Functional Microcapsules with Hybrid Shells Made via SolâGel

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Functional microcapsules with hybrid shells made via sol-gel reaction within double emulsions David G. Moore, Jonathan V. A. Brignoli, Patrick Alberto Rühs, and André R. Studart Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b01503 • Publication Date (Web): 16 Aug 2017 Downloaded from http://pubs.acs.org on August 20, 2017

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Functional microcapsules with hybrid shells made via sol-gel reaction within double emulsions David G. Moore, Jonathan V.A. Brignoli, Patrick A. Rühs, André R. Studart*

André R. Studart, Vladimir-Prelog-Weg 5, HCI G537 Complex Materials, Department of Materials, ETH Zürich, 8093 Zürich, Switzerland *corresponding author: [email protected]

Abstract Microcapsules with organic-inorganic hybrid shells can be used as functionally-responsive delivery systems that are attractive for a broad range of applications. Hybrid-shell microcapsules have often been synthesized by the assembly of solid inorganic nanoparticles and polymers. Efforts to extend this approach to microfluidic emulsification have been hampered by problems with clogging and flow instabilities when utilizing dispersions of solid particles. In this work, hybrid shell microcapsules are synthesized through the reaction of liquid precursors, eliminating the use of solid dispersions. Our microfluidic water-oil-water emulsification technique also enables the preparation of hybrid-shell microcapsules with thicker and more robust shells compared to alternative techniques. By utilizing bridged-silane precursors to form the hybrid material, we demonstrate hybrid-shell microcapsules with independently tunable functional and mechanical/barrier properties. This independent tuning of physical and functional properties allows for the production of functional organic-inorganic hybrid shell microcapsules that can be tailored to meet the demands of a wide range of applications.

Keywords: sol-gel, microcapsules, organic-inorganic hybrid, microfluidics

Introduction Microcapsules made via emulsification are utilized to protect, transport, store or 1

2

deliver functional and active ingredients for applications in cosmetics, pharmaceuticals, and agriculture.3 Beyond established applications, more advanced encapsulation systems allow for the incorporation of responsive functionalities such as externally-triggered release,4 targeted delivery,

5,6

or the selective permeability

7,8

of encapsulants. These added

functionalities, however, are all for naught if the microcapsule lacks the barrier and mechanical properties needed to sustain efficient encapsulation. The importance of mechanical and barrier properties becomes clear when one considers the harsh handling and usage conditions that microcapsules are typically exposed to, such as the acidic environment 5

of the stomach during the passage of encapsulated probiotics, the sustained exposure to direct sunlight of encapsulated pesticides,9 or the high shear forces experienced by encapsulated fragrances used in clothing detergents.10 Additionally, designing a microcapsule

ACS Paragon Plus Environment

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to rupture at a specific stress range is desirable for deodorants and cosmetics4 as well as for the

encapsulation

of

self-healing

11

agents.

Therefore,

the

design

of

advanced

microencapsulation systems for protection and controlled release requires versatile processing platforms that enable tuning of the functional response while maintaining desired 12

mechanical and barrier properties.

Because of their intrinsic materials properties, inorganic particles offer a practical means of achieving functional responses. For instance, magneto-responsive iron oxide particles can be used to manipulate microcapsules and improve local drug delivery,13 whereas light-responsive titanium dioxide particles can be used to trigger release from a capsule upon illumination.

14

Despite the wide range of possible functional properties, the

inherent brittleness of oxide materials of interest makes the microcapsule shell mechanically weak. To address this issue, inorganic particles are normally incorporated into a mechanically robust polymer, forming capsules with hybrid organic-inorganic shells. While hybrid materials offer the combined benefit of tunable functionality and mechanical stability, the very different chemical natures of the organic and inorganic building blocks imposes processing challenges during the formation of the capsule shell. For instance, when pre-formed inorganic nanoparticles are used as building blocks in microfluidic encapsulation devices, one may encounter

issues

like

clogging,

agglomeration,

or

emulsion

destabilization

during

microcapsule formation. Moreover, it is often difficult to predict how inorganic nanoparticles will interact with the other components of the emulsion formulation. As an example, it has been shown that, depending on the surfactants used, gold nanoparticles dispersed in a hexane-in-water emulsion may agglomerate into ordered structures within the oil phase, enable synergistic stabilization of the surfactant-laden interface, or even destabilize the emulsion.15 The processing challenges arising from the combination of organic and inorganic building blocks require the development of alternative controlled assembly strategies to produce capsules with hybrid shells. A possible assembly approach to controllably form hybrid shells is to deposit the organic and inorganic building blocks in a stepwise, sequential manner. This has been realized either through the layer-by-layer deposition of inorganic particles and polymeric species on emulsion templates or by the interfacial assembly of particles on droplets followed by the growth of a polymer layer from the particle surface.

16,17,18

In the case of layer-by-layer techniques, the nanoparticles are adsorbed by electrostatic forces to charged polymers, which form the initial shell. The common strategic feature of these techniques is that particles can be used at lower and therefore less problematic concentrations, and only later be concentrated at the microcapsule shell by electrostatic or interfacial forces, thus making it amenable to bulk emulsification.

19

The sequential nature of approaches relying on the stepwise deposition of organic and inorganic building blocks makes the preparation of capsules very time-consuming and


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inefficient. Moreover, the shells of microcapsules produced with these methods are often very thin, which leads to very permeable and weak capsules. One successful strategy for decreasing the shell permeability of thin-shelled microcapsules is the use of metallic coatings.

20-22

The use of double emulsions made in microfluidic devices as templates for

capsule formation is an effective strategy to address issues of permeability, by generating thick shells with controllable thickness. Such capsule templates are formed through the simultaneous dripping of an aqueous solution within an oil phase in a flow-focusing microfluidic device.23 This dripping behavior is characterized by a critical point where shear forces on the droplet exceed the interfacial tension holding it to the emitter.23 As the amount of capsules produced by microfluidics is limited, current work is focused on the parallelization of microfluidic emulsification devices, which could soon lead to high-throughput production of microcapsules.

24-26

A single-step procedure to form functional microcapsules with thicker and stronger hybrid shells without undergoing agglomeration and device clogging issues would be possible if inorganic and organic precursors in molecular form could be simultaneously incorporated in the middle phase of double emulsion templates made by microfluidics. For water-oil-water emulsions this requires the two precursors to be compatible and dispersible in the middle oil phase of the soft template. An elegant route to implement this strategy is to use metal alkoxide-functionalized precursors as building blocks that would simultaneously carry both inorganic and organic segments within the same molecule. The time-lapsed hydrolysis and condensation of such precursors through established sol-gel reactions could produce microcapsules with hybrid shells in a single-step, particle-free procedure. This approach would benefit from the extensive work that has been conducted on the mechanical and functional properties of solgel based hybrid materials made in bulk form.27,28 On the basis of this previous work, we know that a hybrid material can be directly assembled from a single, well-defined building block, if bridged-silane precursors are used. Bridged-silane precursors are polymers with silane end groups that hydrolyze and condense in the presence of water to form a crosslinked polymer network. Since the modulus of elasticity of a single flexible polymer chain scales inversely with its molecular weight,29 the polymers formed by these precursors usually have very well defined and homogenous mechanical properties, given by the polymer chemistry and chain length. Additionally, a variety of polymers can be synthesized with bridged-silane precursors ranging from self-healing,30 biodegradable,31 and stimuli-responsive materials.32 Bridged-silane precursors have also recently been utilized for the formation of porous microparticles using an emulsion templating technique,33 but no microcapsules have yet been produced with these materials. In this work, we present a method for forming microcapsules with organic-inorganic hybrid shells through the time-delayed sol-gel reaction of a tailored bridged-silane precursor


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within the oil phase of a water-oil-water (w/o/w) double emulsion. An oil phase is chosen that enables full dissolution of the bridged-silane precursor and prevents immediate reaction of this reactive precursor with the continuous aqueous phase. Later, slow evaporation of such oil leads to the hydrolysis and condensation reactions needed for the formation of a network of bridged-silanes in the shell. This technique is amenable to organic-inorganic hybrid syntheses that are driven by the water-induced reaction of bridged-silane precursors. A key facet of this approach is the solubilization of the bridged-silane precursors in an oil phase that can be slowly removed by diffusion and evaporation. The use of a water-immiscible solvent slows the rate of water-catalyzed gelation, preventing gelation within the microfluidic device and producing homogenous polymer shells. Additionally, the sol-gel induced gelation technique demonstrated in this work requires no post-processing steps after emulsification and is therefore a good candidate for scaled-up microfluidic approaches. Bridged-silane precursors ranging in hydrophobicity from fully water-miscible to water-immiscible are investigated here to establish relevant chemical architectures for the design and synthesis of hybrid capsules using this simple, one-step processing route. By changing the length of the bridged-silane precursor, we demonstrate that large ranges of mechanical properties are accessible. The addition of metal alkoxides to the system enables tuning of the inorganic content and particle size, imbuing the polymer with added functionalities such as photocatalytic activity without affecting mechanical properties. This approach offers a flexible and facile technique for the incorporation of sol-gel derived organic-inorganic hybrids into microcapsule shells.

Experimental Section Materials Due to the water sensitivity of silane precursors, all solvents were used in analytical form and dried with molecular sieves. Poly(propylene glycol) diamines, under the trade name Jeffamine, were kindly provided as a sample from Huntsman. Molecular weights of such PPG diamines were taken from the provided technical data sheets. Bridged-silane poly(propylene glycol)s precursors were formed by the reaction of 2.2 molar equivalents of (3Glycidoxypropyl)trimethoxysilane (GPTMS) with poly(propylene glycol) diamines, under reflux conditions, at 110 °C for 4 hours. For PPG triamines, 3.3 molar equivalents of GPTMS were used. For Jeffamines D230, D400, and T403 this was carried out at 80 °C due to their high vapor pressures. Octadecyltrimethoxysilane (ODTMS), tetraethoxyorthosilicate (TEOS), titanium (IV) n-butoxide (TNBT), GPTMS, and silane-modified poly(propylene glycol)s were stored in a nitrogen flooded cabinet when not in use. Fluorescein isothiocyanate-labeled dextrans were utilized at 0.1 weight percent solutions in deionized water. Silver nitrate solutions were also prepared as 0.1 weight percent solutions in deionized water. Poly(vinyl alcohol) (PVA) solutions were prepared by dissolving 31,000–50,000 g/mol (87-89% hydrolyzed) PVA in water at 80 °C for 24 hours.

Preparation of Inorganic-Organic Hybrid Films


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PPG bridged-silane precursors were cast into hydrophobized glass petri dishes and left overnight in a humidity chamber at 37 °C. In order to hydrophobize petri dishes, they were exposed for 4 hours to a toluene solution containing 5 wt % ODTMS and 0.5 wt % n-butyl amine, then rinsed and dried. After overnight gelation, the hybrid films were mechanically punched into dogbones with 12 x 2 x 1 mm gauges and tested under tensile load using a Shimadzu tensile tester (Model AGS-X) with 1 mm/s strain rate. Films for photo absorbance measurements were prepared in a similar fashion to tensile test specimens, however the precursors were cast directly into TPP 24-well culture plates. An appropriate volume was chosen to ensure 2 mm thick films. The absorbance was measured at room temperature with a Tecan Infinite M200 Plate Reader.

Synthesis of Microcapsules Microcapsules were formed using glass capillary microfluidic devices. These microfluidic devices were fabricated and operated according to previously outlined 34

methods. . Briefly, emitter capillaries were fabricated with a 25 micron diameter opening using a microforge and collector capillaries were fabricated with a 250 micron diameter opening by polishing, and the two were spaced approximately 120 microns apart in a 1.05 mm square capillary. The emitters were hydrophobized with a solution of ODTMS, butyl amine, and toluene as mentioned in the previous section. Flow rates varied due to the differences in viscosity of the oil phases, but typical flow rates were in the range of 25 mL/hour for the outer phase, 2 mL/hour for the middle phase, and 2 mL/hour for the inner phase. The outer phase was a 2.0 wt % PVA aqueous solution, which stabilizes the emulsion, the inner phase was pure water, and the middle phase consisted of 25 wt % bridged-silane precursors in a mixture of toluene and dichloromethane at a weight ratio of 3:1, respectively. After producing w/o/w emulsions with the described set-ups, samples were left in open containers in ventilated hoods to allow the evaporation of the toluene and dichloromethane. Microcapsules were rinsed and allowed to dry onto glass slides. In order to examine the interior of the shell, the microcapsules were pressed between two glass slides to break them. Microcapsule SEM samples were sputtered with 3 nm Pt. Micrographs were taken using a Zeiss LEO 1530 SEM at a working distance of between 3 and 5 mm and an acceleration voltage between 2 and 3 kV.

Determination of Lower Critical Solution Temperature In order to determine the lower critical solution temperature (LCST) of the bridgedsilanes the corresponding diamine precursors were examined. Because of the reactivity of bridged-silane precursors, they could not be directly observed, but the solubility of the diamine precursor was assumed to be similar to that of the bridged-silane precursor. Solutions of 1.0 wt % diamine were cooled to 5 °C and slowly heated. The LCST was determined by the temperature at which the solutions clouded.


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Quantifying Permeability and Diffusion Coefficients Permeability and diffusion coefficients were measured using a Zeiss LSM 510 confocal microscope. Diffusion coefficient was determined using fluorescence recovery after photobleaching (FRAP).

35

FRAP could not be performed directly on the microcapsule shell,

because the shells were too thin and diffusion was not symmetric. Therefore, FRAP was performed on microparticles that were made by forming single emulsions instead of double emulsions, using the same precursor/solvent solutions (Figure S1). We created the microparticles by running the microfluidic experiment as a single emulsification step instead of a double emulsification. Because the same devices were used for both experiments, the outer radius of the microparticles was very similar to the outer radius of the microcapsules. Therefore, we assume similar diffusion kinetics to the outside of the capsule and particle. Organic-inorganic hybrid materials formed by sol-gel techniques are generally homogenous as long as all reagent diffusion processes are faster than the gelation processes.

27

We expect

the diffusion of water to be much faster than the gelation process, which takes 24 hours. Therefore, the hydrolysis and condensation of the polymer is not diffusion limited and should be homogenous throughout the microparticle. The microparticles were allowed to come to equilibrium in solutions of FITC-labeled dextrans, were photobleached with a 405 nm laser, and then the recovery of fluorescence was monitored over time to determine the diffusion coefficient. The diffusion coefficient was calculated by using the following curve fitting, where w is the width of the bleached region, and Ifinal is the plateau value for the intensity of fluorescence, I (see Figure S1 for a typical set of experiments): ‫ܫ = ܫ‬௙௜௡௔௟ (1 − ݁ ି௪

మ ௧/ସ஽

)

(1)

In order to measure the permeability, P, of FITC-labeled dextrans through the microcapsule shell, the concentration inside the microcapsule, Cin, and outside the microcapsule, Cout, after introducing the dextrans into the outer phase of a sample well containing microcapsules was monitored. If the fluorescence is assumed to be in a linear regime then the ratio of Cin/Cout can be approximated as Iin/Iout, where Iin is the fluorescence intensity inside the capsule, and Iout is the fluorescence intensity outside of the microcapsule. Finally, permeability is calculated through curve fitting using Equation 2, where m is the capsule thickness, and r is the capsule radius (Figure S2): ௗ

ூ

ܲ = ൬ௗ௧ ቂூ ೔೙ ቃ

೚ೠ೟ ௧ୀ଴

൰

௠௥

(2)

ଷ

Results and Discussion The architecture and properties of the hybrid microcapsules proposed in this work are schematically depicted in Figure 1. The physical and functional properties of the microcapsules can be tuned independently by designing the organic and inorganic precursor


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building blocks added to the oil phase of the templating double emulsion. Using polymers that crosslink through silane end groups, we create an organic-inorganic hybrid material where the inorganic phase is concentrated at the nodes of the polymer network (Figure 1). The size and composition of such inorganic nodes can be tuned through the addition of metal alkoxide precursors, which will undergo condensation with each other and with the silane to form functional nano-clusters between the polymer segments. One of the advantages of this sol-gel technique is that the organic and inorganic moieties generated upon condensation of the silane-functionalized polymers can be tuned in an orthogonal fashion to affect the mechanical and functional properties of the hybrid material independently. For instance, mechanical properties of bridged-silane organic-inorganic hybrids can be tuned by adjusting the molecular weight of the organic bridging group of the precursor, whereas functional properties like magnetism, color or catalytic activity can be implemented through the choice of the metal in the alkoxide. The stiffness of the microcapsule depends on the molecular weight and chemistry of the organic segments between silane end groups. Additionally, both hydrophobic and hydrophilic organic segments may be used, allowing for control over permeability. In the case of poly(propylene glycol), the degree of hydrophobicity of the polymers can be tuned by simply adjusting the molecular weight of the polymer segment, as indicated in Figure 1.

Figure 1. Schematic of organic-inorganic hybrid microcapsules with a shell consisting of a network of functional inorganic nanoclusters interconnected by polymer chains of tunable molecular weight. By increasing the molecular weight of the polymer segments (labeled green), the capsule shell hydrophobicity and elasticity can be tuned. With the addition of metal alkoxides, inorganic nanoclusters (labeled red) are created between the polymer segments, leading to unique magnetic, catalytic, and optical functionalities.

We illustrate the properties and functionalities of the proposed hybrid shell using poly(propylene glycol) as the polymer segment. Poly(propylene glycol) with reactive silane


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end groups was prepared by the reaction of commercially available amine-terminated PPGs, known as Jeffamines, with 3-(glycidoxypropyl)trimethoxysilane (GPTMS), leading to the functional macromolecules shown in Figure 2a. These bridged-silane precursors are formed by combining stoichiometric amounts of reagents and heating to 80-110 °C. An alternative approach

to

functionalize

PPG

diamines

with

silanes

isocyanatepropyltriethoxysilane (ICPTES) instead of GPTMS.

36,37

would

be

to

use

The resulting bridged-silane

precursor is stable under dry conditions for several days. In order to form a gel, the bridgedsilane precursor is exposed to water, leading to hydrolysis and condensation reactions

38

(Figure 2b). Additional metal alkoxide precursors, such as tetraethyl orthosilicate (TEOS), can be added to the mixture to deliberately control the fraction of organic and inorganic phases. Because this material is formed by water-driven assembly under ambient conditions, reproducible and homogenous specimens can be reliably obtained.

Figure 2. Poly(propylene glycol) trimethoxysilanes (PPG-TMS) used as precursors for organic-inorganic hybrids. (a) Silane-functionalized PPG gels are formed by the reaction of a glycidyl-terminated silane with the amines of Jeffamine, a commercially available polymer with various molecular weights and degrees of branching. (b) The silane-functionalized PPGs gel in the presence of water through hydrolysis and condensation. (c) The inorganic phase is concentrated in the nodes of the gel, forming a transparent, rubbery solid. Silanefunctionalized Jeffamine D2000 was used to obtain the shown polymer.

To explore the range of mechanical properties that can be reached with this PPGSiO2 system, we evaluate the effect of the hybrid architecture on the strength, stretchability and stiffness of bulk composite specimens. Homogeneous dogbones for mechanical testing were prepared by casting the sol-gel precursors into films. The hydrolysis and condensation


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of siloxane-crosslinked poly(propylene glycol) (PPG) gels leads to organic-inorganic hybrids in bulk form that are transparent and flexible (Figure 2c). The mechanical properties of hybrid films with different architectures are shown in Figure 3. The Young’s modulus of the bulk material decreases markedly with increasing molecular weight of the organic bridging group (Figure 3a-d). The effect of the molecular weight of the organic bridging segment on the elastic modulus can be interpreted by assuming that the hybrid material is formed by a rubber-like crosslinked network of flexible polymer chains interconnected by inorganic nodes (Figure 1 and 2). By contrast, minor changes in the Young’s modulus were observed when an inorganic precursor such as TEOS is added to the D2000 formulation (Figure 3e and 3f). This suggests that additional inorganic precursors concentrate indeed in inorganic nodes, keeping the molecular weight of the interconnecting polymer segment and thus the mechanical properties unchanged. The decrease in elastic modulus of the hybrid material for increasing polymer molecular weight was accompanied by an expected increase in the maximum deformation at break, εr, and decrease in the tensile strength σr (Figure S3). As observed for the elastic modulus, only a small change in εr and σr was seen upon further addition of inorganic precursor. Overall, the elastic modulus between 0.5 and 50 MPa of the material used in this work is relatively low when compared to previously demonstrated microcapsule materials. For instance, the elastic modulus of acrylates can achieve elastic moduli over one GPa39 and polyurea microcapsules achieve moduli of 500-1000 MPa.1 Thus, the proposed hybrid material extends the available range of mechanical properties achievable within the capsule shell.


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Figure 3. Mechanical properties of siloxane-crosslinked poly(propylene glycol) organicinorganic films with respect to organic and inorganic precursor composition and hybrid architecture. The effect of the addition of (a-d) organic precursors of different molecular weights and (e-f) inorganic precursors (TEOS in D2000) at different concentrations on the mechanical properties of the hybrid films.

The effect of molecular weight of the organic bridging group on mechanical properties of the hybrids films can be quantitatively interpreted in terms of the cross-linking density of the rubber-like network. For entangled and highly crosslinked networks the Young’s modulus, E, typically follows a power law dependence with the crosslink density, Cx: E ≈ Cxn. Cx is defined by the ratio of polymer density, ρ, and functionality, f, to molecular weight of precursor,29 Mw: Cx = fρ/Mw. We find that our Young’s modulus data closely follows this simple relation over more than two orders of magnitude for both the difunctional and trifunctional precursors (Figure 4a). An exponent n = 2.46 is obtained by fitting the above equation to our experimental data, which is in very close agreement with value of 2.5 reported in previous work.40 The tensile strength of films also followed a similar power law dependence relative to the crosslink density with an exponential factor of n = 1.83, which is also close to the scaling law of 1.90 for previously reported work

40

(Figure 4b).

Figure 4. (a) Elastic modulus and (b) tensile strength of PPG bridged-silane organic-inorganic hybrid films show power law dependencies with the density of crosslinks, Cx. Assuming a β

5

power law dependence of the form E,σ = KCx , we obtained K values equal to 2.06×10 and 6

5.00×10 from the fittings to E and σ data, respectively.

The flexibility of the proposed sol-gel chemistry was exploited to tune the mechanical and functional properties of capsule shells by incorporating the PPG bridged-silane precursors within the oil phase of w/o/w double emulsions made in a microfluidic device. In this case, organic-inorganic solid shells were formed through the in situ gelation of the precursors, added to the middle oil phase, when in contact with water (Figure 5a). To demonstrate the microcapsule formation process, double emulsions were prepared using conventional capillary microfluidics (Figure 5b). Briefly, three phases are coaxially flow-


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focused resulting in a dripping behavior, where a water-immiscible middle phase separates 34

aqueous inner and outer phases.

The middle phase is constituted of 25 wt % precursors

that are dissolved in a mixture of toluene and dichloromethane at a 3:1 weight ratio in order to achieve a density slightly less than that of water and prevent coalescence of double emulsions due to large density mismatch. This also results in microcapsules that float to the surface to allow for solvent evaporation. After gelation and solvent evaporation the microcapsules become slightly denser than water and sink, creating a mechanism that ensures the formation of homogenous microcapsules. Because water simultaneously diffuses into the oil phase, the bridged-silane precursors undergo gelation through hydrolysis and condensation reactions. As an example, a bridged-silane precursor with a molecular weight of ~2000 g/mol was emulsified in this manner, and after approximately 24 hours complete gelation occurred, producing rubbery, transparent microcapsules of approximately 200 µm diameter (Figure 5b and 5c). Overall, the diameter and shell thicknesses of the generated droplets could be easily tuned within the ranges 70-250 µm and 4.5-35 µm, respectively.

Figure 5. Synthesis of microcapsules with organic-inorganic hybrid shells. The organic and inorganic precursors are dissolved in solvents with only slight water solubility (shown in yellow). The resulting mixture is used as the middle phase of double emulsions. (a) Interdiffusion between such solvents and the aqueous phase allows the precursors to gel through hydrolysis and condensation. (b,c) w/o/w emulsion templates containing precursors in the oil phase are prepared using capillary microfluidics, leading to rubbery polymer-shell microcapsules after removal of the solvents.

The inter-exchange of silane-functionalized PPG precursor and water molecules between the droplet middle phase and the aqueous phase triggers the gelation reaction and


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determines the final hybrid shell architecture. We observed that the architecture of the hybrid shells is influenced by the molecular weight of the PPG segment (Figure 6a-d). Hybrid capsules made from PPG precursors with lower molecular weight (≤ 430 g/mol) exhibit nanoporous structures in the shell after the sol-gel reaction (Figure 6a-c). By contrast, we observed no porosity down to the tens of nanometer scale for shells generated from PPG segments of higher molecular weight (≥ 2000 g/mol) (Figure 6d). The presence of pores in shells containing low-molecular weight PPG-silane precursors can be tentatively explained by the general phase behavior of PPG-water-oil mixtures.

41

PPG is known to display a lower

critical solution temperature (LCST) in water and an upper critical solution temperature (UCST) in non-polar medium (oils) above and below which the polymer segments precipitate, respectively.41 An increase of the molecular weight of PPG reduces both critical temperatures in the binary systems. When combined with both water and a hydrophobic oil, PPG precipitates can co-exist with the aqueous phase and the oil. Following the trend observed in the binary systems, the position of such a three-phase separated system in the phase diagram is also affected by the molecular weight of the PPG segment.

41

Increasing the

molecular weight of PPG moves the three-phase region to lower temperatures. It is therefore possible that the porous structures observed in systems containing lower molecular weight PPG (≤ 430 g/mol) reflect the formation of such a three-phase heterogeneous mixture during the intermixing of the oil, water and PPG-silane molecules. Drying of such a three-phase mixture should generate porous structures because the evaporated aqueous and oil phases work as templates for the formation of pores within the remaining polymer-rich phase. We expect that systems containing PPG with higher molecular weight show the three-phase mixture below room temperature, thus leading to a more homogeneous binary solution during emulsification and thus non-porous smooth shells after the sol-gel reaction and subsequent drying. While further work is needed to confirm the interpretation above, we carried out cloud point measurements to assess the effect of the molecular weight of the PPG precursor used in this work on its lower critical solution temperature (LCST) in water. In this work, the LCST of the water-PPG binary systems will be referred to rather than the UCST of the PPGs in the oil-water-PPG ternary system because the former transition is more easily observed. The correspondence of the LCST of the PPG-water binary system with the UCST of the three phase region of oil-water-PPG mixtures has been previously shown.41 In order to test this claim, PPG-containing oils were put in contact with water at three different temperature: 10 °C, 25 °C, and 80 °C. In each of the three cases, oil phases containing PPGs with LCST above the fixed temperature became turbid, while the others remained transparent (Figure S4). For instance, at 10 °C the samples D230, D400, T430, and T3000 all showed turbid oil phases, while at 25 °C this was the case only for D230, D400, and T430. At 80 °C all oil phases were transparent. Because of the high reactivity of silanes with water, we measured the LCST of the unmodified amine-terminated PPGs as an indicator of the solubility of the silane-ended PPG molecules. The obtained LCST values were found to decrease with


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increasing molecular weight, in line with the arguments used in the discussion above (Figure 6e).

41

Amine-terminated PPG precursors with LCST below and above room temperature are

labeled green and red in Figure 6e, respectively. Despite their different solubilities in water, all the investigated bridged-silane precursors were soluble in the dichloromethane/toluene solvent mixture, thus enabling their incorporation into the middle oil phase of the double emulsion template.

Figure 6. Microstructure of hybrid shells and thermal response of PPG precursors of different molecular weight. (a-c) Organic-inorganic hybrid shells formed from emulsions that contain PPGs with LCST above 25 °C (MW < 430 g/mol) show porosity after freeze-drying. (d) Shells obtained from polymers with LCST below 25 °C, such as the trifunctional amine-terminated PPG with MW of 3000 g/mol, contain no porosity after freeze-drying. (e) Effect of the molecular weight of amine-terminated PPGs on their lower critical solution temperature (LCST) in water. With an increase in molecular weight of the initial PPG compounds, the LCST decreases.


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In order to test the barrier properties of the microcapsules, confocal microscopy techniques (Figure 7a-b) were utilized. Barrier properties were quantified by measuring the bulk diffusion coefficient, D, and the permeability, P, of the hybrid shell of microcapsules. For this purpose, fluorescein isothiocyanate (FITC)-labeled dextrans were chosen as model diffusants that combine water solubility and availability in a wide range of molecular weights. The measured diffusion coefficient (D) and permeability (P) values are correlated through the following relation:

42

P = KD/h, where h is the shell thickness and K is the partitioning

coefficient. For the three porous shell materials investigated, two distinct behaviors are observed: molecular weight dependent and molecular weight independent diffusion (Figure 7c). Overall, the absolute values of the diffusion coefficient decrease with an increase in molecular weight of the precursor (Figure 7c, inset). Normalizing the absolute values of D by the diffusion coefficients of the diffusants in water, D0, we find that the diffusion coefficient of hybrid polymers obtained from precursors D230 and D400 scale linearly with D0. Thus, the decrease in D with the increase in molecular weight of the precursors is in this case caused solely by the larger size of the diffusant, as predicted by the Stokes-Einstein relation. We interpret this molecular weight independent behavior as diffusion through an open porous network,43 in which the permeable pores are much larger than the size of the diffusing molecule. In this case, the diffusion coefficient is simply given by the diffusion coefficient in water multiplied by a ratio of tortuosity, τ, and open porosity, ϕ: D = (ϕ/τ)D0. For the hybrid material made from the precursor T430, however, a striking decrease in diffusion coefficient with molecular weight was seen (Figure 7c). Because the molecular weight dependence for T430 remains even after the D values are normalized by D0, our results suggest that the diffusant in this case is transported through the microporous network defined by the water-swollen polymer phase. In this case, one should expect the diffusion coefficient to follow a power law dependence, as -β

follows: D/D0 ~ MW with a factor of β = -2 for diffusion by reptation globular diffusion.

45

44

or a factor of β = -1 for

Indeed, a trend in diffusion is observed, which follows a power law with -2

< β < -1 (Figure 7c). Permeability of the microcapsules followed similar trends as diffusion, with decreasing permeability for increasing molecular weight of diffusant (Figure 7d). This result indicates that the effect of the microstructure of the hybrid shell on the bulk diffusion coefficients can be directly translated into capsule shells to create microcompartments with tunable permeability. Overall, the model diffusants permeated the hydrophilic membranes either through interconnected pores or through the water-swollen polymer mesh. Permeability of the inorganic-organic hybrid capsules to dextran (Mw = 10,000–2,000,000 g/mol, h*P ≈ 10-12–10-15 2

m /s) was of the same order of magnitude as the previously measured permeability of porous -12

-13

polymer shell microcapsules to dextrans (Mw = 20,000–150,000 g/mol, h*P ≈ 10 –10 m2/s).

46

However, our capsules are significantly more permeable than fully dense inorganic-9

2

organic hybrid capsules were to fluorescein (Mw = 332 g/mol, h*P ≈ 10 m /s).

47

Explicit

control over polymer mesh size was demonstrated through mechanical testing (Figure 4)


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whereas the specific criteria necessary to control the porosity of the hybrid material remain to be examined. In order to tune the barrier properties of the hybrid shell microcapsules, both porosity and polymer mesh size can be varied through the selection of hybrid precursor. Alternatively, hydrophobic shells formed from the T3000 precursor are expected to be impermeable to hydrophilic diffusants as both components are chemically incompatible and the shells have no porosity (Figure 6d). The measurement of diffusion coefficient and permeability for these hydrophobic shell capsules was, however, hindered by adsorption of the FITC-labeled dyes to the microcapsule shell (Figure S2). By selecting precursors that form either dense or porous shells, the permeability of the capsules can be tuned over several orders of magnitude using the proposed sol-gel route.

Figure 7. Diffusion and permeability coefficients of fluorescein-labeled dextrans within the organic-inorganic hybrids. Bulk diffusion coefficients were measured through fluorescence recovery after photobleaching (FRAP) in microparticles (a). Permeabilities of microcapsule shells were measured by monitoring inner, Cin, and outer, Cout dextran concentrations and tracking their changes over time (b). For precursors with LCST above room temperature (D230, D400 and T430), diffusion and permeability constants decreased with molecular weight (c-d). Diffusion coefficients in (c) are plotted both as normalized by diffusion coefficients of the diffusants in water, D0, and as absolute values, D (inset). One capsule was measured per data point.


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Besides the tunable permeability, the hybrid shells can be functionalized through the addition of inorganic precursors into the double emulsion middle phase (Figure 8). For instance, titanium dioxide precursors, such as titanium (iv) butoxide (TNBT), can be added to the formulation to create hybrid materials with photocatalytic functionality. A large increase in absorbance within the UV range between 300–350 nm was observed for PPG bridged-silane hybrids containing TNBT, confirming the formation of light-sensitive TiO2 within the shell (Figure 8a). Hybrid materials with up to 20 wt% TNBT relative to PPG precursor remained optically transparent and flexible. Due to this lack of optical scattering, we assume that the TNBT precursor reacted to form well-dispersed TiO2 nanoclusters within the hybrid. The photocatalytic activity of such TiO2 nanoclusters was probed by adding the microcapsules to an aqueous solution of silver nitrate and exposing the system to UV illumination. During exposure to light, an electron in the valence band of the titanium dioxide is excited by UV light to the conduction band, which then reduces dissolved silver cations to its charge neutral metallic state (Figure 8b).48 Indeed, the photocatalytic reaction of silver ions on the surface of a titanium dioxide nanocluster was found to generate a layer of silver metal on the shell of the illuminated microcapsule (Figure 8c-f). Overall charge balance is achieved in this reaction by the presence of an electron donor in the system. In this case poly(vinyl alcohol), stored in the microcapsule compartment, may assume the role of electron donor, as has previously been demonstrated.49 The photocatalytic reaction occurs rapidly, and after 24 minutes of exposure to UV, a microcapsule with 8 wt % TNBT relative to PPG precursors appeared dark red, an indication that silver nanoclusters were formed (Figure 8c-e). To verify this, the microcapsule shell was observed using SEM and a large number of ~200 nm nanoclusters were detected in the shell (Figure 8f). Aside from the ability to add photocatalytic functionality to the shell, these results demonstrate the general possibility of adding metal alkoxides in the form of inorganic nanoclusters to the shell material. Exploring other metal alkoxides can potentially 50

lead to many more functionalities,

which should enable the design and fabrication of

transparent functional microcapsules using a simple sol-gel processing route.


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Figure 8. Photocatalytic behavior of microcapsules containing titanium dioxide nanoclusters. (a) Addition of titanium dioxide precursor titanium (iv) butoxide (TNBT) results in UV absorbance. (b) By adding titanium dioxide precursor to the shell of the microcapsule one can incorporate photocatalytic functionality to the shell. (c-e) Capsules synthesized with 8 wt % TNBT, relative to D2000 organic precursor, were submerged in a 0.1 wt % silver nitrate solution and exposed to a mercury lamp for 24 minutes, resulting in the photocatalytic production of colloidal silver in the shell. (f) Scanning-electron microscopy of the microcapsule shell showed nanoscale precipitations of silver after exposure to light.

Conclusions Microcapsules with organic-inorganic shells can be created by the gelation of bridged-silane precursors within the oil phase of water-in-oil-in-water emulsions. This emulsion templating approach enables the production of microcapsules with independently tunable mechanical and functional properties. By varying the length of the organic bridging group of the bridged-silane precursor, the mechanical properties are tunable over several orders of magnitude. The length of the bridged-silane precursor also affects the permeability of the microcapsule through the formation of different shell microstructures during the gelation process. To add functionalities to the hybrid shell, metal alkoxides can also be incorporated within the oil phase of the double emulsions to form metal oxides nanoclusters within the gelled network. As an example, titanium alkoxide was added in order to imbue the hybrid shell with photocatalytic activity without affecting the mechanical properties, since these oxides precipitate at the nodes of the rubbery polymer network. Because of the wide range of metal alkoxides and bridged-silanes available, this technique can be expanded to cover a broad

variety

of

chemistries,

mechanical

properties

and

microcapsules to be finely tuned to specific applications.


functionalities,

allowing

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Acknowledgement This research was supported by the Swiss National Science Foundation through the National Centre of Competence in Research Bio-Inspired Materials. Microscopy experiments were conducted in the ETH Zürich Scientific Center for Optical and Electron Microscopy (ScopeM).

Supporting Information Representative data for fluorescence recovery after photobleaching measurements; representative data for microcapsule permeability measurements; the tensile strength and elongation at break for bulk films; turbidity of oil-water-PPG mixtures

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Table of Contents Graphic


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Figure 1. Schematic of organic-inorganic hybrid microcapsules with a shell consisting of a network of functional inorganic nanoclusters interconnected by polymer chains of tunable molecular weight. By increasing the molecular weight of the polymer segments (labeled green), the capsule shell hydrophobicity and elasticity can be tuned. With the addition of metal alkoxides, inorganic nanoclusters (labeled red) are created between the polymer segments, leading to unique magnetic, catalytic, and optical functionalities. 67x26mm (300 x 300 DPI)


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Figure 2. Poly(propylene glycol) trimethoxysilanes (PPG-TMS) used as precursors for organic-inorganic hybrids. (a) Silane-functionalized PPG gels are formed by the reaction of a glycidyl-terminated silane with the amines of Jeffamine, a commercially available polymer with various molecular weights and degrees of branching. (b) The silane-functionalized PPGs gel in the presence of water through hydrolysis and condensation. (c) The inorganic phase is concentrated in the nodes of the gel, forming a transparent, rubbery solid. Silane-functionalized Jeffamine D2000 was used to obtain the shown polymer. 95x67mm (300 x 300 DPI)


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Figure 3. Mechanical properties of siloxane-crosslinked poly(propylene glycol) organic-inorganic films with respect to organic and inorganic precursor composition and hybrid architecture. The effect of the addition of (a-d) organic precursors of different molecular weights and (e-f) inorganic precursors at different concentrations on the mechanical properties of the hybrid films. 116x79mm (300 x 300 DPI)


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Figure 4. (a) Elastic modulus and (b) tensile strength of PPG bridged-silane organic-inorganic hybrid films show power law dependencies with the density of crosslinks, Cx. Assuming a power law dependence of the form E,σ = KCxβ , we obtained K values equal to 2.06×105 and 5.00×106 from the fittings to E and σ data, respectively. 59x26mm (300 x 300 DPI)


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Figure 5. Synthesis of microcapsules with organic-inorganic hybrid shells. The organic and inorganic precursors are dissolved in solvents with only slight water solubility (shown in yellow). The resulting mixture is used as the middle phase of double emulsions. (a) Interdiffusion between such solvents and the aqueous phase allows the precursors to gel through hydrolysis and condensation. (b,c) w/o/w emulsion templates containing precursors in the oil phase are prepared using capillary microfluidics, leading to rubbery polymershell microcapsules after removal of the solvents. 104x68mm (300 x 300 DPI)


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Figure 6. Microstructure of hybrid shells and thermal response of PPG precursors of different molecular weight. (a-c) Organic-inorganic hybrid shells formed from emulsions that contain PPGs with LCST above 25 °C (MW < 430 g/mol) show porosity after freeze-drying. (d) Shells obtained from polymers with LCST below 25 °C, such as the trifunctional amine-terminated PPG with MW of 3000 g/mol, contain no porosity after freeze-drying. (e) Effect of the molecular weight of amine-terminated PPGs on their lower critical solution temperature (LCST) in water. With an increase in molecular weight of the initial PPG compounds, the LCST decreases. 141x251mm (300 x 300 DPI)


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Cdextran

Cdextran

1-5 min. ct=0

x

b

ct>0

x

cout cin h

c

x

d 10-11

1 10 10-13 10-14 10-15

D (m2/s)

-12

10-2

10-12 104 105 106 MW Dextran (g/mol)

10-3 D230 D400 T430

10-4 10-5

10

3

h*P (m2/s)

10

-1

D/D0 (m2/s)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

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UV Bleach

Cdextran

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10-13 10-14 10-15

10

4

10

5

10-16 3 ACS Paragon Plus Environment 6 7 10 10 10

MW Dextran (g/mol)

D230 D400 T430

104

105

106

MW Dextran (g/mol)

107

a

O.D. (unitless)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

b 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0 300

Langmuir

Page 28 of 29

20% TNBT 8% TNBT 4% TNBT 2% TNBT 1% TNBT 0% TNBT

320

340

360

380

Wavelength (nm)

UV

200 μm

t = 1 min

e

-

Ag+

Eg h+

h+

Ag0 e- Donor + H+ e- Donor

400

d

c

e

-

e

f

200Paragon μm 200 μm ACS Plus Environment

t = 12 min

t = 24 min

250 nm

Page 29 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Table of Contents Graphic. 27x8mm (300 x 300 DPI)


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