Toward the Highly Ordered Lamellar Hybrid Membrane for Molecular Sieving by One-Pot Self-Assembly of Hybrid Hyperbranched Polymer

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLE1 Aug 2020Toward the Highly Ordered Lamellar Hybrid Membrane for Molecular Sieving by One-Pot Self-Assembly of Hybrid Hyperbranched Polymer Jin Li, Zhilong Su, Xiaodong Ma, Jie Yin and Xuesong Jiang Jin Li School of Chemistry & Chemical Engineering, Frontiers Science Center for Transformative Molecules, State Key Laboratory for Metal Matrix Composite Materials, Shanghai Jiao Tong University, Shanghai 200240 , Zhilong Su School of Chemistry & Chemical Engineering, Frontiers Science Center for Transformative Molecules, State Key Laboratory for Metal Matrix Composite Materials, Shanghai Jiao Tong University, Shanghai 200240 , Xiaodong Ma School of Chemistry & Chemical Engineering, Frontiers Science Center for Transformative Molecules, State Key Laboratory for Metal Matrix Composite Materials, Shanghai Jiao Tong University, Shanghai 200240 , Jie Yin School of Chemistry & Chemical Engineering, Frontiers Science Center for Transformative Molecules, State Key Laboratory for Metal Matrix Composite Materials, Shanghai Jiao Tong University, Shanghai 200240 and Xuesong Jiang *Corresponding author: E-mail Address: [email protected] School of Chemistry & Chemical Engineering, Frontiers Science Center for Transformative Molecules, State Key Laboratory for Metal Matrix Composite Materials, Shanghai Jiao Tong University, Shanghai 200240 https://doi.org/10.31635/ccschem.020.201900105 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesTrack Citations ShareFacebookTwitterLinked InEmail Ordered lamellar hybrid membranes are of great interest, due to their application as functional materials, including molecular sieving, biosensors, optical devices, and drug, as well as gene deliveries, but their fabrication remains challenging because three critical issues regarding synthesis of regular nanosheets, interfacing, and their spatial distribution in a polymer matrix are difficult to address simultaneously. Here, we fabricated a well-defined hybrid lamellar membrane through one-pot self-assembly, whose structure consisted of alternating layers of hydrophilic hyperbranched poly(ether amine) (hPEA) and polyhedral oligomeric silsesquioxane (POSS). The POSS-capped photosensitive hPEA initially formed ultrathin sandwich-like hybrid nanosheets (HNS) with a thickness of 5 ± 0.5 nm in an aqueous solution through a living crystallization-driven self-assembly, which subsequently, generated a highly ordered lamellar hybrid membrane with a uniform layer of spacing through direct self-stacking by filtration. The resulting membrane acquired a well-defined lamellar structure, thereby providing an ideal model to study molecular diffusion in confined spaces. Also, it could be used for the separation of molecules of different sizes or charges in water solution and is capable of being efficiently, effectively, and economically (known as 3E) applied for the purification of products of methyl orange and fluorescein from their precursor mixtures. This one-pot assembly of the hybrid polymer provides a facile and essential alternative to fabricate highly ordered hybrid membranes. Download figure Download PowerPoint Introduction Lamellar hybrid membranes, in which inorganic two-dimensional (2D) nanosheets are sheathed or decorated by a polymeric material, have recently gained remarkable interest and exhibited exceptional advantages in molecular sieving for the continuous operating mode of lamellar membranes,1,2 and the synergy of functional hybrid components (1+1>2).3–6 Hybrid membranes with layered configuration promise clear-cut sieving in size exclusion and continuum transportation for the narrow width distribution of tight confined sub-nanometer-scale 2D channels of organic polymers, endowing selectivity property in molecular sieving.2,7 As a fundamental part of hybrid membranes, using high-aspect-ratio 2D nanosheets exhibit resultant exotic physicochemical properties, different from those of their isotropic counterparts due to their extended lateral dimensions and nanometer thickness.8 At present, high-performance hybrid lamellar membranes created using various 2D nanostructures,9–11 including metal-organic framework (MOF),12 zeolites,13 transition-metal dichalcogenides (XS2),14 graphenes,15 and graphene oxide (GO)16,17 as the building blocks and other supramolecular porous membranes18 could achieve molecular sieving properties. Their ability to form intrinsic porous structure or nanocapillaries/channels between individual layers exhibit inherent advantages in molecular sieving. Although significant achievements have been made for hybrid membranes used in molecular sieving in the past few years, the realization of complex hybrid hierarchical structures implies a perfect integration of the chemistry and the process,3 which plays a central role in "integrative chemistry" in the field of advanced hybrid materials.19 However, during the fabrication of these lamellar hybrid membranes, three critical issues are inevitably considered when combining the organic and the inorganic aspects at the molecular or nanoscale levels in one material.3 First, regular nanosheets with controlled size, thickness, and shape are required and must qualify in forming ordered lamellar structures whose fabrication remains challenging. Potential defects might occur in membranes fabricated by detached sheets for shortcomings such as low-aspect-ratio, fragmentation, morphological defects, and reaggregation, accompanying the traditional top–down exfoliation process.20 Second, a spatially uniform distribution of inorganic nanosheets and polymers is critical for hybrid membranes employed in molecular transportation.21 Rapid permeation is well recognized to rely on continuous pore–flow transportation and ordered molecular flow through interlayer spaces.22 However, currently available hybrid lamellar membranes possess irregular channels generated by the heterogeneous distribution of nanosheets in polymeric matrices. Achieving a spatially uniform hybrid membrane with a settled thickness of organic or inorganic layers by direct incorporation 2D nanosheets or decorating with organic polymers is difficult. Third, the interfacial compatibility between polymers and inorganic nanosheets is an unavoidable issue in the hybrid membrane4,23 as it plays a predominant role in modulating properties such as separation and stability. The phase separation and inevitable slip of two components by simply incorporating 2D nanostructures with polymeric matrices during filtration might negatively affect the stability and mechanical properties of the physical blending with weak bond connections such as van der Waals forces, hydrogen bonds, or electrostatic bonds. Thus three critical issues of regular nanosheets fabrication are desirable, viz, interfacing, spatial distribution, and polymers that affect the performance of hybrid membranes, synergistically and cooperatively. To address these critical issues, here, we developed a one-pot assembly strategy to fabricate a well-defined hybrid lamellar membrane by using a polyhedral oligomeric silsesquioxane (POSS) end-capped hyperbranched poly(ether amine) (POSS/TX-hPEA) as the building block (Scheme 1a), which initially, formed 2D hybrid nanosheets (HNS) through living crystallization-driven self-assembly (CDSA) in an aqueous solution. The uniform square HNS with a tunable size was sandwich-like and comprised hPEA as the outer layer and a POSS aggregation as the inner layer.24 The uniform sandwich-like nanosheets permit a unique construction of alternating organic-inorganic lamellar structure through a direct self-stacking process of filtration. Different from the reported hybrid membranes that POSS nanoparticles physically blended with polymer matrices,25,26 the HNS-M fabricated by HNS could avoid the mismatched interfacial compatibility effectively, as the hPEA layer is covalently bonded to the organic POSS at a molecular scale. In the stacking process, the photo-crosslinked and intertwined hPEA chains occupied the interlayer space of the POSS aggregation forming in-plane 2D nanochannels, thereby endowing the membrane with precise molecular sieving in the filtration process. Experimental Method Preparation of POSS and TX-functionalized hyperbranched poly(ether amine) We synthesized amphiphilic hPEA-containing POSS and thioxanthone (TX), according to previous reports.27,24 Briefly, the hydrophilic hPEA was synthesized through a one-pot nucleophilic addition/ring-opening reaction of commercial poly(ethylene glycol) diglycidyl ether (PEG-DE) and N-ethylethylenediamine (NEED). The molecular weight of the obtained hPEA was 0.8 × 104 g/mol (Mw/Mn = 1.44) measured by gel permeation chromatography (GPC). POSS and TX-functionalized hPEA (POSS/TX-hPEA) were synthesized through the "click chemistry" reaction between amino groups and epoxy groups.24 The whole synthesis route and the detailed structure of POSS/TX-hPEA are shown in Supporting Information Figure S1. Specifically, hPEA (1.23 g, 0.002 mol, in terms of secondary amino groups), E-POSS (0.93 g, 0.001 mol), and 15 mL alcohol were added into a two-necked flask and refluxed for 24.0 h with N2 protection. Then, epoxythioxanthone (ETX; 0.284 g, 0.001 mol) was added to the mixture and refluxed for another 12.0 h. After cooling to room temperature (RT), the mixture was dialyzed against tetrahydrofuran (THF) for 3 days, using cellulose ester dialysis membranes (MCWO 3500; Sigma-Aldrich, Hong Kong, China), following the THF removal through reduced pressure distillation and drying in a vacuum oven. Preparation of 2D HNS Initially, the 2D HNS were fabricated using the CDSA of POSS/TX-hPEA.24 Amphiphilic POSS/TX-hPEA (100 mg) was dissolved in 1,4-dioxane (20 mL), a suitable solvent for the hPEA, POSS, and the TX moieties, at a final concentration of 5 mg/mL. Afterward, the solution was allowed to equilibrate at 30 °C. Simultaneously, Milli-Q water (80 mL) was added at the speed of 5 mL/h to 1 mL of the polymer solution with gentle stirring. The aqueous solution of HNS obtained was at a concentration of ∼ 1 mg/mL. Subsequently, vacuum-assisted filtration of a specific volume (5.0, 10.0, 15.0, or 20.0 mL) of the HNS aqueous solution directly, HNS-M membranes of different thicknesses were obtained, which were subjected further to irradiation using 365 nm UV light with an intensity of 8.4 mW/cm2, for 10 min for cross-linking. Results and Discussion Preparation of POSS/TX-hPEA The hPEA possesses a large number of secondary amino groups in the periphery ( Supporting Information Figure S1b). Through the click reaction with POSS and TX, hPEA could be presented to the periphery to obtain amphiphilic hyperbranched poly(ether amine) (POSS/TX-hPEA). Preparation of 2D hybrid HNS We prepared a series of square HNS with a tunable size using the CDSA of amphiphilic hyperbranched polymer POSS/TX-hPEA, which comprised hydrophilic hPEA as the backbone and hydrophobic POSS and TX moieties in the inner core. A detailed description of the synthesis27 and characterization of the POSS/TX-hPEA are shown in Supporting Information Figures S1 and S2. The planar TX was photosensitive, and thus, could cross-link with the resulting HNS by UV light irradiation. Self-assembly was carried out by gradually adding water to a homogeneous 1,4-dioxane solution of POSS/TX-hPEA according to a previously reported method.24 AA filtered water, prepared using the AA nanofiltration system (Milli-Q gradient; Millipore, USA), was added, during which the solvent became progressively unsuitable for the hydrophobic POSS and TX moieties because of solubility differences among the hydrophilic hPEA and hydrophobic POSS and TX in water. Consequently, a microphase separation occurred when the water content reached a critical value. Induced by the crystal of the hydrophobic POSS and π–π stacking of the TX moieties, the core hydrophobic POSS and TX moieties aggregated and formed an inner hydrophobic layer along the x–y plane directed by the backbone hydrophilic hPEA, resulting in the generation of regular sandwich-like HNS (Scheme 1a). The nanosheets obtained ( Supporting Information Figure S3a–c) could well disperse in an aqueous solution with a hydrodynamic size of 812 nm and low the polydispersity index (PDI) of 0.24 ( Supporting Information Figure S3e), determined by dynamic light scatting (DLS) using a 3D LS Spectrometer (PD2000DLS; Precision Detectors, Milpitas, CA, USA). The bright cyan fluorescence emission ( Supporting Information Figure S3c) and the blue fluorescent image ( Supporting Information Figure S3d) of the divided nanosheet ascribed to the fluorescence of the TX moieties on the hPEA backbone in the resulting nanosheets. The Tyndall effect ( Supporting Information Figure S3b and Scheme 1b, inserted image) and the resultant Zeta potential (+37.5 ± 0.75 mV) of the homogenous solution indicated high stability of the HNS in an aqueous solution for covalent bonding between hard hydrophobic POSS and soft hydrophilic hPEA on the molecular scale. A series of HNS with regular square shapes, different sizes, and an average edge length of 0.5–2.5 μm were prepared using CDSA of the POSS/TX-hPEA and by controlling the CDSA time, revealed by transmission electron microscopy (TEM) (Scheme 1b and Supporting Information Figure S4). With regard to the size of the POSS (∼ 1 nm),28 and the molecular weight of the hPEA,27 uniform dispersion and thickness of 5 ± 0.5 nm were obtained (Scheme 1c and d), suggesting that the HNS was in monolayer form, in which the hydrophobic POSS and TX formed the inner layer, while the hydrophilic hPEA chains contacted with water to stabilize the assemblies. Accordingly, sandwich-like organic–inorganic 2D HNS with a high-aspect-ratio (∼ 460) in the extended lateral dimensions and nanometer (nm) thickness were fabricated successfully through CDSA. As Supporting Information Figure S4c shows, the selected area electron diffraction (SAED) pattern of the HNS obtained in Scheme 1 and Supporting Information Figure S4b (red dotted cycle) exhibited a diffraction ring corresponding to the interplanar spacing, d, of 1.17 nm along the z-axis, which bore the same size as that of the POSS nanoparticles.28 In other words, a sandwich-like ultrathin HNS, whose structure comprised a versatile hPEA as the outer layer on both sides and a POSS aggregation as the inner plane, was developed by the living CDSA of the hPEA-capped POSS. Preparation and characterization of a hybrid HNS-based membrane (HNS-M) An advantage of this uniform ultrathin nanosheet with a high-aspect-ratio is that it could be stacked layer-by-layer to form a lamellar membrane with a considerably enhanced transparency. The layer-stacked HNS-M was prepared using a vacuum-assisted filtration of the homogeneous HNS solution onto an anodized aluminum oxide (AAO; pore diameter = 0.22 μm) substrate (Scheme 1a). Contrary to the random stacking of nanosheets with narrow x–y planes ( Supporting Information Figure S5a), no vertical nanosheets were found on the surface morphology after examination by atomic force microscopy (AFM; Fuzhou University, Fujian, China). The height and in-phase images for the direct volatilization of the HNS solution on mica plates are shown in Supporting Information Figure S5b–d, where the divided nanosheet is marked with green dotted lines. During filtration, HNS with micrometer lateral x–y widths and nanometer thickness regularly stacked along the z-axis, forming a self-standing transparent membrane (Scheme 1a), which exhibited a hydrophilic surface for the coverage of hPEA chains on the sandwich-like HNS, with a contact angle of ≈ 46° ( Supporting Information Figure S6d). Upon exposure to UV light, TX moieties could extract hydrogen from hPEA backbones to produce radicals, likely to enhance the stability of the resulting cross-linked HNS-M through the coupling of the radicals ( Supporting Information Figures S6a). After the asymmetric (as)-prepared membranes were irradiated using a UV light for 10 min, the intensity of the absorption peak of TX decreased substantially ( Supporting Information Figure S6c), and the bright cyan fluorescence of the membranes weakened significantly ( Supporting Information Figure S6b), suggesting photocrosslinking of the HNS-M had occurred. Cross-sectional TEM and scanning electron microscopy (SEM) images of the HNS-M were analyzed to gain insight into their morphology (Figure 1a–d). The calculations of both the alternating thickness (~ 5.4 nm) in the TEM image of the cross-section of the HNS-M and the corresponding interplanar spacing (d = 5.4 nm) were obtained by fast Fourier transformation (FFT) algorithm displayed as the middle two spots in Figure 1b, confirming the existence of a long-range, highly ordered, lamellar structure, comparable with the measured thickness (5 ± 0.5 nm) using AFM (Scheme 1c and d). For the high-magnification view in Figure 1d, the cross-sectional bright, darkfield, and the corresponding high-angle annular dark-field (HAADF) images of the HNS-M exhibited complementary sequences, indicating its lamellar structure. The dark arrays in the bright-field image showed the POSS layer, which corresponded with the bright arrays in both the dark-field and HAADF images (Figure 1d) for a larger atomic mass of Si (POSS cages) than of C (hPEA chains). The overview image of the silicon (Si) element mapping confirmed that the fabricated HNSs were homogeneous across the whole membrane, and Si was almost located in the layered stacking (Figure 1d). Therefore, we inferred that all the nanosheets were stacked together along the z-axis and oriented in the x–y plane of the membrane, resulting in an ordered lamellar membrane with alternating organic–inorganic layers. The highly aligned lamellar microstructures guaranteed a high light transmittance even with a thickness of 22.0 μm for a well-oriented POSS building block, which was conducive to decreasing the light scattered between the nanosheets (Scheme 1a, an optical photograph of the transparent HNS-M). Moreover, the modulus of the HNS-M was higher than that of the POSS/TX-hPEA solid ( Supporting Information Figure S7) for this highly ordered structure, evidenced by the long-range ordered lamellar structure and a cross-sectional view of the SEM image (Figure 1c). Accordingly, the relatively high-aspect ratio between the edge length and thickness (∼ 460) of the resulting nanosheet produced a membrane with alternating organic–inorganic layered structures. Additionally, the high surface area coated with large amounts of functional hPEA polymer chains of the HNS led to the formation of numerous recognition sites in the 2D plane throughout the membrane for molecular sieving. Scheme 1 | Illustration of the preparation process for the lamellar hybrid membranes and characterization of the obtained HNS. (a) A fabrication mechanism for the highly ordered lamellar hybrid membrane by one-pot self-assembly of POSS/TX-hPEA. (b) TEM, (c) AFM images, and (d) height profile of the uniformed square HNS, digital photos of the Tyndall effect of the HNS solution, and the HNS-M (b, inserted image). Download figure Download PowerPoint The structure of the HNS-M was investigated further using differential scanning calorimeter (DSC) and X-ray diffraction (XRD) ( Supporting Information Figure S8). Two exothermic peaks in the DSC thermogram of pure POSS powder indicated the coexistence of its two crystalline phases, supported by the XRD data.29 On the contrary, only a single endothermic peak at the lower temperature of 120 °C and a lower enthalpy of melting (ΔHm) was observed for the HNS-M. This might be ascribed to the 2D crystallized aggregation of the POSS moieties in the ultrathin layer with a thickness of several nanometers in the HNS-M, which was confirmed by small-angle X-ray scattering (SAXS) measurements (Figure 1e). The distinct reflection peak at d spacing of 1.07 nm (Point II, q = 6.26 nm−1; Figure 1e) was consistent with the fingerprints of POSS crystals ( Supporting Information Figure S8c),30 indicating that the triangular pyramidal molecule of POSS/TX-hPEA took face-to-face assembly with the crystallized aggregation of the POSS in the middle layer, forming the sandwich-like HNS, thereby, resulting in an ordered lamellar HNS-M. The discrete scattering maxima located at q = 1.25 nm−1 SAXS (Point I) of the HNS-M (Figure 1e) confirmed the existence of a long-range ordered structure. The corresponding d spacing of 5.0 nm was located in the HNS thickness, t, range of 5 ± 0.5 nm, determined by AFM (Scheme 1c and d), and was consistent with the interplanar spacing, d, of 5.4 nm calculated by the corresponding FFT displayed in Figure 1b. Thus, concerning the size of the POSS nanoparticles,31 the HNS-M had a highly ordered lamellar structure comprising alternating POSS and hPEA layers whose structures are shown in Figure 2b. Figure 1 | Characterization and structure of HNS-M. Cross-sectional TEM (a) and SEM (c) of the HNS-M, and corresponding fast Fourier transformation (b) in (a) (white dotted circle), cross-sectional STEM bright-field image, dark-field image, corresponding HAADF image, and Si mapping of the formed HNS-M (d), membranes were produced using cryo-ultramicrotomy; detailed cross-sectional 1D SAXS profiles of POSS and HNS-M (e) and 2D SAXS profile of HNS-M (insert, e). Download figure Download PowerPoint Water flux of the HNS-M The uniformed hydrophilic hPEA layer in the resulting lamellar hybrid membrane provided an ideal confined 2D nanochannel for molecular transportation, which is critical in the design of layered membranes for filtration and separation techniques for any unusual fundamental behavior that might arise at the molecular scale.1,32 Water fluxes (Figure 2a) were to the performance of hybrid HNS-M of different We observed a in with membrane with flow we employed the used to the water flow between membranes by that water the 2D interlayer channels as a = ( d × ( × × × h d is the vertical space between nanosheets, the average lateral length of nanosheets, the of water, and h the thickness of the lamellar the HNS-M = 22.0 μm) as an this a flux of which was 104 of than ± is the made in the which are those of the flow and at the layer flow with The between the flux and this that the of water flow through the HNS was to a transportation of water through confined 2D was ascribed to water the hydrophilic area of nanosheets and the ordered hydrogen formed by the single of water resulting in and flow in the were observed in the of with a diameter than 10 nm, which indicated a flow of for water Figure | and molecular sieving properties of the HNS-M. (a) of water permeation with membrane all the fluxes were was reached and at three (b) the structure and of the separation of the HNS-M. Download figure Download PowerPoint separation of the HNS-M on the well-defined lamellar structure with 2D and high water we that the HNS-M might be a potential for molecular sieving and for the removal of organic during water For from and are and to because of their complex To the performance of a HNS-M in a series of organic as well as its we several organic both and negatively to filtration Specifically, we selected the following methyl blue methyl orange fluorescein and The van der Waals sizes of these molecules were calculated using CA, USA), as shown in Figure was used to concentration and after the filtration process. For the negatively with different sizes such as and almost all of were from the water after filtration (Figure In with a size as but exhibited no concentration during the filtration process, and the which was confirmed by the optical displayed as an in Figure The detailed separation are in Supporting Information S1 and Supporting Information Figure with of and for and the HNS-M could negatively and from water, while the and exhibited no concentration during the separation This distinct was due to the exhibited electrostatic by the HNS-M, which of the negatively and that at the of the 2D of the membranes, resulting in a space for the molecular Figure 3 | molecules model and performance of the HNS-M. (a) Molecular model calculated from displayed and and corresponding structure of the molecules of of and fluorescent of and and other of and The concentration of different and organic in mL and as evidenced by the and optical during filtration, (b) (c) (d) (e) and Download figure Download PowerPoint As Figure shows, hPEA chains for the of amino groups and occupied the of the POSS layers During filtration a pressure of one (∼ 1 hPEA chains or molecules along the of polymer while the POSS as a and the layer by the molecules from through the size of POSS nanoparticles and nm by a at using the the HNS-M for transportation of molecules was to be Here, the effect of electrostatic between the and the groups of hPEA could be to the electrostatic the hPEA chains the negatively and molecules As all the amino moieties were located at the backbone of the hPEA, which occupied the

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