Polymer Nanosheets from Supramolecular Assemblies of Conjugated

Sep 8, 2017 - We present a strategy for robustly cross-linking self-assembled lamellar mesophases made from plant-derived materials to generate polyme...
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Polymer nanosheets from supramolecular assemblies of conjugated linoleic acid - high surface area adsorbents from renewable materials Xunda Feng, Kohsuke Kawabata, Dylan M. Whang, and Chinedum O. Osuji Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b02467 • Publication Date (Web): 08 Sep 2017 Downloaded from http://pubs.acs.org on September 9, 2017

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Polymer nanosheets from supramolecular assemblies of conjugated linoleic acid – high surface area adsorbents from renewable materials

Xunda Feng,1 Kohsuke Kawabata,1 Dylan Whang,2 and Chinedum O. Osuji1*

1. Department of Chemical and Environmental Engineering, Yale University, 9 Hillhouse Avenue, New Haven, Connecticut 06511, USA 2. The Dalton School, 108 E 89th St, New York, New York 10128, USA E-mail: [email protected]

* To whom correspondence should be addressed.

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Abstract

We present a strategy for robustly crosslinking self-assembled lamellar mesophases made from plant-derived materials to generate polymer nanosheets decorated with a high density of functional groups. We formulate a supramoleclar complex by hydrogen-bonding conjugated linoleic acid moieties to a structure-directing tribasic aromatic core. The resulting constructs self-assemble into a thermotropic lamellar mesophase. Photo-crosslinking the mesophase with the aid of an acrylate crosslinker yields a polymeric material with high-fidelity retention of the lamellar mesophase structure. Transmission electron microscopy images demonstrate the preservation of the large area, highly ordered layered nanostructures in the polymer. Subsequent extraction of the tribasic core and neutralization of the carboxyl groups by NaOH result in exfoliation of polymer nanosheets with a uniform thickness of ~3 nm. The nanosheets have a large specific area of ~800 m2/g, are decorated by negatively charged carboxylate groups at a density of 4 nm-2, and exhibit the ability to readily adsorb positively charged colloidal particles. The strategy as presented combines supramolecular self-assembly with the use of renewable or sustainably-derived materials in a scalable manner. The resulting nanosheets have potential for use as adsorbents and, with further development, rheology modifiers.

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Introduction

Two-dimensional (2D) materials comprise a wide variety of constructs in which the characteristic system size is on nanometer or sub-nanometer lengthscales in one dimension. This rich and expanding class of materials has attracted intense interest as the reduced dimensionality results in a host of unique and useful properties, in several contexts. The extraordinary optoelectronic and catalytic properties of graphene and transition metal dichalcogenide (TMDC) monolayers are well-known and highly visible examples, with promising applications in areas ranging from molecular electronics and energy storage to health-care and membrane-based separations.1 Their use as antimicrobial agents is also being contemplated.2-5 Polymer nanosheets are likewise of interest for membrane applications, and additionally for use as high surface area adsorbents, catalyst supports, and sensors.6,7 They have also found application in biomedical settings.8 In a similar manner as 2D clay particles9-11 and graphene oxide12,13, polymer nanosheets may offer a potentially novel route for rheology modification in complex fluids14-18 and, as recently demonstrated, mechanical reinforcement of hydrogels19. Monolayers of covalent organic frameworks (COFs)20 and coordination polymers such as metal organic frameworks (MOFs) are molecularly thin 2D materials which offer unique opportunities for catalysis and molecular separations.21

The ability to fabricate and manipulate 2D materials over device or application-relevant length scales and geometries is essential for exploiting the above-mentioned functional properties. Chemical and mechanical exfoliation from bulk crystals with layered structures is 3 ACS Paragon Plus Environment

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a commonly utilized approach for generating graphene and TMDC 2D materials, in addition to chemical vapor deposition and wet chemical synthesis methods.22,23 The extension of these methods to enable large area fabrication and facile manipulation of materials is a critical goal of current research. Fabrication of polymer nanosheets has been accomplished by polymerization of densely packed monomer24-28 or polymer29,30 monolayers at air-fluid interfaces, often produced using the Langmuir-Blodgett scheme, as well as by polymerization of solid surface-adsorbed monomers such as pyrrole.31,32 While they are useful, particularly for in-place generation of functional polymer nanosheets in devices, as surface-based methods, these approaches are limited in their scalability.

Polymerization of appropriate self-assembled small-molecule systems possessing layered structures is an alternate route that enables scalable fabrication of polymer nanosheets by exfoliation of the resulting layered polymer. Polymerizable lyotropic lamellar mesophases and polymerization of secondary species within such mesophases are of interest in this respect. While there have been successes19,33-36, structural disruption by polymerization induced phase separation in lyotropic systems presents a considerable challenge.37-40 Thermotropic mesophases offer considerably more stability as demonstrated in early work by Stupp et al,41 as well as in more recent work in aligned discotic systems.42,43 Successful fabrication of polymer nanosheets using a scalable exfoliation approach would open up new opportunities for these 2D materials, for example as practical high specific surface area adsorbents. The production of polymer nanosheets using sustainably-derived, i.e. renewable, materials would represent an additional compelling benefit. 4 ACS Paragon Plus Environment

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Here we present an approach for generating polymer nanosheets using sustainably-derived materials. We utilize supramolecular self-assembly to construct a thermotropic lamellar liquid crystal (LC) mesophase with conjugated linoleic acid (CLA) derived from vegetable oils. Although there is longstanding interest in the production of polymeric materials from renewable feedstocks such as vegetable oils and their fatty acids derivatives, the production of functional nanostructured polymers from such renewable materials has not been widely considered. Our approach relies on the use of a precisely tailored structure directing species or templating molecule to direct the assembly of the comparatively simple plant-derived monomer, CLA. The thermotropic rather than lyotropic nature stabilizes the system against loss of structural order during polymerization. Polymerization of the mesophase in the presence of a crosslinking agent and subsequent exfoliation leads to the formation of large quantities of ~3 nm thick polymer sheets. The sheets have lateral dimensions exceeding several tens of microns (aspect ratios on the order of 104) and display carboxylic acid functional groups at an exceptionally high density of ~4 nm-2 with overall specific surface area of ~800 m2/g. This exceeds the range of commonly observed values (~200-750) for the specific surface area of graphene oxide.44,45 We anticipate that these materials may be useful as renewably-derived and biodegradable adsorbents .

Experimental Section

1. Synthesis 5 ACS Paragon Plus Environment

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All reagents were purchased from Sigma-Aldrich and used as received unless otherwise noted. The conjugated linoleic acid (CLA) monomer is a mixture of cis- and trans-9,11- and -10,12-octadecadienoic acid isomers as specified by the supplier. Silica nanoparticles with a diameter of approximately 12 nm as verified by electron microcopy were purchased from Sigma-Aldrich

Synthesis of 1,3,5-tris(1,4,5,6-tetrahydropyrimidin-2-yl)benzene (TPB) TPB was synthesized using a single-step reaction adopted from literature. 46 The product was purified twice by sublimation prior to use. NMR spectra (Supporting Information, Figures S1 and S2) were obtained with Agilent DD2 400 MHz NMR spectrometer using methanol-d4 as solvents. Chemical shifts (δ) are reported in parts per million (ppm) relative to the peak at 3.35 ppm (quintet) and 49.3 ppm (septet) of methanol-d4 for 1H and 13C, respectively. 1H NMR (CD3OD, 400 MHz): δ (ppm) 7.94 (s, 3H), 3.49 (t, 12H, J = 5.8 Hz), 1.90 (quinted, 6H, J = 5.8 Hz). 13C NMR (CD3OD, 400 MHz): δ (ppm) 158.16, 138.73, 127.92, 43.09, 21.75. Preparation of TBP/(CLA) complex TBP and CLA with a molar ratio of 1:3, respectively were dissolved in a co-solvent of chloroform/methanol (weight ratio of 90:10). The supramolecular complex was obtained by evaporation of the solution under nitrogen atmosphere. The complex was then completely dried in vacuum overnight. Preparation of mesophases doped with 1,6-hexanediol diacrylate (HDA) The TBP/CLA complex was doped with HDA with a weight ratio of 20 wt% to form a homogenous mesophase. The mixture was homogenized by vortexing and centrifugation 6 ACS Paragon Plus Environment

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several times. Polymerization/crosslinking of mesophases A small amount of radical photo-initiator (~1 wt%) 2-methoxy-2 phenylacetophenone was added to the HAD containing mesophase for polymerization. The mesophase was sandwiched by two glass slides coated by a sacrificial layer of polyvinyl alcohol (with molecular weight of 30000 to 70000 as specified by Sigma Aldrich), followed by thermal annealing with heating above the isotropic transition and cooling to room temperature at a rate of 1 °C/min. The annealed sample was exposed to 365 nm UV light using a focused spot UV beam (100 Watt Sunspot SM Spot Curing System at a distance of ~10 cm) for 1 h followed by a benchtop lamp (8 Watt UVL-18 EL lamp at a distance of ~2 cm) for 24 h. Extraction of TBP and nanosheet exfoliation Crosslinked samples were carefully immersed into methanol for 24 h to extract TBP molecules. After extraction, the polymers were rinsed by water several times. TBP-free, polymeric samples were immersed into a 0.1 wt% NaOH aqueous solution for 12 h followed sonication for 1 min.

2. Characterization

Polarized optical microscopy (POM) and conoscopy POM imaging was performed using a Zeiss Axiovert 200 M inverted microscope equipped with a hot stage coupled with a Linkam Temperature Control Stage TMS 94. Conoscopic images were obtained by a Zeiss Axio Imager M2m microscope equipped with a 40× 7 ACS Paragon Plus Environment

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objective and a Bertrand lens introduced between the analyzer and the ocular. X-ray scattering X-ray scattering was conducted by a Rigaku 007 HF+ instrument equipped with a rotating anode Cu Kα X-ray source (λ = 1.542 Å) and a 2-D Saturn 994+ CCD detector. The resultant 2-D X-ray data was calibrated using a silver behenate standard (d-spacing of 58.38 Å). All the 2-D scattering patterns were integrated into 1-D plots of scattering intensity (I) versus q, where q = 4πsin(θ)/λ and the scattering angle is 2θ. Transmission Electron Microscopy (TEM). Crosslinked mesophase samples were embedded in an epoxy resin for sectioning. The epoxy resin was then thermally cured for 12 h. The epoxy resin block containing polymer samples was sectioned by a diamond knife mounted on a Leica EM UC7 ultramicrotome, which resulted in thin specimens of a thickness around 60 nm. The sectioned specimens were further stained by vapor of a 0.5% aqueous solution of RuO4 for 10 min. Exfoliated nanosheets were stained with AgNO3 to provide appropriate contrast. TEM imaging was performed using an FEI Tecnai Osiris TEM operated at an accelerating voltage of 200 kV. Atomic force microscopy (AFM) AFM images were obtained in tapping mode on a Bruker Dimension Fastscan instrument.

Results and Discussion

The molecular templating concept for the formation of the lamellar liquid crystal is described in Figure 1. Figure 1a shows the molecular structures of the two constituent isomers in the 8 ACS Paragon Plus Environment

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vegetable oil derived CLAs employed in this study, i.e., cis- and trans-9,11- and -10,12-octadecadienoic acids. A supramolecular complex is formed by hydrogen-bonding of CLA and a tribasic core, 1,3,5-tris(1,4,5,6-tetrahydropyrimidin-2-yl)benzene (TPB) with a molar ratio of 3:1 in a cosolvent of chloroform and methanol with a weight ratio of 9:1. The synthesis of TPB was conducted as previously reported.46 Characterization data are provided in the Supporting Information. The molecular design of this template is based on the following considerations. First, the template should be able to complex with CLA by the carboxylate acid groups. Second, the bonding of CLAs with the template should be weak which can be broken for the removal of the template after polymerization. Therefore, hydrogen bonding represents a viable choice, which can be realized by interaction of basic nitrogens with carboxylic acids groups. Finally, the complexation of CLAs to the template should facilitate strong nanophase-segregation into lamellar structures. Despite the discotic nature of the core species, the supramolecular complex self-assembles into a lamellar mesophase at room temperature, rather than a columnar mesophase, as schematically illustrated in Figure 1b.

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Figure 1. Schematic for fabrication of polymer nanosheetes from vegetable oil-derived fatty acids. (a) Molecular structures of polymerizable CLA isomers: cis- and trans-9,11- and -10,12-octadecadienoic acids. (b) Supramolecular complexation by hydrogen-bonding of CLA and a template molecule leads to a lamellar mesophase. (c) Polymerization of the lamellar mesophase results in a polymer with layered nanostructures. Polymer nanosheets with surface enriched –COOH groups can be obtained by removal of the template molecules. (d) Exfoliated polymer nanosheets bearing sodium carboxylate groups.

Formation of lamellar mesophases in discotic mesogens is uncommon, but there have been reports of lamellar mesophase occurrence driven by strong nanophase segregation of aromatic discotic cores from non-aromatic peripheries.47,48 The lamellar morphology is confirmed by the X-ray scattering data that shows three orders of scattering peaks with the characteristic q ratios of 1: 2: 3 and an interlayer spacing (d001) of 3.1 nm (Figure 2a). The presence of several higher-order peaks in the X-ray diffractogram qualitatively differentiates the mesophase from the classic thermotropic smectic-A phase formed by calamitic mesogens that often lack higher-order diffraction peaks due to weaker positional correlation of their layered structures.49,50 The pronounced structural order in the present system likely originates due to the strong nanophase segregation of the rigid core, with its partial ionic character due 10 ACS Paragon Plus Environment

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to strong hydrogen bonding, and the liquid-like aliphatic periphery. The broad hump or amorphous halo found in the wide-angle region of the 1D diffractogram corresponds to a spacing of 0.47 nm and is attributed to the amorphous packing of the aliphatic chains of the CLA molecules. The system shows no evidence of sharp Bragg diffraction from periodic π-π stacking of the core molecules. This suggests that there is limited orientational correlation of the aromatic cores which is consistent with the formation of a lamellar mesophase, rather than a columnar-lamellar or pure columnar system.

Figure 2. (a) X-ray scattering data of the supramolecular complex shows the characteristic scattering peak location ratios of 1:2:3 exhibited by lamellar structures. Inset: 2-D scattering pattern. (b) POM image of bâtonnet texture exhibited by the mesophase upon cooling from the isotropic state. (c) POM showing oily streak texture at room temperature.

POM imaging (Figure 2b) reveals temporary formation of Bâtonnet texture upon cooling from elevated temperatures, a characteristic of a lamellar structure growing from the isotropic melt.51,52 The Bâtonnet texture slowly transformed into oily streak texture at room temperature within 1 hour (Figure 2c). This suggests that the lamellar layers preferentially orient parallel to the film plane (i.e. the lamellar normal is parallel to the film normal).53 The lamellar-isotropic phase transition is located at 75 °C on heating, as determined by polarizing 11 ACS Paragon Plus Environment

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optical microscopy (POM) of a sandwiched film sample (Figure 3a).

Figure 3. Determination of isotropic transitions by temperature-resolved POM of 400 µm thick samples. (a) Neat TPB/CLA complex. (b) TPB/CLA mixed with 20 wt% HDA. The heating rate is 0.5 °C/min. The clearing points for the neat and HAD doped samples are 75 and 40 °C, respectively, as defined by the inflection point of the transition region.

CLA is amenable to polymerization and crosslinking using a range of chemistries including diene metathesis with Grubbs catalysts54, controlled radical polymerizations55 and thiol-ene click chemistry.55 Here, we pursued a straightforward free-radical route simply for proof-of-concept purposes. This necessitated the use of a comonomer to crosslink the system due to the limited free-radical reactivity of CLA. 1,6-hexanediol diacrylate (HDA) was used as the crosslinking comonomer as previous reports have shown that acrylate monomers or crosslinkers

can

be

successfully

copolymerized

with

CLAs

by

free

radical

polymerization.56,57 The mesophase stability was reduced by HDA addition, with a reduction of the clearing temperature observed with increasing HDA content. This was accompanied by a slight decrease of the d001 spacing of the lamellar morphology. The lamellar mesophase was stable to HDA incorporation up to 23 wt%. Beyond this amount, the system showed an isotropic phase at room temperature. 0.5 wt.% of 2-methoxy-2-phenylacetophenone was used 12 ACS Paragon Plus Environment

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as a photo-initiator for the crosslinking reaction.

We systematically investigated the structural properties of the polymer obtained by polymerization of a mesophase containing 20 wt% HDA. The clearing point of this mesophase is 40 °C, as measured by POM (Figure 3b). Before UV-exposure, a 20 µm thick thin film of the mesophase sandwiched by two glass slides was thermally annealed by heating to the isotropic phase and slowly cooling back to room temperature at a rate of 0.2 °C/min. The surfaces of the glass slides were coated by a sacrificial layer of polyvinyl alcohol to enable release of the sample after processing. The thermal annealing helped to uniformly align the mesophase and reduce structural defects generated from random domain orientation. The annealed sample was then polymerized by exposure to 365 nm UV. Dissolution of the polyvinyl alcohol sacrificial layer in water enabled detachment of the sample from the glass slides, yielding a free standing polymer film (Figure 4a).

Figure 4. (a) Photo of a 20 µm thick polymerized film containing ~6,000 layers as calculated on the basis of the layer thickness of 3.2 nm. The 3-D model illustrates the ordered lamellar 13 ACS Paragon Plus Environment

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structures in the film. (b) POM and conoscopic (inset) images of the polymerized lamellar mesophase showing the homeotropic LC director and thus the parallel alignment of layered structure. (c) 2-D X-ray scattering pattern confirms the ordered layered structures. Inset: Gaussian fit of azimuthal intensity. (d) 1-D SAXS data of the mesophase before and after UV-initiated polymerization. (e) TEM micrograph of a specimen sectioned along the film normal showing the lamellar structures.

The polymerized film shows optical extinction under POM and a characteristic Maltese cross in conoscopic imaging, respectively (Figure 4b). This indicates that that the optic axis of the mesophase is along the film normal and therefore that the lamellae are uniformly well-aligned parallel to the plane of the film. X-ray scattering studies on the polymer film were performed, with X-rays incident parallel to the film plane, i.e. providing a cross-sectional view. The observed anisotropic meridional scattering in the 2-D pattern corresponds to a d001-spacing of 3.2 nm and demonstrates that the ordered layered structure was well-preserved after polymerization (Figure 4c). The azimuthal distribution of the scattering intensity is narrow, with a full width half maximum (fwhm) of 7.5°, indicative of a high degree of lamellar orientational order. This is consistent with the observed uniform optical extinction under POM imaging. On the basis of the lamellar d001-spacing of 3.2 nm and the film thickness of 20 µm, roughly 6,000 layers or polymer sheets were stacked along the film normal. The lamellar order after polymerization is effectively unchanged as demonstrated by comparing the scattering peak profiles in the 1-D SAXS data obtained before and after polymerization of the mesophase (Figure 1d). The (001) scattering peak of the polymerized sample displays only a slight decrease of the intensity in relative to that of the non-polymerized one, indicative of a strong preservation of the layered structure. Samples were embedded into epoxy and microtomed along the film normal to provide thin 14 ACS Paragon Plus Environment

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sections for TEM imaging. The sections were vapor stained using RuO4 to provide contrast by preferential uptake of the staining agent by the aromatic cores. The layered structure of the system is clearly observed by high resolution TEM imaging (Figure 4e), further demonstrating the strong preservation of the mesophase structure after polymerization. The sharp contrast of the alternating dark-bright stripes shown in the TEM image reflects the strong nanophase segregation of the aromatic core molecules and the polymerizable CLAs. TEM micrographs obtained at lower magnification show the uniformity of the structure over larger areas (Supporting Information, Figures S3).

The system comprises an alternating stack of hydrogen bonded core molecules and crosslinked aliphatic chains. Extraction of the core molecules from the material should therefore yield stacked polymer nanosheets of crosslinked CLA. To this end, polymer films were soaked in methanol, a good solvent for the TPB core, for 24 h. Atomic force microscopy (AFM) was utilized to visualize the nanosheet morphology. Figure 5a displays a 3-D topographic AFM image of a cleaved film after TPB removal. The terrace-like structure reflects the preservation of individual nanosheets of crosslinked CLAs and their stacking. As measured by the AFM cross-sectioned height profile (Figure 5b), each polymer nanosheet has a thickness of ~2.9 nm. This is smaller than twice the projected length of the fully extended C18 fatty acid chain, ~4.7×2=9.4 nm. The difference is attributed to a combination of inter-digitation of the CLA chains, tilt of the chains away from the lamellar normal due to the Y-shaped structure of the supramolecular complex, and, importantly, significant flexibility of the chain due both to its length and its content of cis-conjugated bonds 15 ACS Paragon Plus Environment

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Based on the periodicity of the polymerized lamellar system we can estimate the areal density of CLA in the exfoliated nanosheets,  =   / and their specific surface area,

= 2/( ), where  is the periodicity of the nanostructure (3.2 nm),  is the volume fraction of CLA in the system ( =0.58, assuming equal mass densities of the core and polymerized constituents),  is Avogadro’s number, is the molar mass of CLA (280 g.mol-1) and is the mass density of homopolymer or copolymerized CLA (estimated at 1 g.cm-3, vs. 0.9 and 1.01 g/cm3 for the monomeric CLA and HDA liquids, respectively).

 =   is the thickness of the nanosheet (2.5 nm) where  is the volume fraction of CLA/HDA copolymer (0.78). The specific surface area S~800 m2.g-1 and the areal density

σ~4 nm-2.

For comparison, the areal density of COOH groups in the molecular crystal of trans-9, trans-11 linoleic acid58 (monoclinic unit cell, a=9.57;b=0.495; c=0.731 nm;  =1.01 g.cm-3) is ~21 nm-2 along (h00) cleavage planes at the midpoint of the hydrogen bonded dimer that populates the lattice. The areal density produced in Langmuir-Blodgett monolayers of mixtures of linoleic acid isomers at air-water interfaces at the collapse pressure is considerably lower, around 2 nm-2.59,60 Areal densities in all-trans conjugated, or non-conjugated fatty acids, are roughly a factor of 1.5-2 larger.61,62 For example, stearic acid (C17COOH) has an areal density of 3.5 nm-2.62 Our polymer nanosheets therefore display a remarkably high density of carboxylic acid groups with ~25% of the absolute value, and 72% on a per mass basis, compared to the surfaces which would be produced by hypothetical cleavage or exfoliation of the molecular crystal, seen as a layered material. The areal density 16 ACS Paragon Plus Environment

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is larger than would be possible by polymerization of CLA monolayers in general, i.e. monolayers

containing

cis

conjugated

isomers

or

even

CLA

monolayers

of

stereoisometrically pure all-trans species.

The dense decoration of the polymer nanosheet with COOH groups, as schematically illustrated in Figure 1c, makes them useful for binding a variety of colloidal and molecular species in solution. Moreover, it provides a useful handle for subsequent surface modification through simple chemical modifications to display other functional groups and provide new functionalities for the nanosheets. Here, we neutralized the COOH using NaOH to produce negatively charged surfaces from fully dissociated functional groups, in preference to the pH dependent surface charge that would occur by deprotonation of COOH. This conversion results in stronger electrostatic repulsion between nanosheets in an aqueous solution and therefore aids nanosheet exfoliation. Samples were carefully immersed into a 0.1 wt% NaOH solution for 24 h, followed by rinsing and mild sonication (1 min) in water. The exfoliated nanosheets can be clearly seen from TEM imaging of a specimen prepared from a drop of aqueous suspension of nanosheets (Figure 5c).

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Figure 5. (a) Left: 3-D AFM image of stacked nanosheets with a terrace-like structure. Right: Height profile along the arrowed line indicated (inset) shows the height variations of the terrace-like structure and reveals a nanosheet thickness of ~3 nm. (b) TEM images of polymer nanosheets exfoliated by sonication in a 0.1wt % NaOH solution. Scale bar: 1 micron. (c) TEM images of exfoliated nanosheets exposed to Ag+ salt solutions to enhance electron density contrast. The nanosheets (likely stretched by capillary stress during drying) are the dark patches in the image as indicated by the red arrows.

The abundant presence of negatively charged carboxylate groups at the surfaces provides a mechanism for adsorption of positively charged species. We demonstrated such adsorption by incubating nanosheets in solution with positively charged alumina-coated silica particles63, with a 12 nm diameter. Figure 6 shows a TEM image of the polymer nanosheet densely covered by the silica nanoparticles, demonstrating effective adsorption of the colloids to the nanosheets. The uptake of positively charged species is also apparent in the Figure 5c where staining by Ag+ was used to generate electron density contrast for TEM imaging.

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Figure 6. TEM images showing positively charged ~12 nm diameter alumina-coated silica particles adsorbed onto the nanosheet surface. Scale bar: 50 nm.

Conclusion

We have developed a scalable approach for fabricating polymer nanosheets using sustainably-derived materials. The approach utilizes a template species that binds polymerizable CLA moieties to form a supramolecular complex that self-assembles into a highly ordered lamellar mesophase. Structural characterization by SAXS, TEM and POM show that the mesophase can be crosslinked to form polymeric materials with high-fidelity retention of the mesophase’s ordered nanostructure. Polymer nanosheets can be exfoliated from the material using a simple solution process that permits recovery of the template species for reuse in subsequent fabrications. The approach demonstrated here is therefore both scalable and sustainable. The nanosheets have a large specific surface area of 800 m2/g and display functional groups, carboxyl species, at a very high areal density of 4 nm-2. The 19 ACS Paragon Plus Environment

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carboxyl groups can be derivatized for many practical applications. We demonstrated the ability of the sheets to adsorb positively charged species by NaOH neutralization of the acid to form negatively charged sodium carboxylate groups. We anticipate that these polymer nanosheets can be customized to address a broad range of potential uses such as high capacity adsorbents, or with further development, as rheology modifiers.

Acknowledgements

We

gratefully

acknowledge

financial

support

from

NSF

(CMMI-1246804). The authors thank Gilad Kaufman, Brandon Mercado (Yale CBIC), Aniko Bezur (Yale IPCH) for technical assistance. K.K. acknowledges additional financial support from a JSPS Overseas Research Fellowship. C.O. acknowledges additional financial support from NSF (DMR-1410568) and from a 3M Nontenured Faculty Award.

Author Contributions: X. Feng and K. Kawabata contributed equally to this work.

Supporting Information: Additional data regarding characterization of core species and TEM of a polymerized mesophase are available in Supporting Information.

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