Article pubs.acs.org/Langmuir
Encapsulation of Aliphatic Amines Using Microfluidics Philipp W. Chen, Gian Cadisch, and André R. Studart* Complex Materials, Department of Materials, ETH Zurich, 8093 Zurich, Switzerland S Supporting Information *
ABSTRACT: The encapsulation of amines by emulsification and interfacial polymerization is important for smart adhesives and selfhealing materials but has been challenging because of the reactive nature of amines and their wide miscibility range. In this study, we propose a new method to encapsulate amines using double-emulsion templates made in a microfluidic device. The double emulsions contain an aqueous solution of the amine of interest in the innermost phase and a reactive mixture of acrylic monomers and initiator in the middle phase. Polymerization of the middle phase leads to acrylic microcapsules containing highly concentrated nonvolatile amine upon removal of water via evaporation. The presence of the amine inside the capsules is confirmed with NMR spectroscopy, and their reactivity is demonstrated by showing their effectiveness as cross-linkers of liquid epoxy resins.
1. INTRODUCTION Aliphatic amines are widely used as hardeners of epoxy resins in the manufacture of polymer-based fiber-reinforced composites. Such epoxy-based composites are attractive for the aerospace and automobile industries as a result of their light weight and high strength and stiffness. However, the polymeric epoxy matrix is inherently brittle, thus exhibiting limited resistance to crack formation and growth. Inspired by the autonomic selfrepair of natural systems, microcapsule-based self-healing concepts have been proposed, where microscopic damage in synthetic composites is repaired by releasing healing agents from capsules ruptured by an advancing crack.1 So far, microcapsules for self-healing materials are typically prepared through in situ polymerization in liquid emulsion templates, using, for example, urethane2 or urea/formaldehyde3 polycondensation reactions. In this process, the deposition of prepolymers on the surface of oil droplets generates microcapsules with submicrometer shell thickness. The first successful self-healing composite exploited the polymerization of encapsulated dicyclopentadiene initiated by Grubbs’ catalyst as the healing mechanism.1 However, the high cost of the catalyst prohibits its widespread application. A costeffective alternative system could consist of separately encapsulated liquid epoxy and amine hardeners, which mix and cross-link after the rupture of their respective capsules. Although several groups have reported the preparation of epoxy-filled capsules,4−6 the direct encapsulation of liquid amines has so far only been feasible using high-viscosity amine mixtures,7 which reduces their healing efficiency. Low-viscosity amines have been encapsulated by the vacuum infiltration of empty capsules8 or by a combination of solvent evaporation and phase separation,9 but those display higher permeability and loss of amine at elevated temperatures. Here, we present a microfluidic approach to encapsulating triethylenetetramine (TETA) and diethylenetriamine (DETA), © 2014 American Chemical Society
two common low-viscosity epoxy cross-linkers, using doubleemulsion templates. Because both TETA and DETA are miscible over a wide range of both hydrophilic and hydrophobic liquids, we initially encapsulate aqueous solutions of the amines to facilitate emulsification. The laminar flow conditions enabled by the microfluidic approach also help to circumvent the miscibility issues encountered using less-controlled conventional emulsification techniques. By using acrylate monomers as the middle oil fluid and loading it with a photoinitiator, the double emulsions can be consolidated into solid capsules through UV polymerization. Drying of the collected polymerized capsules removes most of the water content, leaving the less-volatile amine in the core. We investigate the encapsulation efficiency of this microfluidic approach using NMR spectroscopy, thermogravimetric analysis, and FT-IR spectroscopy. For TETA, we evaluate the reactivity of the encapsulated amine through mechanical tests of epoxies cured in the presence of the microcapsule extract. The results are discussed with regard to the potential of this controlled emulsification process in generating microcapsules for the fabrication of cost-effective self-healing epoxy-based composites.
2. EXPERIMENTAL SECTION 2.1. Microfluidics and Capsules. In the microfluidic process, an aqueous amine solution is first dripped from the emitter capillary into an oil fluid, which is then engulfed by an aqueous solution containing surface-active molecules. All liquids are flow-focused into the collector capillary to enable the controlled formation of double emulsions. We build glass capillary microfluidic devices consisting of two aligned microcapillaries inside a square glass tube following the procedure Received: July 11, 2013 Revised: February 14, 2014 Published: February 21, 2014 2346
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outlined by Utada et al.10 The inner tip diameters of the capillaries are polished to 180 μm for collectors and around 40−50 μm for emitter capillaries, whereas the intercapillary distance is 100−120 μm. To ensure efficient dripping from the emitters, we first hydrophobize them in a solution of octadecyltrimethoxysilane and butylamine in toluene. Unless otherwise noted, all chemicals and solvents are purchased from Sigma-Aldrich and used without further purification. The outer fluid is a mixture of a 2 wt % solution of poly(vinyl alcohol) (PVA, Mw = 31 000−50 000 g/mol, 87−89% hydrolyzed) and CaCl2·2H2O (Merck, Germany). The concentrations of calcium chloride are 1.2 and 2.0 M for the TETA and DETA experiments, respectively. The calcium salt is added to reduce the osmotic pressure across the middle monomer phase. For the middle fluid, we disperse 2 vol % hydrophobized silica particles (Fiber Optic Center, USA) in a solution of 50 wt % 1,6-hexanediol diacrylate (Alfa Aesar, USA) and 50 wt % bisphenol A glycerolate dimethacrylate. We also add 2 mol % of photoinitiator hydroxy-2-methylpropiophenone relative to the acrylates to enable shell formation via UV polymerization.11,12 The silica particles have a monodisperse diameter of 500 nm and are hydrophobized by refluxing overnight in a solution of 3(trimethoxysilyl)propyl methacrylate and butylamine in isopropanol at 90 °C. We disperse the particles in the monomer solution using a Vibra-Cell VCX 130 sonicator (Sonics & Materials, USA). The inner fluid is either a 50 wt % solution of TETA (96%, Dow Chemical Company, USA) or a 60 wt % solution of DETA (99%) in water. Double emulsions are formed by pumping the three fluids through the microfluidic device at appropriate rates. For the devices used in this study, typical flow rates are in the ranges of 5000−7000, 200−400, and 500−700 μL/h for the outer, middle, and inner fluids, respectively, and are chosen to maximize the amount of the encapsulant while maintaining a continuous production of stable double droplets. In particular, we typically set the inner flow rate to at least twice the middle flow rate. The double emulsions are collected in glass vials and simultaneously irradiated with UV light from an Omnicure 1000 light source (APM Technica, Switzerland) with an iris opening of 100% for at least 2 min. After polymerization, the capsules are washed with water and acetone, dried in air at 70 °C for 5 min, and stored in a desiccator under vacuum for at least 2 days. 2.2. Capsule Properties. We investigate the microcapsules under a DM6000 B microscope (Leica, Germany) and with a LEO 1530 scanning electron microscope (SEM, Carl Zeiss SMT, Germany), which is also equipped for energy-dispersive X-ray spectroscopy (EDX). To view cross-sectional areas, SEM samples are first crushed between two glass surfaces. To render the samples conductive, a 5 nm platinum layer is first deposited on them using an SCD 050 sputter coater (Bal-Tec, Liechtenstein). For size analysis, we measure at least 50 capsules optically. However, the inner diameters cannot be accurately obtained optically because of the different refractive indices of the fluids, especially in the case of thin shell walls. Therefore, the shell thickness is calculated from the fluid flow rates using a simple volume balance and verified using SEM analysis. To assess the mechanical properties of the microcapsules, we conduct compression tests with a custom-made device based on a setup by Keller and Sottos.13 An individual capsule is deformed by a dc motor actuator and controller (M-230.25 dc and C-863 Mercury, Physik Instrumente, Germany) at a deformation rate of 4 μm/s while a 30 g load cell (GSO, Transducer Techniques, USA) measures the applied load. The acquisition of load and displacement data is performed with an NI-9237 DAQ module (National Instruments, USA) and a motor controller, respectively. 2.3. Amine Content and Reactivity. To verify the presence of amines in the polymerized microcapsules, we crush capsules in a pestle and mortar. The released capsule contents are extracted with ethanol, filtered, dried under vacuum, and analyzed via NMR and FT-IR spectroscopy. The 1H and 13C NMR spectra are recorded using a Bruker Avance 500 spectrometer operating at 500.13 MHz using CDCl3 as the solvent. FT-IR analysis is conducted on a Tensor 27 spectrometer (Bruker, USA). To quantify the amount of amine and residual water within the capsules, we conduct thermogravimetric analysis (TGA) on dried capsules in a nitrogen atmosphere using
TGA/SDTA 851e equipment (Mettler Toledo, Switzerland) with a heating rate of 1 °C/min. For the investigation of reactivity, we focus on the TETA capsules. We again rupture capsules and then expose the extract to a stoichiometric amount of bisphenol A-based epoxy Araldite GY 250 ES (Huntsman, USA). The slurry of shell fragments, encapsulant, and epoxy is mixed and used to glue together two testing fixtures (inset of Figure 5) attached to a 4411 tensile testing device (Instron, USA). The cylindrical fixtures have a diameter of 25 mm and a top face cut at an angle of 45°. We allow the mixture to set for 3 days and then conduct a tensile test to demonstrate epoxy hardening qualitatively. To verify the stability of the capsules, this test is conducted 2 weeks after their production. We also evaluate the effect of residual water on the curing properties of the encapsulated TETA compared with as-received amine. For this, we purposely mix a 10:1 (w/w) solution of amine and water with an equivalent amount of Araldite epoxy and observe the curing kinetics of the resin using a Cary 5000 spectrometer (Varian, USA). The mixture is smeared onto KBr disposable IR cards with 0.5 mm PTFE spacers, and the absorbance between 2000 and 2500 nm is recorded every 5 min. A mixture of as-received TETA and epoxy serves as a control sample. For the epoxy conversion, we track the area of the epoxy peak at 2205 nm, which is normalized with respect to the phenyl peak at 2163 nm following the protocol suggested by Choi and co-workers.14 The epoxy conversion is then calculated as EC = 1 − A(t)/A(0), where A(t) is the normalized peak area at time t.
3. RESULTS AND DISCUSSION We form double emulsions consisting of an aqueous TETA or DETA solution as the inner phase and an acrylic monomer mixture as the middle phase with nearly 100% yield at a rate of 100−150 double droplets per second, depending on the flow rates. The controlled laminar flow conditions inherent in microfluidics are crucial to avoiding excessive mixing of the amine into the oil phase. In comparative experiments, attempts to obtain double emulsions through the standard emulsification of two phases followed by a second emulsification with the third phase proved inefficient, probably because of the extensive mixing between the fluids caused by the turbulent flow introduced during bulk emulsification. The double emulsions made by microfluidics are effectively stabilized by the combination of surface-active PVA molecules in the outer aqueous medium and modified silica nanoparticles in the middle oil phase. Because of their stability under the given flow conditions, the double-emulsion templates are successfully polymerized into capsules with a yield of almost 100%. Typical capsules exhibit outer diameters of around 160−170 μm (Figure 1a), which is comparable to the sizes of epoxy capsules reported in the literature for self-healing systems. Because of the well-controlled flow conditions provided by the microfluidic setup, the capsule polydispersity is just 2%. The shell thickness of the capsules in Figure 1 is around 15 μm. As is typical with microfluidic double emulsification, we can modify both the shell thickness and the capsule size by changing the fluid flow rates and using devices with different capillary dimensions.10,12,16 In the following experiments, the capsules have average diameters of 172 ± 2.9 μm and shell thicknesses of around 10 μm, which we achieve by increasing the flow rate of the inner liquid relative to that of the middle fluid. For these capsules, we obtain an average failure load of 75 ± 25 mN (N = 21), which is very high compared to that of other self-healing capsules13 but still allows for crack-induced rupture in a typical epoxy specimen.15 Optical microscopy and SEM micrographs clearly show the capsules’ hollow core−shell 2347
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Figure 2. 1H NMR spectra of commercial, as-received amines and of extracted residues from ruptured (a) TETA and (b) DETA capsules.
The 13C NMR spectra also conform to the expected structures (Supporting Information). To prove the existence of amine groups, we collect FT-IR spectra, which display N−H stretch signals between 3200 and 3500 cm−1 for both as-received and extracted TETA as well as DETA (Figure 3). Thermogravimetric analysis (TGA) of TETA capsules dried in a desiccator shows three significant mass losses at 50−100, 120−180, and 250−430 °C, which correspond to the evaporation of water, the decomposition of TETA, and the decomposition of the shell, respectively (Figure 4). Similarly, TGA of DETA capsules displays mass losses at 50−100, 100− 140, and again at 250−430 °C. The assignment of these three main mass losses to water evaporation, amine decomposition, and shell degradation is validated by testing control mixtures of water and the respective amine as well as a bulk sample of the capsule shell material, which display analogous curves (Figure 4). After TETA capsules are dried under 15 mbar of vacuum in a desiccator for 4 days, the TGA data indicate approximately 41−42 wt % TETA and 0.5 wt % residual water within the capsules, with the balance given by the shell. From the experimental flow rates used during emulsification, we expect a core-to-shell volume ratio of 2.08:1, which corresponds to a theoretical weight fraction of TETA of 48% relative to the total capsule mass after the complete removal of water. We therefore estimate an encapsulation efficiency of 88% relative to the original amount of TETA added. The partial loss of encapsulant can be attributed to slight diffusion and evaporation of the amine during emulsification and drying, respectively. DETA capsules were also evaluated using TGA after drying at 50 mbar for 7 days. Their TGA is more difficult to interpret
Figure 1. (a) Monodisperse microcapsules in water after polymerization. The average diameter in this batch is 164 ± 3.8 μm. (b) SEM micrograph of a dried and ruptured microcapsule. (c) Close-up of the capsule shell.
structure, as depicted in Figure 1. To rule out excessive diffusion or the reaction of the amine in the shell, we perform EDX scans on the cross-section of capsule shells, which show no or only negligible nitrogen signals compared to the visible lines of carbon, oxygen, silicon or even the sputtered platinum layer (Supporting Information). In these proof-of-principle experiments, the rate of production is limited to approximately 1 g of capsules/h as a result of the serial emulsification process and low fluid flow rates. However, ongoing efforts to upscale and parallelize microfluidic processes have shown that it is possible to produce well-controlled double emulsions at rates on the order of 1 kg/ day.17,18 Other techniques with similarly well-defined flow conditions such as membrane emulsification could push the production rate even further.19 The manual rupture of dried capsules shows the presence of a nonvolatile liquid left in the core. For both TETA and DETA, 1 H NMR analysis of the residue shows that the resonance data for the ethyl groups are identical to those of the pure amines (Figure 2). The amine signals differ slightly in shape and chemical shift, which may be attributed to concentration effects. 2348
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higher vapor pressure, which could lead to more evaporative losses during drying. The TETA remaining inside the capsules is still reactive and capable of hardening epoxy, as demonstrated by the strong film formed upon mixing the capsule extract with a stoichiometric amount of epoxy resin (Figure 5). The tensile test of the film
Figure 5. Force−displacement curve obtained from the tensile test of a cured film formed upon mixing the capsule extract with an epoxy resin. The inset shows the fixtures for the tensile tester and the ruptured epoxy film after the test.
Figure 3. FT-IR spectra of encapsulated and neat (a) TETA and (b) DETA. All spectra clearly exhibit the N−H stretch signals at around 3300 cm−1.
cured between the two fixtures (Figure 5 inset) shows a peak force of 825 N, which is followed by rupture and partial delamination of the film. In comparison, repeating the experiment using only epoxy without the capsules yields an adhesion force of only 4 N. The white color of the film results from the capsule shell fragments that were mixed into the epoxy along with the liquid extract. Because around 1−10 wt % of residual water relative to the amine may be retained within the capsules, it is also important to assess the effect of water on the curing properties of the encapsulated amine. Near-infrared spectra of both as-received TETA and the 10:1 (w:w) TETA−water mixture reveal conversion curves typical of epoxy curing at room temperature, which level out at a conversion of about 60% (Figure 6).
Figure 4. TGA curves of desiccated (a) TETA and (b) DETA microcapsules. Curves for the bulk shell material (dashed lines) and amine/water mixtures (dotted lines) are added for reference.
Figure 6. Epoxy conversion against curing time for epoxy mixed with as-received TETA and a 10:1 mixture of TETA/water.
because the mass losses of water and DETA overlap. We find approximately 2 wt % water, 41 wt % DETA, and 57% shell material, which corresponds to an encapsulation efficiency of around 80%. This lower efficiency compared to TETA encapsulation could be attributed to the smaller molecular weight of DETA, which results in a higher diffusivity and therefore higher losses during emulsification. DETA also has a
Interestingly, both the curing rate and the final conversion value actually increase with the addition of water. This is in line with previous findings reported in the literature, which show that molecules containing hydroxyl groups such as water or alcohols can act as catalysts for the epoxy curing reaction.14,20 2349
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(7) McIlroy, D. A.; Blaiszik, B. J.; Caruso, M. M.; White, S. R.; Moore, J. S.; Sottos, N. R. Microencapsulation of a reactive liquidphase amine for self-healing epoxy composites. Macromolecules 2010, 43, 1855−1859. (8) Jin, H.; Mangun, C. L.; Stradley, D. S.; Moore, J. S.; Sottos, N. R.; White, S. R. Self-healing thermoset using encapsulated epoxy-amine healing chemistry. Polymer 2012, 53, 581−587. (9) Li, Q.; Mishra, A. K.; Kim, N. H.; Kuila, T.; Lau, K.-t.; Lee, J. H. Effects of processing conditions of poly(methylmethacrylate) encapsulated liquid curing agent on the properties of self-healing composites. Composites, Part B 2013, 49 (), 6−15. (10) Utada, A.; Lorenceau, E.; Link, D.; Kaplan, P.; Stone, H.; Weitz, D. Monodisperse double emulsions generated from a microcapillary device. Science 2005, 308, 537−541. (11) Oh, H.-J.; Kim, S.-H.; Baek, J.-Y.; Seong, G.-H.; Lee, S.-H. Hydrodynamic micro-encapsulation of aqueous fluids and cells via ‘on the fly’ photopolymerization. J. Micromech. Microeng. 2006, 16, 285− 291. (12) Chen, P. W.; Erb, R. M.; Studart, A. R. Designer polymer-based microcapsules made using microfluidics. Langmuir 2012, 28, 144−152. (13) Keller, M. W.; Sottos, N. R. Mechanical properties of microcapsules used in a self-healing polymer. Exp. Mech. 2006, 46, 725−733. (14) Choi, S.; Janisse, A. P.; Liu, C.; Douglas, E. P. Effect of water addition on the cure kinetics of an epoxy-amine thermoset. J. Polym. Sci., Part A: Polym. Chem. 2011, 49, 4650−4659. (15) Neuser,, S.; Chen, P. W.; Studart, A. R.; Michaud, V. Fracture toughness healing in epoxy containing both epoxy and amine loaded capsules. Adv. Eng. Mater. 2013, doi: 10.1002/adem.201300422. (16) Erb, R. M.; Obrist, D.; Chen, P. W.; Studer, J.; Studart, A. R. Predicting sizes of droplets made by microfluidic flow-induced dripping. Soft Matter 2011, 7, 8757−8761. (17) Rotem, A.; Abate, A. R.; Utada, A. S.; Van Steijn, V.; Weitz, D. A. Drop formation in non-planar microfluidic devices. Lab Chip 2012, 12, 4263−4268. (18) Romanowsky, M. B.; Abate, A. R.; Rotem, A.; Holtze, C.; Weitz, D. A. High throughput production of single core double emulsions in a parallelized microfluidic device. Lab Chip 2012, 12, 802−807. (19) van der Graaf, S.; Schroën, C. G. P. H.; Boom, R. M. Preparation of double emulsions by membrane emulsification-a review. J. Membr. Sci. 2005, 251, 7−15. (20) Chen, J.; Nakamura, T.; Aoki, K.; Aoki, Y.; Utsunomiya, T. Curing of epoxy resin contaminated with water. J. Appl. Polym. Sci. 2001, 79, 214−220.
4. CONCLUSIONS AND OUTLOOK Microcapsules filled with a reactive amine can be successfully formed by encapsulating an aqueous amine solution using double-emulsion templates generated in microfluidic devices. The water can be removed from the as-fabricated microcapsules by evaporation, leaving an amine-filled core that is still reactive and capable of curing epoxy systems. This versatile technique also allows for the precise production of capsules with defined sizes and shell thicknesses and therefore defined mechanical properties. Although the throughput of capsules made from the glass capillary microfluidic devices utilized in this proof-ofconcept study is significantly lower than the quantities typically achieved in bulk emulsification processes, our results demonstrate the feasibility of encapsulating reactive amines under the well-defined flow conditions enabled by microfluidics. Actual self-healing of an epoxy resin containing such amine capsules as well as epoxy capsules produced with traditional methods has also been recently demonstrated.15 Translating our strategy to highly parallelized processes using microfluidics or membrane emulsification techniques should enable the preparation of much larger quantities of amineloaded microcapsules, with the ultimate goal of producing costeffective self-healing composites based on amine−epoxy curing.
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ASSOCIATED CONTENT
* Supporting Information S 13
C NMR spectra of encapsulated and extracted DETA and TETA compared to as-received amines. Representative EDX spectra measured across a fracture surface of a capsule shell indicating the absence of nitrogen and thus amine within the shell. This material is available free of charge via the Internet at http://pubs.acs.org/.
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AUTHOR INFORMATION
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS We thank Véronique Michaud, Sam Neuser, and Tobias Niebel for constructive discussions; Doris Sutter, Michael Heinrich, and Eve Loiseau for technical assistance, and ETH Zurich for financial support through an ETHIIRA grant (no. 0-20676-10).
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REFERENCES
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