Toward Microcapsule-Embedded Self-Healing Membranes

Letter pubs.acs.org/journal/estlcu

Toward Microcapsule-Embedded Self-Healing Membranes Sang-Ryoung Kim,† Bezawit A. Getachew,† Seon-Joo Park,† Oh-Seok Kwon,† Won-Hee Ryu,† André D. Taylor,† Joonwon Bae,‡ and Jae-Hong Kim*,† †

Department of Chemical and Environmental Engineering, Yale University, New Haven, Connecticut 06511, United States Department of Applied Chemistry, Dong Duk Women’s University, Seoul 131-714, Republic of Korea

‡

S Supporting Information *

ABSTRACT: We herein report the first instance of a self-healing water treatment membrane that restores its water flux and particle rejection properties autonomously. The self-healing membrane is fabricated by embedding microcapsules with a polyurethane shell and an isophorone diisocyanate core within a conventional poly(ether sulfone) membrane. A dual-surfactant system and polydopamine coating were used to control the size of these microcapsules and avoid capsule buckling. When the membrane structure is physically damaged, the microcapsules release a reactive isocyanate healing agent that reacts with the surrounding water to form a polyurea matrix that plugs the damage. The self-healing was found to recover the water flux and particle rejection of the membrane to 103 and 90% of the original membrane’s performance, respectively. The results of this study show that microcapsule-embedded membranes are a promising approach to fabricating versatile, next-generation membranes that can self-heal.

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We herein present the first instance of a membrane that is capable of healing physically induced damage without any intervention, i.e., locating the damage sites or introducing chemicals during operation. Our central idea originates from the microcapsule approach that has been widely explored in the production of self-healing structural and coating materials.11−16 In this approach, healant-loaded microcapsules are embedded in the matrix of a material. Damage to the material causes the shells of these microcapsules to break and release healing materials that polymerize on site to seal the damage.14−17 While it is conceptually straightforward, this approach presents a unique set of challenges when applied to the fabrication of membranes because the microcapsules need to be smaller than 100 μm and stable under exposure to polar aprotic solvents. This study reports our approaches for overcoming the complex combination of challenges associated with incorporating microcapsules in a membrane structure and our successful proof of concept for the first time in the literature.

INTRODUCTION Membranes are widely used in water treatment as a nearabsolute barrier against particulate, soluble, and microbial contaminants depending on their pore size.1,2 Commercial lowpressure membranes are mostly based on polymeric materials. They are manufactured through a phase inversion process to form an asymmetric structure with a thin size-exclusion layer reinforced by an underlying porous support.3,4 Because of the inherent limits in the physical/chemical robustness of polymeric materials and a typical membrane’s thin-layer configuration, the membrane active layer can be damaged during installation and operation. Damage due to air-scouring, vibration and stretching, the cleaning process, and untreated objects in the feedwater and the resulting detrimental loss of product water quality have been widely reported.5−8 However, precisely locating the damage sites and fixing breaches present significant challenges in large-scale systems, often requiring substantial process downtime and replacement of an entire module.6 Accordingly, the question regarding the integrity of membranes is often listed as one of the key restraints of the membrane technology. Similar challenges are also present in smaller-scale membrane systems deployed for point-of-use water treatment that are often used in developing countries. To address this problem, we recently reported a technique for healing damage in a membrane by dispatching chitosan agglomerates and cross-linking them using dilute glutaraldehyde.9,10 While this in situ healing technique was found to effectively restore the performance of damaged membranes, an autonomously self-healing membrane would be a more ideal solution because it would not require the addition of extra chemicals to the membrane system. © XXXX American Chemical Society

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EXPERIMENTAL SECTION Microcapsule Preparation. Microcapsules with an isophorone diisocyanate (IPDI) core and a polyurethane shell were synthesized via interfacial polymerization of oil-in-water (O/W) emulsions as schematically illustrated in Figure S1.14−18 The dispersed oil phase was prepared by first dissolving toluene diisocyanate (TDI) prepolymer (4.5 g, Desmodur L, Bayer) in Received: February 4, 2016 Revised: March 1, 2016 Accepted: March 2, 2016

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DOI: 10.1021/acs.estlett.6b00046 Environ. Sci. Technol. Lett. XXXX, XXX, XXX−XXX

Letter

Environmental Science & Technology Letters

Figure 1. (a) Optical microscopic images of dried microcapsules and (b) size distribution of microcapsules prepared using a varying amount of DTAB. (c) SEM images of a crushed microcapsule and (d) cross section of a microcapsule shell. Samples in images c and d were washed with toluene to remove core solvent. Optical microscope images were obtained using an AmScope Microscope and SEM images using a Hitachi SU-70 SEM after chromium coating. The size distribution was determined using a dynamic light scattering analyzer (Microtrac S3500 SI). Scale bars represent (a) 100, (c) 50, and (d) 5 μm.

subsequently placed in a water bath without water flow at room temperature for 24 h.

chlorobenzene (CB; 5 g) under vigorous stirring and subsequently adding IPDI (10 g, Desmodur I, Bayer) to the mixture. The oil phase was then dispersed into the continuous water phase (60 mL) that contained gum arabic (GA; 3.75 g) and a varying amount of dodecyltrimethylammonium bromide (DTAB) under gentle mixing. The suspension was gradually heated from room temperature to 50 °C, and 1,4-butanediol (BD; 3.09 mL) was slowly added as a chain extender to form a polyurethane shell at the O/W interface via the reaction between TDI prepolymer and BD. The suspension was further heated to 70 °C and allowed to react for 1 h for complete shell formation. After the suspension had cooled to room temperature, the microcapsules were rinsed five times through centrifugation, collected by filtering, and air-dried.15,16,18 For select microcapsules, the surface was further coated with polydopamine (PDA) by suspending them in a pH 8.5 Tris buffer solution containing dopamine hydrochloride (2 mg/mL) under gentle mixing.12 After the suspension had been mixed for a few minutes, it turned brown, indicating successful dopamine polymerization and capsule surface coating.12,19,20 Self-Healing Membrane Fabrication and Characterization. The microcapsule-embedded membranes were fabricated by the non-solvent-induced phase separation method.3,4 Varying amounts of the prepared microcapsules were dispersed in a casting solution [18 wt % PES (Ultrason E6020P, BASF) in dimethylacetamide (DMAc)] under vigorous vortexing. After vacuum degassing, the suspension was cast using a doctor blade to form a 200 μm thick film on a polyester nonwoven fabric. The film was instantly immersed in a precipitation water bath for phase inversion. The water flux and rejection rate of a circular membrane coupon were measured using a stirred cell (Amicon 8010, Millipore) pressurized with a nitrogen tank (Figure S2). The filtration of 1 μm and 50 nm fluorescent micro/nanospheres (Polyscience, Inc.) was performed under 100 kPa and with 200 rpm stirring. Select filtration experiments were performed using membranes whose surface was physically damaged using a microtome blade (Figure S3) and

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RESULTS AND DISCUSSION We first employed O/W emulsification procedures that have been established for preparing microcapsules to be embedded in self-healing structural materials.14,15,18 Our initial attempts to prepare microcapsules using hexamethylene diisocyanate as core and methylene diphenyl diisocyanate as shell materials showed that these chemicals were too reactive for proper microencapsulation (Figure S4). We therefore employed IPDI and TDI for the encapsulation process, which formed stable microcapsules. However, these microcapsules were found to be too large and polydisperse to be effectively embedded within a thin (100−300 μm) membrane structure, leading to protrusion of some oversized capsules above the membrane surface (Figure S5). We therefore controlled the microcapsule size during the emulsification step by augmenting the commonly used anionic surfactant (GA) with a cationic cosurfactant (DTAB) to contract emulsions through electrostatic interaction between surfactants.21,22 At all DTAB concentrations, stable O/ W emulsions were formed with decreasing size at increasing DTAB concentrations (Figure S6a). The resultant microcapsules showed consistent size dependency with the mean diameter decreasing to approximately 40 μm at 10.8 mM DTAB, beyond which no further size reduction was observed (Figure 1a). The size distribution was also significantly decreased as more DTAB was added (Figure 1b). Scanning electron microscope (SEM) images (Figure S6b) confirmed the spherical, smooth-surfaced morphology of the microcapsules, while some indentations that were not present in optical microscopic images emerged. These are believed to have resulted from the vacuum environment required during SEM sample preparation. The core−shell structure of the microcapsules consisting of a hollow core (Figure 1c) and approximately 3 μm thick shell (Figure 1d) was confirmed by examining partly crushed B


Letter


Figure 2. (a) FTIR spectra (Thermo Nicolet 6700) of the core material (IPDI), shell precursor (TDI prepolymer), shell-only capsule, crushed capsule, and crushed capsule after ethylenediamine addition. Optical microscope images of microcapsules (b) with and (c) without PDA coating in the membrane casting solution after a 5 min suspension. Inset SEM images in panels b and c show microcapsule shells before and after PDA coating, respectively.

and was found to prevent the buckling phenomenon (Figure 2c). We then prepared a microcapsule-embedded membrane through the phase inversion of a casting solution containing PDA-coated microcapsules. While the control membrane exhibited a typical asymmetric configuration with a thin active layer and a fingerlike structure underneath, the microcapsuleembedded membrane displayed increasingly deformed fingerlike structures (Figure 3a). This is believed to be the result of an increase in casting solution viscosity due to microcapsules4,25 and the subsequent obstruction of water inflow and delay in solvent/nonsolvent exchange during phase inversion.26,27 The water flux through the microcapsule-embedded membrane was also slightly lower than that of the control membrane presumably because of increased physical obstruction of flow through the membrane (Figure 3b), while the particle rejection remained virtually constant (Figure 3c). Microcapsules added at concentrations higher than 10 wt % were found to induce defects in the membrane surface as more microcapsules migrate toward the surface along with the casting solution during phase inversion, significantly impacting both water flux and particle rejection. The concentration of microcapsules was therefore set at 5 wt %, the highest concentration that can be added without disturbing the intrinsic properties. To examine the self-healing property of the microcapsuleembedded membranes, sample membranes were damaged using a microtome with an approximate width of 37 μm, resulting in damage that is 0.40% of the total membrane area. The damage resulted in an expected increase in the water flux from 90.7 ± 5.0 to 109.0 ± 8.7 L m−2 h−1 and a decrease in particle rejection from 99.9 ± 0.1 to 81.3 ± 3.6% (Figure 4a). More than 24 h after the damage, the defect sites filled up with a newly formed material (Figure 4c,d) and the depth of the damage became shallower [from ∼37 μm for the damaged membrane to ∼10 μm for the healed membrane (Figure 4d)], effectively healing the damage site. We presume that this efficient plugging of the large damage area is a result of the

microcapsules by SEM. Thermogravimetric analysis suggested that the microcapsules contain approximately 78 wt % core material. See Figure S7 for a detailed discussion of these results. The release of IPDI from the core when the shell breaks was visibly confirmed by placing microcapsules between glasses and gently pressing the glasses (Figure S8) as well as by the appearance of an isocyanate stretching peak at 2275−2250 cm−1 in the Fourier transform infrared spectroscopy (FTIR) spectrum (Figure 2a).23 The released IPDI quickly reacts with nucleophiles such as amines or water to form a sturdy polyurea matrix (Figure S9a,b).14,16,18 The isocyanate stretching peak at 2275−2250 cm−1 concurrently disappears because of the exhaustion of reactive isocyanate. Additionally, the strong N− H absorption peak at 3320 cm−1 and the CO peak at 1664 cm−1 emerge, indicating the presence of urea linkages.16,18,23 These results collectively suggest that the IPDI core is successfully encapsulated within a polyurethane shell, and when released from the core, the shell reacts with amines/water to form polyurea that acts as the healing agent in the context of this study. Microcapsules prepared using 10.8 mM DTAB, the concentration that leads to sufficiently small microcapsules compared to the dimension of membrane thickness, were further treated to coat their surfaces with PDA. This step was necessary to avoid significant buckling (capsule shrinking) that occurs when microcapsules were dispersed in the membrane casting solution (Figure 2b). Buckling, or the loss of core solvent, occurs because of the difference in osmotic pressure between the core phase and the concentrated casting solution across the microcapsule shell. This structural deformation is known to depend on several factors, including the chemical composition, thickness, and permeability of the capsule shell.24 PDA, a versatile surface coating agent with a strong adhesion property, effectively covers the surface of the microcapsules to lower solvent permeability through the shell and possibly enhance structural robustness.12,19,20 The presence of PDA coating was confirmed through SEM analysis (Figure 2c, inset) C


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Figure 3. (a) Cross-sectional and top-view SEM images of membranes embedded with a varying amount of PDA-coated microcapsules. The crosssectional images were obtained by freezing the membrane in liquid nitrogen and instantly fracturing. All scale bars represent 100 μm. (b) Pure water flux of the membranes at 100 kPa and room temperature. (c) Rejection of 1 μm fluorescence microparticles and 50 nm fluorescence nanoparticles. Error bars represent the standard deviation (n = 3). The water flux was measured as filtrate weight over time using an analytical balance and the rejection by measuring the concentration of fluorescence particles in permeate using a spectrofluorophotometer (Shimadzu RF-5031PC; λex = 470 nm, and λem = 505 nm).

to be overcome to apply this technology in real-world practices. First, the performance of microcapsule-embedded membranes under more realistic damage scenarios needs to be explored. Because the healing performance was found to be dependent on the magnitude of the damage (Figure S10) and curing temperature, the effect of these parameters on the healing performance also needs to be studied. Second, improvements that use more benign chemicals and allow effective self-healing to occur under the flow of water would be optimal for the application of this technology in water treatment. Our ongoing research focuses on improving the healing performance by encapsulation of more reactive diisocyanates, augmenting the healing kinetics with catalysts (e.g., reactive diamine), optimizing the mode of incorporation of microcapsules into the membrane matrix (e.g., covalent boding between the microcapsule surface and membrane matrix), and exploring microvascular composite membranes for repeated healing, while searching for an environmentally benign healing agent with enhanced healing performance.

expansion of the polyurea matrix due to CO2 evolution as diisocyanate and water react to form urea linkages.14,17,18,28 A similar phenomenon of healing matrix expansion has previously been reported in other self-healing materials.14−16,18 The selfhealing restored the water flux to 93.7 ± 5.6 L m−2 h−1 and rejection to 90.0 ± 1.7%, which correspond to 103.2 and 90.4% performance recovery, respectively, compared to the undamaged membrane. A faster self-healing was observed when the temperature during the self-healing period was increased to 50 °C.29 The results presented here collectively suggest that we were able to fabricate a porous membrane with self-healing capability by embedding microcapsules that release a healing agent upon membrane damage. This successful proof of concept lays out the first groundwork for the development of next-generation membranes with unexplored functionality. Because polyurea is very stable against hydrolysis at room temperature, we expect that the healed membranes will have an adequate life span.30 However, this study still presents several challenges that need D


Letter


Figure 4. Change in (a) water flux and (b) rejection of 50 nm fluorescence nanoparticles due to membrane damage and self-healing. Error bars represent the standard deviation (n = 3). (c) SEM and (d) confocal laser scanning microscope (Nikon C2) images of pristine, damaged, and healed membranes. For CLSM images, membranes were immersed in ethanol containing 0.03 wt % rhodamine B for 30 min and rinsed with pure ethanol for 5 min. The images were obtained using a 10× objective lens and 4× software zoom at an excitation wavelength of 514 nm with depth scanning at 5 μm resolution. The three-dimensional images were constructed using IMARIS version 6.1.5 (Bitplane).

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ASSOCIATED CONTENT

and by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2014R1A1A2056302).

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.estlett.6b00046. Detailed experimental procedures (Figures S1−S3) and results of SEM, microscope, CLSM, and TGA analyses (Figures S4−S10) (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Telephone: +1-203-432-4386. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the International Cooperation Program for Industrial Technologies (N0001232) funded by the Korea government Ministry of Trade, Industry and Energy E


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Toward Microcapsule-Embedded Self-Healing Membranes

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