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Article Cite This: ACS Omega 2018, 3, 16127−16133
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Salt Water-Triggered Ionic Cross-Linking of Polymer Composites by Controlled Release of Functional Ions Brian M. Mosby, Sachit Shah, and Paul V. Braun* Department of Materials Science and Engineering, Department of Chemistry, Materials Research Laboratory, Beckman Institute for Advanced Science and Technology, University of Illinois at Urbana−Champaign, Urbana, Illinois 61801, United States
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S Supporting Information *
ABSTRACT: A composite that undergoes ionic cross-linking in the presence of salt water is presented as a viable strategy for the development of chemically responsive materials. The permeation of salt water through the composite activates embedded inorganic fillers, resulting in the release of functional ions and subsequent cross-linking with the functional groups of the polymer matrix. The release of a cross-linking agent from the inorganic filler and composite is evaluated along with the impact of the cross-linking on composite properties. The new methodology is then coupled with a dopamine-functionalized polymer in order to evaluate the potential of this approach for environmentally triggered self-healing materials.
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INTRODUCTION Nonlinear responses to small-input stimuli are common and important in the natural world, yet rare in synthetic systems. One area where significant advances have been made in imparting nonlinear responsivity is self-healing materials, where a small damage event (e.g., crack formation) can trigger extensive chemical reactions which can heal the crack. Such self-healing materials have been the subject of considerable attention because of their breadth of potential applications.1−9 To date, most stimuli-responsive systems are polymer based, and generally fall into one of two categories. The most prevalent approach, and the one closest to commercialization, involves embedding capsules which contain functional molecules within a matrix.2,10,11 Mechanical damage to the matrix ruptures the capsule, releasing the molecules, providing functionalities such as damage indication and matrix selfhealing.2,11,12 In the other prevalent approach, the polymer is chemically derivatized to undergo a mechanically triggered chemical transformation or re-bond when fractured and returned to contact.13−15 Although capsule-based systems are effective for providing mechanical damage-triggered agent release, there are limited options for driving agent release via chemical triggers, with the exception of microcapsules that undergo depolymerization and core release in the presence of a trigger.16,17 In contrast to capsule-based systems, where core release is driven by damage to the capsule shell, a porous inorganic material-based agent release strategy offers a unique opportunity for active material release, assuming there exists a trigger that upon entering the porous inorganic particle drives release of functional chemicals. From a practical standpoint, inorganic materials are generally structurally robust and thermally stable, offering potential for © 2018 American Chemical Society
self-healing materials compatible with aggressive processing such as high shear, something difficult to realize using microcapsules. In our search for inorganics which could provide triggered release of functional molecules, we were inspired by the controlled release of chemical agents from metal-organic frameworks, zeolites, zirconium phosphates, and layered double hydroxides for applications such as drug delivery.18−21 Critical for self-healing is a release system capable of triggering by an environmentally applied trigger; we chose to focus on developing systems triggered by salt water, given the abundance of NaCl(aq) in both oceanic (∼500 mM) and physiological (e.g., blood, ∼150 mM) environments. Although salt water is an appealing chemical trigger, its intrinsic functionality is limited because it is monovalent and not generally catalytic for important reactions; thus, it alone has limited applicability for driving self-healing. One study attempted to show salt water-triggered self-healing; however, healing was not triggered by the salt, but rather by the water, which promoted mobility of ions, which led to cross-linking.22 Here, we advance the concept of environmentally triggered materials by preparing a composite that undergoes ionic crosslinking exclusively in the presence of a common trigger, salt water. The composites consist of a multivalent ion-responsive polymer and an inorganic filler, which upon exposure to salt water releases multivalent ions. The released multivalent ions then coordinate with the functional groups of the polymer matrix, resulting in the ionic cross-linking of the matrix. This concept is specifically demonstrated in a composite consisting Received: October 12, 2018 Accepted: November 14, 2018 Published: November 28, 2018 16127
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Figure 1. (a) Representation of salt water-triggered cross-linking within Ca−Y zeolite/PAA composites and its effect on swelling. (b) Optical images of dry and triggered disk-shaped PAA composites loaded with Ca−Y or Na−Y after 8 min immersion in a 0.5 M NaCl solution, showing the reduced swelling of the Ca−Y-loaded composite because of cross-links formed by salt water-triggered released Ca2+.
of polyacrylic acid (PAA) and Ca2+-loaded zeolite Y particles (Ca−Y). As salt water enters the composite, it displaces Ca2+ from zeolite Y, which forms ionic cross-links between carboxylate groups in the PAA, significantly changing the swelling behavior (Figure 1a).
mide was used as the Ca−Y pore modifier for all subsequent experiments. The salt water-responsive composite was formed by mixing an aqueous solution of PAA (100 mol %) with an aqueous solution of tetrabutylammonium (TBA) hydroxide to deprotonate the majority (70 mol %) of the carboxylic acid sites (Ca2+ only cross-links deprotonated carboxylate groups). An aqueous suspension of pore-modified Ca−Y was then added to the aqueous polymer solution and mixed thoroughly. Finally, 1,4 butanediol diglycidyl ether (0.1−5 mol %) was added as a covalent cross-linker. The resulting solution was cast into a silicone mold, dried, and cross-linked by a 5 min thermal treatment (90 °C). The composite compositions can be found in Table SI 1. As a first step, Ca2+ release from Ca−Y-containing composites was evaluated. To perform this study, composites with protonated PAA were prepared. The protonated PAA contains carboxylic acid groups which do not form ionic crosslinks with multivalent ions; thus, ions released from the zeolite can leave the polymer. By measuring the concentration of Ca2+ in 0.5 M NaCl(aq) containing pieces of the composite over time, the Ca2+ release behavior of Ca−Y particles contained within the composite can be monitored. Figure 3 shows the release of Ca2+ from both carboxylic acid (PAA) and deprotonated carboxylate (TBA−PAA)-based composites over time with varying degrees of cross-linking, normalized to the maximum release from Ca−Y in 0.5 M NaCl(aq). The carboxylic acid composite displays a high release of calcium into aqueous solution, indicating dissolved Na+ can drive release of most the Ca2+ contained within Ca−Y in the polymer matrix. Initially, Ca2+ release from all the composites is comparable regardless of polymer cross-link density; however, over time both the 0.1 and 1 mol % cross-linked composites display lower levels of Ca2+ in solution, indicating partial reabsorption of some ions into the polymer. Regardless, all composites display a release of more than 70% of the Ca2+ load within 30 min. Upon triggering of the carboxylate composites with NaCl solution, divalent ions released from the Ca−Y are expected to coordinate to the ionized carboxylate groups, leading to a significant retention of Ca2+ in the composite. In this system, after 60 min, the carboxylate composites show a maximum Ca2+ release of only ∼40% (comparable to the release observed in the carboxylic acid form of the composite in ∼7.5 min), indicating that the bulk of the ions released from the Ca−Y are in fact retained by carboxylate groups of the polymer. To further characterize the Ca2+ release behavior, the Ca−Y carboxylate composites were evaluated via 29Si and 23Na
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RESULTS AND DISCUSSION Initially, Ca−Y was formed by loading divalent Ca2+ ions inside the zeolite Y framework through conventional ion exchange.23 Ion exchange was confirmed through the X-ray diffraction pattern and inductive coupled plasma mass spectroscopy (ICPMS) analysis (Figure S1). To improve the specificity of small ion triggers such as Na+, relative to large ions that may be present in aqueous solutions, the pores of Ca−Y were functionalized with silane-coupling agents, a known method for decreasing the effective diameter of zeolite pores and controlling the uptake and release of species within the framework.24−27 Pore modification was confirmed by thermogravimetric analysis (Figure S2) and its effect on the salt water-triggered release kinetics of Ca2+ from Ca−Y evaluated (Figure 2). The amount of Ca2+ released from pore-modified
Figure 2. Ca2+ release of Ca−Y and two pore-modified Ca−Y zeolites (PM1: modified with n-(3-triethoxysiylpropyl)-4-hydroxybutyramide and PM2: modified with 3-aminopropyldiethoxymethyl silane functionalized with glycidol).
zeolites upon Na+ triggering is slightly reduced relative to free Ca−Y, but in all cases the majority of the Ca2+ was released within 10 min. Because the goal was to maximize the Na+triggered release of Ca2+ in a pore-modified system, while minimizing the Ca2+ release because of larger species (as already discussed), n-(3-triethoxysiylpropyl)-4-hydroxybutyra16128
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Figure 3. Release of Ca2+ into solution triggered by 0.5 M NaCl(aq) from the (a) carboxylic acid composite and (b) carboxylate composite relative to the maximum release from Ca−Y under similar conditions. The covalent cross-link densities of the composites are 0.1, 1, and 5 mol % as indicated by the legend.
Figure 4. (a) 29Si DPMAS NMR and (b) 23Na MAS NMR of the 0.1 mol % cross-linked Ca−Y-loaded carboxylate composite before and after triggering with 0.5 M NaCl(aq).
magic-angle spinning (MAS) NMR. The 29Si spectra before and after triggering display the characteristic resonances of zeolite Y pertaining to the Si atoms bonded to different numbers of O−Al and O−Si groups (Figure 4a).28 It is well known that the position of these resonances is dependent on the Si−O−Si and Al−O−Si bond angles associated with tetrahedral sites in the framework, which fluctuate based on the degree of hydration and presence of cations within the framework.29 Comparison of 29Si NMR of the composite before and after Na+ ion triggering reveals several shifts. Replacement of Ca2+ with Na+ cations changes the tetrahedral bond angle primarily at the Si(3Al) and Si(2Al) sites.30 The resonances observed in our system at these positions are 3.3 and 1.6 ppm, respectively. Previous studies indicate that shifts of 1−2 ppm represent a change in the tetrahedral angle by roughly 6°.29 The observed shift in the resonances before and after the trigger suggests an increased tetrahedral bond angle because of ion-exchange as the level of hydration is equivalent in both cases. Additionally, the positions of the two intense resonances of Ca−Y and Na−Y match our composites before and after the trigger, respectively.29 After triggering the Ca−Y carboxylate composite, two resonances can be observed in the 23Na spectra (Figure 4b), both a new resonance at ∼7.6 ppm corresponding to solid NaCl derived from the NaCl used for triggering and a broad resonance centered at around −7 ppm corresponding to the sodium within the framework (see Figures S3 and S4 for details).31 The presence of solid NaCl suggests NaCl is present in excess, and the broadness of the second resonance corresponds well with the monovalent or a mixed divalent−
monovalent form of the zeolite expected after a successful exchange reaction. The combination of the NMR results supports that the propagation of sodium ions throughout the composite leads to displacement and release of Ca2+ from Ca− Y, which also agrees with the observed release behavior of the carboxylic acid and carboxylate composites in the presence of salt water. We then proceeded to evaluate the effect of the released Ca2+, with the goal of determining if it can serve as a salt watertriggered cross-link between carboxylate groups on the polymer backbone, by monitoring the swelling behavior of composites in aqueous salt solutions. Formation of Ca2+centered bridges between two carboxylic acid groups on the polymer backbone will increase the cross-link density of the polymer, which should change its swelling behavior relative to a polymer without ionic cross-linking. Composites with 0.1, 1, and 5 mol % covalent cross-linked PAA and sufficient Ca−Y to provide 45 mol % Ca2+ relative to the molar concentration of carboxylate groups (assuming complete release of Ca2+ from the zeolite) were cut into circles of known diameter using a punch. Figure 5a shows the degree of swelling of dry composites after immersion in the 0.5 M NaCl(aq) triggering solution as a function of time. Note, the degree of swelling depends on the ionic strength of the media,32 and thus it is important to compare the degree of swelling after triggering with control composites made using Na+-loaded zeolites exposed to an identical trigger [0.5 M NaCl(aq)] as Na+ does not form ionic cross-links. In all cases, the composites loaded with Na−Y swell faster and to a higher degree than the Ca−Y-loaded composites. The 16129
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Figure 5. Behavior of dry (a) and fully swollen (b) composites after treatment with a salt solution (0.5 M NaCl). The cross-link densities of the samples are as indicated.
Figure 6. Optical images of a damaged Ca−Y-loaded DOPA−PAA composite. (a) In the dry state; (b−d) following exposure to a drop of salt solution, (b) 10, (c) 20, and (d) 30 min. All composites are 8 mm in diameter. The arrow indicates the damaged region.
reswelling after ∼10 min, whereas the 5 mol % cross-linked sample takes at least 30 min to start reswelling. From the release properties from Figure 3, it can be seen that the 5 mol % cross-linked system displays a continual release of calcium for at least 30 min, which accounts for the observed stability of the composite. In this case, as Ca2+ is displaced from carboxylate sites, it is replaced with fresh Ca2+ from Ca−Y, keeping the ionic cross-link density approximately constant. Deionized (DI) water-swollen composites were also triggered with excess 0.5 M NaCl(aq) (Figure 5b). Here, the composites began in their fully swollen state and contracted upon the addition of the triggering solution. In the lightly cross-linked case, the salt water-triggered contraction of the Ca−Y composite is not significant compared to the Na−Y material, perhaps because carboxylate groups are too far apart to effectively cross-link before the Ca2+ diffuses out of the composite. The more rigid Ca−Y-loaded composites display a shrinkage of roughly double the Na−Y-loaded composites upon exposure to salt water, indicating successful formation of cross-links between polymer chains. The observed swelling behavior for both the dry and swollen composites clearly indicates that the Ca−Y-loaded composites undergo ionic cross-linking in the presence of the salt water-trigger because of release of Ca2+ ions from the zeolite. Upon verification of the salt water-triggered release of the calcium ions and corresponding cross-linking, a system was designed to investigate the plausibility of using such an approach for self-healing applications. To do so, a functional ligand capable of forming cross-links that lead to healing behavior with a number of different multivalent metal ions,
lower degree of swelling of the Ca−Y composites relative to the control suggests that the released divalent ions form ionic cross-links rapidly. The kinetics of the ionic cross-linking observed in the swelling experiments correlates well with the rate of Ca2+ release observed from both Ca−Y and Ca−Yloaded composites exposed to a 0.5 M NaCl(aq) solution (in the composite case, full release was achieved in under 30 min). We propose that salt water first causes an initial swelling of the composite. Shortly afterwards, Na+ begins to enter the zeolite, triggering release of Ca2+. The released Ca2+ then forms ionic cross-links, driving deswelling of the composite (note the reduction in diameter of the Ca−Y disks in Figure 5a after the initial rapid swelling). As shown in Figure 5, the Na−Y- and Ca−Y-loaded composites exhibit distinctly different behavior. The time required for the initial Ca2+ crosslink-induced deswelling (order 5 min) is reasonable based upon the initial release studies from the carboxylic acid composite, where release of Ca2+ is observed in as little as 2.5 min. The 0.1 mol % cross-linked Ca−Y composite reaches its lowest degree of swelling at 9 min, whereas the 1 and 5 mol % cross-linked composites take 10 and 21 min, respectively, to reach their minimum degree of swelling. This is consistent with the previously discussed release studies where both the rate of release of Ca2+ from Ca−Y and retention of Ca2+ within the composite are dependent on the degree of covalent crosslinking. After the minima is reached, the composite slowly swells, likely due to displacement of Ca2+ cross-linkers from the composite by the effectively infinite Na+ in the triggering solution. The displacement rate is directly correlated with the composite cross-link density. The 0.1 mol % composite begins 16130
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under a nitrogen atmosphere. The silane coupling agent (20 mL) was then added and the mixture allowed to stir at room temperature for 24 h. The resulting solution was filtered and rinsed with toluene and methanol to remove excess silane. The recovered solid was heated at 50 °C for 1 h to dry. In the case of APDEMS, the dried pore-modified product was re-dispersed in toluene and treated with an excess of glycidol to functionalize the amino group of the silane. Preparation of Dopamine-Functionalized PAA. Dopamine-functionalized PAA synthesis was adapted from the procedure of Chung and co-workers.33 PAA (4.5 g) was dissolved in 100 mL of dimethylformamide along with 1.15 g NHS and 2.06 g of DCC. The mixture was stirred for 3 h in an ice bath and an additional 3 h at room temperature. Dialysis of the product yielded PAA−NHS, which was subsequently reacted with dopamine hydrochloride in a phosphate-buffered saline solution at pH 8.5. The product was purified by dialysis and freeze-dried to produce a solid. Preparation of Ca−Y/PAA Composites. In a typical experiment, 100 mg of polymer was dissolved in 2 mL of DI water. A stoichiometric amount of TBAOH was then added to deprotonate the desired amount of acid sites. Ca−Y was dispersed in ∼2 mL of DI water, sonicated, and added to the PAA mixture. This solution was mixed until homogeneous and a stoichiometric amount of 1,4 butanediol diglycidyl ether was then added as a cross-linking agent. The solution was cast into silicone molds and allowed to dry. After drying, the composites were heated at 90 °C for 5 min for covalent cross-linking. The composition of the prepared composites can be seen in Tables S1 and S2. Characterization. Thermogravimetric analysis was carried out on a TA Q50 TGA at a heating rate of 10 °C per minute from room temperature to 800 °C under a nitrogen atmosphere. ICP analysis was carried out by the UIUC SCS Microanalysis laboratory using a PerkinElmer Elan DRCe ICPMS. 23 Na and 29Si solid-state NMR experiments were conducted at 7.04 T on a Varian Unity Inova spectrometer at the SCS NMR Facility at the University of Illinois at Urbana− Champaign, operating at a resonance frequency of ν0 (23Na) = 79.4 MHz and ν0 (29Si) = 59.6 MHz at room temperature. A Varian/Chemagnetics 4 mm double-resonance APEX HX MAS probe was used for all MAS experiments under a spinning rate of 10 kHz and TPPM 1H decoupling. Experimental sodium chemical shift referencing, pulse calibration, and setup were done using 1 M NaCl solution, which has a chemical shift of 0.00 ppm. The 23Na pulse width used was 2.25 μs, which is the calibrated selective π/2 pulse width for I = 3/2. A recycle delay of 1 s was used and a number of transients of 1024 scans were acquired for each sample. Experimental silicon chemical shift referencing, pulse calibration, and setup were done using powdered octakis(dimethylsilyloxy)silsesquioxane (Q8M8), which has a chemical shift of 11.45 ppm, relative to the primary standard, tetramethylsilane at 0 ppm. The 29Si π/4 pulse width used was 1.0 μs. A recycle delay of 30 s was used and a number of transients of 2000 scans were acquired for each sample. All optical microscope images were acquired with a Zeiss SteREO Discovery V20 Microscope.
dopamine, was incorporated onto PAA using DIC coupling.33,34 NMR analysis showed that ∼15% of the acrylic acid groups were modified with the ligand (Figure S5). The resulting dopamine-functionalized PAA (DOPA−PAA) was then used to make composites identical to those presented earlier (Table SI 2). Swelling experiments indicated that the composites containing dopamine swell to a lesser degree than composites made from the nonfunctionalized polymers when treated with salt solution (Figure S6) and that the presence of Ca2+ ions in the composites decreases swelling relative to control samples that only contain Na+ (Figure S7). In both cases, this indicates an increase in cross-link formation within the composites because of the presence of the dopamine groups and Ca2+ ions. The potential for self-healing was then evaluated by damaging the dry composites with a razor blade and treating them with a drop of salt water solution (Figure 6). Upon addition of the salt water solution, the composite swells slightly as it hydrates. Simultaneously, the calcium ions are released and coordinate with the dopamine, leading to the healing of the damaged area, which persists even after drying.
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CONCLUSIONS In conclusion, a stimuli-responsive composite was prepared that undergoes salt water-triggered ionic cross-linking. The combination of functional fillers with functional polymers enables a sequence of reactions starting with activation of the inorganic filler, which drives release of Ca2+. The released Ca2+ subsequently coordinates and cross-links carboxylate groups of the polymer. The cross-linking is observed as a reduction of swelling in the composite relative to control samples and healing behavior is possible if the polymer contains appropriate functional groups. Salt water-triggering of responsive polymers may find application in mechanical reinforcement or self-healing, and experiments on using this new paradigm for realizing stimuli-responsive materials with such functionalities are planned in the near future.
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EXPERIMENTAL SECTION
Materials. Poly(acrylic acid) (Mw ≈ 450 000), TBA hydroxide (40% in H2O), calcium nitrate tetrahydrate (99%), 1,4-butanediol diglycidyl ether, glycidol, aminopropyldiethoxymethylsilane, dopamine hydrochloride (98%), n-hydroxysuccinimide (98%), and dicyclohexylcarbodiimide (98%) were purchased from Sigma-Aldrich. Potassium chloride was purchased from Fisher Scientific. Sodium chloride was purchased from EMD chemicals. N-(3-Triethoxysiylpropyl)4-hydroxybutyramide and 3-[bis(2-hydroxyethyl)amino]propyl triethoxysilane were purchased from Gelest. Na−Y was purchased from Alfa Aesar. All chemicals were used as purchased, without further purification. Synthesis of Ca−Y Zeolite. Ca−Y was prepared by ion exchange reactions of Na−Y, according to the literature.23 Na−Y (5 g) was added to a 500 mL round-bottom flask containing 100 mL of 0.5 M Ca(NO3)2. The resulting mixture was heated at 60 °C for 12 h and the solid was recovered by centrifugation. The solid paste was re-dispersed in 100 mL of 0.5 M Ca(NO3)2, reacted for 12 additional hours, filtered, rinsed with DI water, and dried in an oven overnight at 60 °C. Pore Modification of Ca−Y Zeolite. Pore modification reactions were adapted from previously published procedures.26 Initially, 5 g of Ca−Y was added to 1 L of toluene in a 2 L round-bottom flask. The solution was degassed and put 16131
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(13) Ying, H.; Zhang, Y.; Cheng, J. Dynamic urea bond for the design of reversible and self-healing polymers. Nat. Commun. 2014, 5, 3218. (14) Li, J.; Nagamani, C.; Moore, J. S. Polymer Mechanochemistry: From Destructive to Productive. Acc. Chem. Res. 2015, 48, 2181− 2190. (15) Brown, C. L.; Craig, S. L. Molecular engineering of mechanophore activity for stress-responsive polymeric materials. Chem. Sci. 2015, 6, 2158−2165. (16) Esser-Kahn, A. P.; Sottos, N. R.; White, S. R.; Moore, J. S. Programmable Microcapsules from Self-Immolative Polymers. J. Am. Chem. Soc. 2010, 132, 10266−10268. (17) Esser-Kahn, A. P.; Odom, S. A.; Sottos, N. R.; White, S. R.; Moore, J. S. Triggered Release from Polymer Capsules. Macromolecules 2011, 44, 5539−5553. (18) Oh, J.-M.; Choi, S.-J.; Lee, G.-E.; Han, S.-H.; Choy, J.-H. Inorganic Drug-Delivery Nanovehicle Conjugated with Cancer-CellSpecific Ligand. Adv. Funct. Mater. 2009, 19, 1617−1624. (19) Díaz, A.; Saxena, V.; González, J.; David, A.; Casañas, B.; Carpenter, C.; Batteas, J. D.; Colón, J. L.; Clearfield, A.; Hussain, M. D. Zirconium phosphate nano-platelets: a novel platform for drug delivery in cancer therapy. Chem. Commun. 2012, 48, 1754−1756. (20) Taylor-Pashow, K. M. L.; Della Rocca, J.; Xie, Z.; Tran, S.; Lin, W. Postsynthetic Modifications of Iron-Carboxylate Nanoscale Metal−Organic Frameworks for Imaging and Drug Delivery. J. Am. Chem. Soc. 2009, 131, 14261−14263. (21) Horcajada, P.; Serre, C.; Maurin, G.; Ramsahye, N. A.; Balas, F.; Vallet-Regí, M.; Sebban, M.; Taulelle, F.; Férey, G. Flexible Porous Metal-Organic Frameworks for a Controlled Drug Delivery. J. Am. Chem. Soc. 2008, 130, 6774−6780. (22) Xia, N. N.; Xiong, X. M.; Wang, J.; Rong, M. Z.; Zhang, M. Q. A seawater triggered dynamic coordinate bond and its application for underwater self-healing and reclaiming of lipophilic polymer. Chem. Sci. 2016, 7, 2736−2742. (23) Bae, T.-H.; Hudson, M. R.; Mason, J. A.; Queen, W. L.; Dutton, J. J.; Sumida, K.; Micklash, K. J.; Kaye, S. S.; Brown, C. M.; Long, J. R. Evaluation of cation-exchanged zeolite adsorbents for post-combustion carbon dioxide capture. Energy Environ. Sci. 2013, 6, 128−138. (24) Anwander, R.; Nagl, I.; Widenmeyer, M.; Engelhardt, G.; Groeger, O.; Palm, C.; Röser, T. Surface Characterization and Functionalization of MCM-41 Silicas via Silazane Silylation. J. Phys. Chem. B 2000, 104, 3532−3544. (25) Chudasama, C. D.; Sebastian, J.; Jasra, R. V. Pore-Size Engineering of Zeolite A for the Size/Shape Selective Molecular Separation. Ind. Eng. Chem. Res. 2005, 44, 1780−1786. (26) Li, Y.; Guan, H.-M.; Chung, T.-S.; Kulprathipanja, S. Effects of novel silane modification of zeolite surface on polymer chain rigidification and partial pore blockage in polyethersulfone (PES)− zeolite A mixed matrix membranes. J. Membr. Sci. 2006, 275, 17−28. (27) Zhang, H.; Kim, Y.; Dutta, P. K. Controlled release of paraquat from surface-modified zeolite Y. Microporous Mesoporous Mater. 2006, 88, 312−318. (28) van Bokhoven, J. A.; Roest, A. L.; Koningsberger, D. C.; Miller, J. T.; Nachtegaal, G. H.; Kentgens, A. P. M. Changes in Structural and Electronic Properties of the Zeolite Framework Induced by Extraframework Al and La in H-USY and La(x)NaY: A 29Si and 27Al MAS NMR and 27Al MQ MAS NMR Study. J. Phys. Chem. B 2000, 104, 6743−6754. (29) Chao, K. J.; Chern, J. Y. Silicon-29 magic angle spinning NMR spectra of alkali metal, alkaline earth metal, and rare earth metal ion exchange Y zeolites. J. Phys. Chem. 1989, 93, 1401−1404. (30) Jiao, J.; Altwasser, S.; Wang, W.; Weitkamp, J.; Hunger, M. State of Aluminum in Dealuminated, Nonhydrated Zeolites Y Investigated by Multinuclear Solid-State NMR Spectroscopy. J. Phys. Chem. B 2004, 108, 14305−14310. (31) Hunger, M.; Engelhardt, G.; Koller, H.; Weitkamp, J. Characterization of sodium cations in dehydrated faujasites and zeolite EMT by 23Na DOR, 2D nutation, and MAS NMR. Solid State Nucl. Magn. Reson. 1993, 2, 111−120.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b02786. Synthesis of composites, characterization of poremodified Ca−Y samples, release studies of poremodified zeolites, 23Na MAS NMR of zeolites and composites, and swelling behavior of DOPA−PAA composites (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (P.V.B.). ORCID
Paul V. Braun: 0000-0003-4079-8160 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors would like to acknowledge the funding and technical support from BP through the BP International Centre for Advanced Materials (BP-ICAM), which made this research possible. The authors would also like to thank Prof. Nancy Sottos, the UIUC Microanalysis Laboratory (Rudiger Laufhutte and Kiran Subedi), and the UIUC SCS NMR Laboratory (Andre Sustrino) for experimental assistance and helpful discussions.
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REFERENCES
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ACS Omega
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DOI: 10.1021/acsomega.8b02786 ACS Omega 2018, 3, 16127−16133
