solution was added into DMSO with a methanol/DMSO weight ratio of 1/99. The polymer samples were then rinsed with deionized water to eliminate any residual NaOH/DMSO solution.

dramatic effect on permeability in membranes, with improvements ranging from 10 to 85× reported for membranes based on aligned cylindrical pores, relative to nonaligned samples.24,62,63 We therefore anticipate that the ability to align the nanopores as conducted here will have important implications for realizing membranes with attractive transport properties.

CONCLUSION We have developed a facile approach for fabricating polymer films with highly aligned, vertically oriented nanopores using sustainably derived materials. Our approach relies on the use of a molecular template to guide the self-assembly of polymerizable fatty acids into a hexagonally packed columnar mesophase, that yields nanopores upon removal of the templating species. The template species can be recovered from solution by crystallization and reused for subsequent fabrications. The alignment methods are highly scalable and we envision facile production of large area thin films for membrane applications using film confinement alone, or of thicker materials by combining confinement with magnetic field alignment. The nanoporous materials produced here demonstrate remarkable size and charge selectivity in adsorption experiments, and accessibility of the pore surfaces. The existence of highly ordered and aligned nanostructures with well-defined dimensions allows robust quantification of parameters of interest for applications of these materials, including functional group density and accessible area. We expect that these aligned nanoporous polymers will be useful in a wide range of applications from analytical chemistry to nanofiltration and lithographic pattern transfer. We anticipate that further improvements to the system can be made by optimizing cross-linking density for particular applications of interest, and by the utilization of epoxidized CLA which is more reactive. The additional reactivity of epoxidized CLA may be sufficient to preclude the use of comonomers for cross-linking. Additional experiments would be required to verify whether comonomers can be avoided with epoxidized CLA, or to determine suitable comonomer compositions for such systems. Work is ongoing to quantitatively determine the accessible pore surface area as well as the permeability and solute rejection characteristics of these materials as nanofiltration membranes.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b00304. Additional data regarding characterization of structure and phase behavior of mesophases and descriptions of experimental methods (PDF)

EXPERIMENTAL SECTION All reagents used in this study were purchased from Sigma-Aldrich and used as received unless otherwise noted. The CLA monomer is a mixture of cis- and trans-9,11- and -10,12-octadecadienoic acid isomers as specified by the supplier. Butyl acrylate (BA), 1,6-hexanediol diacrylate (HDA), and divinylbenzene (DVB) were distilled under reduced pressure to remove radical inhibitors before use. Synthesis of the Template Molecule 1,3,5-Tris(1H-benzo[d]imidazol-2-yl)benzene (TBIB). TBIB was synthesized using a singlestep reaction adopted from literature.33 The product was purified twice by sublimation prior to use. NMR spectra (Supporting Information, Figures S1 and S2) were obtained by using an Agilent DD2 500 MHz NMR spectrometer using deuterated chloroform (CDCl3), deuterated dimethyl sulfoxide ((CD3)2SO) or deuterated methanol (CD3OD) as solvents. Chemical shifts (δ) are reported in parts per million (ppm) relative to the singlet at 0.00 ppm of tetramethylsilane as the internal reference or the peaks at 2.49 ppm of (CH3)2SO for 1H and 39.7 ppm of (CD3)2SO for 13C. Preparation of the TBIB/(CLA)3 Complex. CLA and TBIB with a molar ratio of 3:1, respectively, were dissolved in chloroform. A small amount of methanol (∼5 wt %) was then added to the solution to

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Xunda Feng: 0000-0002-4528-0769 Menachem Elimelech: 0000-0003-4186-1563 Chinedum O. Osuji: 0000-0003-0261-3065 Author Contributions

X.F. and K.K. contributed equally to this work. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors thank Douglas L. Gin for helpful insight and discussions during the preparation of this manuscript. Financial support from NSF (CMMI-1246804) is gratefully acknowl3919


Article

ACS Nano

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edged. Facilities use at Yale was supported by the YINQE and NSF MRSEC program (DMR-1119826). The authors thank Mike Degen (Rigaku, Inc.) and Brandon Mercado (Yale CBIC) for technical assistance. C.O.O. acknowledges additional financial support from NSF (DMR-1410568). K.K. acknowledges support from a JSPS overseas research fellowship.

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Highly Selective Vertically Aligned Nanopores in Sustainably Derived

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