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A new approach for the synthesis of nano-zirconia fortified microcapsules Chia-Chen Li, Dzu-How Yu, Shinn-Jen Chang, and Jia-Wei Chen Langmuir, Just Accepted Manuscript • Publication Date (Web): 17 May 2017 Downloaded from http://pubs.acs.org on May 18, 2017

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A new approach for the synthesis of nano-zirconia fortified microcapsules

Chia-Chen Li†*, Dzu-How Yu†, Shinn-Jen Chang‡, Jia-Wei Chen†

†

Institute of Materials Science and Engineering, and Department of Materials & Mineral

Resources Engineering, National Taipei University of Technology, Taipei 10608, Taiwan ‡

Material and Chemical Research Laboratories, Industrial Technology Research Institute,

Hsinchu 30011, Taiwan

* Correspondence author. E-mail: [email protected]

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ABSTRACT Robust poly(urea-formaldehyde) (PUF) microcapsules with composite shells comprising zirconia (ZrO2) nanopowder incorporated in PUF were fabricated via a novel and facile one-pot synthesis. ZrO2 nanopowder was chosen because it owns one of the highest mechanical strength among ceramics. The nanopowder was pre-dispersed in the core material to combine encapsulation and fortification into a single process. In the core, the well-dispersed nanopowder migrated to the interface, where PUF polymerization took place. The mechanical strength of the microcapsule with nano-ZrO2 incorporated in the shell (42% by weight) is three times greater than that of the microcapsule without ZrO2. In a preliminary application wherein the microcapsules were embedded in a model of poly(vinyl alcohol) (PVA) membrane, the PVA specimen exhibited a higher ultimate tensile strength when fortified microcapsules were embedded than when unfortified microcapsules were used.

KEYWORDS:

Robust microcapsules; Self-healing; Nano; Dispersion; Zirconia

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INTRODUCTION Owing to their hollow structures that allow loading with a variety of core species, microcapsules have received considerable interest since the early 2000s [1] and currently receive attention from international researchers in the fields of novel, advanced, and intelligent materials [1-28]. Microcapsules are appropriate for application in extensive industrial applications, such as pharmaceutical and biomedical products [7-10], self-healing reagents [1,11-15], anti-corrosion coatings [16,17], energy storage materials [18-23], and gas-sensor materials [24,25]. In these applications, microcapsules are embedded in or cast on substrates to provide the required functions. Without a triggering force, the reactive reagent has to remain stable in the core of the microcapsule, which the guarantee of functioning only at the right moment is the value of the developing of microcapsules. Although microcapsules have great industrial potential, the shells of microcapsules are primarily polymeric materials that are mechanically soft and thermally unstable, limiting the practical applications of microcapsules. To prevent the premature rupture of microcapsules during synthesis and processing, several strategies for fortifying the shell to make the microcapsule more robust have been proposed and evaluated. Three major types of braced microcapsules with different shell compositions have been fabricated to improve microcapsule robustness [29-32]: polymer-, inorganic-, and hybrid-braced microcapsules (Fig. 1). In polymer-braced microcapsules, the bracing polymer can be the same as or different from the original shell species. The bracing materials used in inorganic- and hybrid-braced microcapsules are frequently ceramics. Most fortifications are achieved using layer-by-layer assembly [33], physical or chemical coating [34], electrostatic adsorption [35], or sol-gel processing [36,37]. To achieve the required thickness and/or complete coverage, the fortifying steps are generally employed repeatedly.

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Fig. 1. Schematic of the three common types of fortified microcapsules.

The techniques commonly used to produce stronger composite shells include multiple complex steps, often making these techniques expensive and uneasily controlled. Particularly for the inorganic- and hybrid-braced microcapsules, the large differences between the surface tensions of most inorganic materials and polymers make it difficult to produce a complete coating or incorporate inorganic particles into or onto the polymeric shell. To improve the fortification of microcapsules, a new method involving the formation of a Pickering emulsion has been proposed in the literature [38-42]. In a Pickering emulsion, the emulsified droplets are stabilized by inorganic particles instead of organic surfactants, and particles are incorporated in the shell during encapsulation. Although Pickering microcapsules can be produced through one-pot syntheses, it is extremely difficult to control the stability of the particle-dispersed droplets. Pickering emulsions are easily demulsified as a result of the re-agglomeration of inorganic particles during mechanical agitation. To successfully achieve the fabrication of robust microcapsules with nanocomposite shells, we propose a new one-pot process that is scalable and easy to carry accomplish in this research. To guarantee the presence of nanoparticles at the interfaces of emulsified droplets 4


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during encapsulation, nanoparticles are pre-dispersed inside instead of outside the droplets using an appropriate surfactant or dispersant. The advantages of this approach are summarized as follows: (1) the emulsified droplets are more easily and better stabilized in than in the Pickering process; (2) the inorganic nanoparticles can be chosen from a wide range of materials, including carbons, ceramics, and metals, with any morphology; (3) dispersion techniques for most inorganic particles are well-established, and a great number of dispersants are available; and (4) when the residual, unincorporated nanoparticles that are left in the core come out with spilled sealant from the ruptured microcapsules, they may function as a filler to provide excess reinforcement for the cracks of the matrix.

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EXPERIMENTAL PROCEDURES Raw Materials: High-purity ZrO2 nanopowder (99.9%, amorphous, SERIN international LLC, Kentucky, USA) was used. Transmission electron microscopy (TEM; JEM-2100, JEOL, USA) revealed that the particle size ranged from 20 to 30 nm. Urea (99%, Showa, Japan) and formaldehyde (37%, Echo, Taiwan) were the primary chemical reagents used in the synthesis of the shell wall material of the microcapsules. Resorcinol (98%, Acros, USA), ammonium chloride (99.5%, Showa, Japan), and triethanolamine (TEA; 99%, Sigma-Aldrich, USA) were used as additives for synthesis. n-Octane (Echo Chemical Co., Ltd, Taiwan) was used as the core material, and poly(ethylene-alt-maleic anhydride) (EMA; 99%, Sigma-Aldrich, USA) was used as the surfactant for encapsulation. The dispersant, oleyl phosphate (OP, 85%; TCI, Tokyo, Japan), and a surface modifier, octadecyltrimethoxy silane (OTMS, 90%, Acros, USA), were used to disperse the ZrO2 nanopowder. All the above chemical reagents were used as received without further purification. Encapsulation: Encapsulation was performed using in situ polymerization. The additives (0.25 g resorcinol and 0.25 g ammonium chloride) were mixed with 2.5 g urea and dissolved in 100 ml deionized water. This aqueous solution was then warmed to 40oC, and the pH was adjusted to 3.5 by the addition of TEA. n-Octane (5.5 ml) with or without well-dispersed ZrO2 was dropped into the aqueous solution and emulsified using a homogenizer (T25, IKA, Germany) at a speed of 13,000 rpm for 5 min. After homogenization, 6.33 g formaldehyde was instantly added, and the emulsion was heated to 55oC and stirred on a hot plate for an additional 2 h. After reaction, the microcapsules were removed from the suspension and washed repeatedly using deionized water. To prepare a well-dispersed suspension of ZrO2 in n-octane, ZrO2 was first surface-modified by OTMS under reflux at 100oC for 4 h. The nanopowder was then dispersed in n-octane with the addition of various concentrations of OP and de-agglomerated using a high-speed three-dimensional mixer (Power Mixer-CM S200,

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Chia Mey, Taiwan) with Y2O3-stabilized ZrO2 media at 1000 rpm for 1 h. Characterizations: The dispersion properties of ZrO2 nanopowders in n-octane with the addition of various concentrations of OP were analyzed using a concentric cylinder rheometer (AR1000, TA Instruments Ltd., UK). The contents of nano-ZrO2 incorporated in the shells of the microcapsules were analyzed by thermogravimetric analysis (TG; Q50, TA instruments Ltd., Crawley, UK). For TG analysis, the microcapsules were smashed and washed several times with ethanol and deionized water to completely remove the core materials. The washed fragments were then dried in oven at 80°C for at least 24 h. The microstructures were characterized using TEM, field-emission scanning electron microscopy (FE-SEM; S-470, Hitachi, Tokyo, Japan), and OM (Primo Star, Zeiss, Jena, Germany). Nanoindentation and tensile tests: The mechanical properties of the microcapsules were evaluated by nanoindentation (TI 950, Hysitron, Minneapolis, USA) using a measuring system equipped with a spherical indenter tip with a radius of 20 µm. The microcapsules selected for measurement were of a constant size of 20 µm. In the tensile tests, 5 vol% red-dyed microcapsules were dispersed into an aqueous solution of 10 wt% poly(vinyl alcohol) (PVA; BP-05, Chang Chun PetroChemical Co., Ltd, Taipei, Taiwan). The obtained aqueous suspension of microcapsules was cast in a home-made glass cell for dying. After drying, PVA specimens were cut with the following dimensions: length = 25 mm, width = 5 mm, and thickness = 1 mm. Tensile testing was conducted by a tensile testing machine (JSV-500D, Algol Instrument Co., Ltd, Taoyuan, Taiwan) with a tensile speed of 1 mm s-1.

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RESULTS AND DISCUSSION In this study, the core of the model microcapsule was composed of n-octane, and the shell was made of PUF. To have zirconia (ZrO2) nanopowder incorporated into the PUF shell to result in a fortified microcapsule, the n-octane core contains pre-dispersed nano-ZrO2. We found that the nano-ZrO2 was spontaneously incorporated with a significant amount into the shell during encapsulation. Figure 2a shows a successfully fortified microcapsule with a shell containing 30 wt% nano-ZrO2; the inset reveals that the nano-ZrO2 was well-dispersed in the shell with sizes ranged from 20 to 50 nm. If the nanopowder is not well-dispersed in the core, the nanopowder would remain agglomerated or transfer from the droplets into the surrounding aqueous phase during encapsulation, as illustrated in Fig. 2(b1–b3). Figure 2c shows the experimental results for poorly dispersed nano-ZrO2; in this case, mostly nano-ZrO2 agglomerates were present in the core, which would lead to a low content of nanopowder in the shell. In addition, some agglomerates remained outside of the microcapsules and formed precipitates in the aqueous phase (as shown in Fig. 2c). In contrast, well-dispersed nano-ZrO2 resulted in good incorporation efficiency, as illustrated in Figs. 2(b4–b6). Because the well-dispersed nanopowder was homogeneously distributed in the droplets, the nanopowder near the interface could be incorporated into the shell during encapsulation. In addition, the well-dispersed nanopowder is smaller and thus faces less resistance to diffusion; this helps compensate for nanopowder depletion at the interface, thereby increasing nanopowder incorporation. Figure 2d shows an optical microscopy (OM) image of microcapsules in the well-dispersed case; the opacity of the microcapsules in this image is attributed to the high level of nanopowder incorporation.

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Fig. 2. (a) TEM image of an enucleated microcapsule with a shell containing 30 wt% nano-ZrO2; the inset is a zoomed-in image of the shell surface. (b) Schematic showing the effect of powder dispersity on incorporation efficiency; b1–b3 and b4–b6 correspond to the poor- and well-dispersed cases, respectively. OM images of microcapsules with initially (c) poor and (d) good dispersions of nano-ZrO2 in the core.

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Fig. 3. (a) Apparent viscosity as a function of shear rate for microcapsules containing 5 wt% ZrO2 in n-octane at various [OP]; inset shows the apparent (left) and relative (right) viscosities obtained at a shear rate of 50 s−1 as functions of [OP]. (b) Potential energy between ZrO2 nanoparticles as a function of OP adsorption thickness. (c) Concentration of nano-ZrO2 in microcapsule shells as a function of [OP]. 10


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The dispersion behavior of the ZrO2 nanopowder in the core suspension of n-octane was evaluated because nanopowder dispersity is critical for the successful fabrication of robust microcapsules. Rheological analysis is generally the most appropriate way to evaluate dispersity. Figure 3a shows the apparent viscosity as a function of shear rate for the ZrO2/n-octane suspension with the addition of various concentrations of dispersant, oleyl phosphate (OP). The added concentration of OP ([OP]) was based on the weight of n-octane. The suspension without OP exhibited clear shear thinning with a power index (α) of 0.75, indicating the presence of soft agglomerates of nano-ZrO2 [43,44]. Notably, an α of 1 indicates a suspension with well-dispersed Newtonian flow containing no agglomerates. Upon the addition of 2 wt% OP, the apparent viscosity decreased significantly, and the powder index increased to 0.92, suggesting improved dispersity. The decreased viscosity and power index close to 1 both evidence that OP is an efficient dispersant for nano-ZrO2. For easier discrimination, the apparent viscosity obtained at a shear rate of 50 s−1 was re-plotted as a function of [OP] in the inset of Fig. 3a. This plot clearly shows that viscosity was minimized at [OP] = 2 wt% and inversely to increase at [OP] > 2 wt%, although α remained at approximately 0.90–0.92 for [OP] from 2 to 40 wt%. There are two possible explanations for the inversely increased apparent viscosity: (1) excessive OP led to specific interactions such as hydrogen bonding and polymer bridging between particles [45,46], causing nanopowder agglomerates to form and increasing the apparent viscosity; and (2) the intrinsic viscosity of the added OP enhanced the overall apparent viscosity. To exclude the rheological contribution of the added OP, the relative viscosity (ηr) of each suspension was obtained by dividing the apparent viscosity by the viscosity of the continuous phase. Extracting the rheological contribution of [OP] revealed that ηr values of the suspensions remained nearly constant for [OP] ≥ 2 wt% (inset of Fig. 3a). Therefore, the 11


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inverse increase in apparent viscosity at [OP] > 2 wt% was mainly attributed to the rheological contribution of OP. That is, increasing the amount of OP did not cause any specific interactions resulting in the agglomeration of nano-ZrO2. Thus, OP was confirmed to be an efficient dispersant, and the least required concentration was 2–5 wt%. The good stability of nano-ZrO2 in n-octane with added OP was also verified by theoretical calculations of Derjaguin–Landau–Verwey–Overbeek energy (see Supplementary Information). The resulted stabilizing potential energy (Vt) as a function of the adsorption thickness of OP on nano-ZrO2 is shown in Fig. 3b. Vt was calculated as the sum of the attractive (VA) and repulsive (VR) energies. The minimum potential energy required for the stabilization of particles is generally 15 kT [43,46,47]; thus, in theory, the ZrO2 nanopowder can be dispersed and stabilized if the adsorption thickness (δ) of OP is larger than 1.2 nm. According to the saturated adsorbance of OP (As; Fig. S1), the projected radius (1/2) of adsorbed OP on ZrO2 was determined to be 0.5 nm by Eq. (1) [48].

As =

SAM π( < S 2 >1/2 )2 N

(1)

where M is the molecular weight of OP, SA is the specific surface area of powder, and N is Avogadro’s number. This indicates that OP was probably adsorbed vertically on the particle surface, and the thickness of the adsorption layer should be approximately the molecular length of OP (~2 nm) [45,46]. Thus, OP was theoretically available for the stabilization and dispersion of ZrO2 nanopowder. To demonstrate the effect of [OP] on the powder incorporation efficiency, Fig. 3c shows the amount of nano-ZrO2 incorporated in the shell as a function of [OP] in the core. Interestingly, the powder contents of the shell were distinct for different suspensions, even though the suspensions had similar dispersities (inset of Fig. 3a). The maximum powder content was 42 wt% at [OP] = 5 wt%, and the lowest content was 16 wt% at [OP] = 40 wt%. Since dispersity was approximated at [OP] in the range of 2 to 40 wt%, the different powder 12


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contents should be related to the distinct apparent viscosities of the suspensions. According to Einstein’s law of diffusion, particle diffusivity increases as the viscosity of the suspension or particle size decreases. The diffusion coefficient (D) of a spherical particle can be determined by the Stokes-Einstein equation [43,49] as

D=

kT 6πηa

,

(2)

where k is Boltzmann’s constant, T is the absolute temperature, η is the viscosity of the dispersion medium, and ɑ is the particle diameter. The calculated value of D for nano-ZrO2 in n-octane with [OP] = 5 wt% is five times larger (9.3 × 10−12 m2/s) than that for [OP] = 40 wt% (1.9 × 10−12 m2/s); that is, the ZrO2 nanopowder exhibits a higher diffusivity in the suspension with lower [OP]. Since higher diffusivity facilitates the migration of powder, in the suspension with higher diffusivity, nano-ZrO2 should more easily migrate to the area of powder depletion where the nanopowder is consumed during encapsulation, leading to a higher content of nanopowder in the shell.

Fig. 4. SEM images showing the (a and b) cross-sectional and (c and d) zoomed-in images of microcapsules (a and c) without and (b and d) with the incorporation of 42 wt% nano-ZrO2. 13


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Figures 4a and 4b show cross-sectional images of the microcapsules without and with the incorporation of 42 wt% ZrO2 in the shell, respectively; Figs. 4c and 4d show the respective zoomed-in images of the shells. In Figs. 4a and 4c, the microcapsule presents a smooth and thin shell with a thickness of approximately 100 nm. The shell thickness of the microcapsule shown in Figs. 4b and 4d is significantly higher (250 nm) because of the incorporation of nano-ZrO2. The exterior surface of the shell remains flat and smooth, whereas the interior is uneven and rough. This indicates that a composite shell formed via the migration and incorporation of nanopowder from the core.

Fig. 5. (a) Powder content of the microcapsule shell as a function of the initial concentration of nano-ZrO2 in the core. OM images of fortified microcapsules with ZrO2 contents of (b) 22, (c) 30, and (d) 42 wt% in the shell.

The fact that an [OP] of 5 wt% is sufficient to obtain well-dispersed nano-ZrO2 in the 14


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core and maximizes the content of ZrO2 in the shell (42 wt%; Fig. 3c) has been revealed in Fig. 3. The incorporating efficiency was further explored by varying the nanopowder loading in the core at a constant [OP] of 5 wt%. Notably, both the viscosity and ηr of all suspensions remained nearly constant for nanopowder loadings from 0.1 to 5 wt% when [OP] is 5 wt% (Fig. S2). The dispersity and nanopowder diffusivity should not be factors in this experiment; the only factor is the initial amount of nanopowder in the core. Figure 5a shows that the amount of nano-ZrO2 incorporated in the shell increases with increasing initial solid loading of nano-ZrO2 in the core. Figures 5b, 5c, and 5d show images of the microcapsules with powder contents of 22, 30, and 42 wt% in the shell, corresponding to initial loadings of 0.5, 2, and 5 wt% in the core, respectively. As expected, a higher nanopowder loading in the core resulted in a higher nano-ZrO2 incorporation in the shell. The relative strengths of microcapsules containing different contents of nano-ZrO2 in the shell were analyzed by nanoindentation [50,51]. Figure 6a shows the loading-displacement curves of the microcapsules with different contents of nano-ZrO2 in the shell. Different nano-ZrO2 contents clearly resulted in different mechanical behaviors. Deviation in the curves from a concave-up shape that is typical for elastic materials was especially significant at high contents of nano-ZrO2. The complicated curves observed in this study can be attributed to the composite microcapsules composed of both organic and inorganic materials along with both solid and liquid components. For the microcapsule without nano-ZrO2, deviation from the elastic curve appeared at a load of approximately 30 µN, representing the onset of plasticity (i.e., the transition from elastic to plastic deformation) [52], and the maximum loading force (Pmax) was 286 µN. With the incorporation of 10 wt% nano-ZrO2, no significant yield point was found with the exception of a slight fluctuation during the initial stage of loading. The slope of the loading curve decreased remarkably when the loading exceeded ~120 µN, and failure occurred at Pmax = 197 µN. This Pmax is slightly lower than 15


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that of the unfortified microcapsule (286 µN). The curve of the microcapsule containing 22 wt% nano-ZrO2 was comparable to that of the unfortified microcapsule. When the nanopowder content was increased to 30 wt%, the slope of the curve increased, suggesting that the microcapsule became harder. This result may be attributed to the greater amount of ZrO2 nanopowder in the PUF shell (Fig. 5c). At the maximum powder content of 42 wt%, the curve exhibits the highest Pmax of 919 µN and can be described as a combination of two curves intersecting at ~280 µN. The two sections of the curve are both concave-up; the first curve presents a pop-in event at 33 µN, and the second one shows a continuous increase in slope. Since Pmax is directly related to the strength of the material, Fig. 6b re-plots Pmax as a function of powder content of the shell for various microcapsules. The Pmax remained low at approximately 200−300 µN for powder contents less than 25 wt%, suggesting that these ZrO2 contents are insufficient for microcapsule fortification. When the content was increased to 30 and 42 wt%, Pmax increased to 616 and 919 µN, respectively. These drastic increases in Pmax clearly resulted from the greater amount of ZrO2 incorporation and higher homogeneity of the formed composite shell (Figs. 5c and 5d). Evidently, having sufficient ZrO2 incorporation is essential for achieving the robustness of microcapsules. A too-low nanopowder content in the shell would negatively affect the mechanical strength of the microcapsules, likely because of the incorporation of discrete island-like nanoparticles (Fig. 5b), which may present as structural defects and weaken the material.

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Fig. 6. (a) Loading-displacement curves of various microcapsules. (b) Pmax as a function of ZrO2 content in the shell. OM images of (c) unfortified and d) ZrO2-fortified microcapsules. (e) Schematic of the instrumental setup for tensile testing. (f) Stress-strain curves for unfortified microcapsules (black) and microcapsules fortified with 42 wt% ZrO2 (purple). OM images of PVA membranes embedded with (g and h) unfortified and (i) fortified 17


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microcapsules corresponding to the samples in (f).

Microcapsules are generally embedded in another material for application. Thus, the mechanical strength of a model specimen consisting of a PVA-based membrane embedded with 5 vol% red-dyed microcapsules was subjected to tensile testing. PVA was selected because of its high toughness; PVA will rupture after the embedded microcapsules, thereby reducing the mechanical interference of the matrix during the observation of microcapsules. Figures 6c and 6d show the morphologies of microcapsules without ZrO2 and with 42 wt% nano-ZrO2 embedded in PVA, respectively. The experimental setup used for tensile testing is schematically shown in Fig. 6e; the resulting stress-strain curves are shown in Fig. 6f. Figure 6g presents an image of the PVA specimen embedded with homogeneously distributed unfortified microcapsules before stretching. Both the unfortified and fortified microcapsules were compatible with the PVA matrix. Figure 6f indicates that embedded microcapsules without nano-ZrO2 began to rupture at a stress of 45 kPa; the corresponding image is shown in Fig. 6h. The rupture of microcapsules was indicated by a variation in color from bright red to dark red. For the embedded microcapsules containing 42 wt% nano-ZrO2 in the shell, the stress at which the microcapsules ruptured was higher (54 kPa); the corresponding image is shown in Fig. 6i. Furthermore, the ultimate tensile strength (UTS) was higher for the specimen embedded with fortified microcapsules than the specimen embedded with unfortified microcapsules. At constant stress, the PVA specimen embedded with fortified microcapsules exhibited less deformation, indicating reduced ductility and increased hardness.

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CONCLUSIONS In summary, we have successfully demonstrated a facile, one-pot synthesis for the fabrication of ZrO2-fortified microcapsules. Conventional methods for synthesizing robust microcapsules with composite shells are generally complex or difficult to carry out. For instance, in the Pickering emulsion technique, a one-pot method is employed to stabilize core droplets with an inorganic nanopowder, allowing the nanopowder to be incorporated in the shell during encapsulation. However, it is difficult to obtain stable nanopowders on the droplet surfaces using this method because the nanopowder tends to agglomerate rather than adsorbing on other materials to minimize its own surface energy. In this study, the nanopowder to be incorporated into the microcapsule shell is pre-dispersed inside the droplets, facilitating the controlled synthesis of robust microcapsules. This two-in-one process, which combines encapsulation and fortification, reduces the complexity inherent in previous techniques and allows the efficient formation of robust microcapsules with homogeneous composite shells. Using nanoindentation, we compared the mechanical strengths of the fortified and unfortified microcapsules. Interestingly, the maximum loading force was enhanced by more than three times when the microcapsule shells were incorporated with 42 wt% nano-ZrO2. In addition, when the microcapsules were embedded in a polymeric matrix, the fortified microcapsules resulted in a better UTS than the unfortified ones. The new one-pot synthesis presented herein represents a breakthrough in the fabrication of robust microcapsules with composite shells and has the potential to be applied in large-scale production.

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ASSOCIATED CONTENT Supporting Information Supporting Information Available: Additional experimental detail, experimental methods, calculation methods, supplemental figures and supplemental discussion. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS The authors are grateful for the financial supports from the Ministry of Science and Technology of the ROC under Grant No: 103-2221-E-027-011- and by the Material and Chemical Research Laboratories of Industrial Technology Research Institute. ABBREVIATIONS PUF, poly(urea-formaldehyde); ZrO2, zirconia; PVA poly(vinyl alcohol); OP, oleyl phosphate; TEM, Transmission electron microscopy; OM, optical microscopy; Pmax, maximum loading force; UTS, ultimate tensile strength.

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Fig. 1. Schematic of the three common types of fortified microcapsules. 76x67mm (600 x 600 DPI)


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Fig. 2. (a) TEM image of an enucleated microcapsule with a shell containing 30 wt% nano-ZrO2; the inset is a zoomed-in image of the shell surface. (b) Schematic showing the effect of powder dispersity on incorporation efficiency; b1–b3 and b4–b6 correspond to the poor- and well-dispersed cases, respectively. OM images of microcapsules with initially (c) poor and (d) good dispersions of nano-ZrO2 in the core. 72x46mm (600 x 600 DPI)


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Fig. 3. (a) Apparent viscosity as a function of shear rate for microcapsules containing 5 wt% ZrO2 in noctane at various [OP]; inset shows the apparent (left) and relative (right) viscosities obtained at a shear rate of 50 s−1 as functions of [OP]. (b) Potential energy between ZrO2 nanoparticles as a function of OP adsorption thickness. (c) Concentration of nano-ZrO2 in microcapsule shells as a function of [OP]. 229x503mm (600 x 600 DPI)


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Fig. 4. SEM images showing the (a and b) cross-sectional and (c and d) zoomed-in images of microcapsules (a and c) without and (b and d) with the incorporation of 42 wt% nano-ZrO2. 106x74mm (600 x 600 DPI)


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Fig. 5. (a) Powder content of the microcapsule shell as a function of the initial concentration of nano-ZrO2 in the core. OM images of fortified microcapsules with ZrO2 contents of (b) 22, (c) 30, and (d) 42 wt% in the shell. 137x131mm (600 x 600 DPI)


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Fig. 6. (a) Loading-displacement curves of various microcapsules. (b) Pmax as a function of ZrO2 content in the shell. OM images of (c) unfortified and d) ZrO2-fortified microcapsules. (e) Schematic of the instrumental setup for tensile testing. (f) Stress-strain curves for unfortified microcapsules (black) and microcapsules fortified with 42 wt% ZrO2 (purple). OM images of PVA membranes embedded with (g and h) unfortified and (i) fortified microcapsules corresponding to the samples in (f). 103x125mm (600 x 600 DPI)


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