J. Phys. Chem. B 2007, 111, 8879-8884
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Fabrication of Hollow Melamine-Formaldehyde Microcapsules from Microbubble Templates Hirofumi Daiguji,*,† Toshinori Makuta,‡,§ Hiroki Kinoshita,† Takayuki Oyabu,† and Fumio Takemura‡ Institute of EnVironmental Studies, Graduate School of Frontier Sciences, The UniVersity of Tokyo, Kashiwa, 277-8563, Japan, and National Institute of AdVanced Industrial Science and Technology (AIST), Tsukuba 305-8564, Japan ReceiVed: March 16, 2007; In Final Form: May 15, 2007
A fabrication method for hollow melamine-formaldehyde microcapsules from microbubble templates is presented. This method is based on the direct encapsulation of microbubbles, and thus does not require a liquid- or solid-core decomposition process. This study determined the conditions for controlling the surface morphology, shell thickness, and diameter distribution of hollow microcapsules. Results showed that the surface morphology of these hollow microcapsules depended on the reaction time, glycine concentration (pH of aqueous continuous phase) and pre-polymer concentration. The capsule shell thickness could be controlled by adjusting the concentration of aniline that had adsorbed on the microbubble surface and reacted with pre-polymer. The capsule diameter depended on the dissolution rate of gases, and the diameter of the hollow microcapsules fabricated from air microbubble templates ranged from 5 to 200 µm.
1. Introduction Hollow microcapsules are gas-filled spherical particles with diameters between 1 and 1000 µm. They show promising potential due to their advantageous properties, such as low effective density and high specific surface. Widely used applications include reduction in the weight of material, encapsulation and immobilization of bioactive and catalytically active substances, customization of the impact strength of compounds, and improved thermal and acoustical insulation (e.g., diagnostic ultrasound contrast agent). Medical or pharmaceutical applications include drug-delivery systems and the synthesis of artificial cell structures.1 Fabrication of tailor-made hollow spherical structures by processes such as spray-drying as well as dripping,2,3 emulsion,4 fluidized bed,5,6 and suspension7-9 techniques has recently been gaining attention. Typically, these processes involve template techniques in which, first, solid layers are deposited on dispersed templates such as liquid droplets or solid particles, and then, hollow spheres are fabricated by decomposing the liquid or solid core inside the layer either by dissolution, evaporation, or thermolysis. However, the capsule shell is often damaged during this core decomposition process. If microbubbles instead of liquid droplets or solid particles can be used as templates, such damage can be eliminated. Furthermore, if capsules have an impermeable wall, liquid or solid core materials could not go out through the wall. In such a case, the encapsulation of microbubbles is essential to fabricate hollow microcapsules. Another advantage of the microbubble template technique is its versatile application to other capsule-shell component materials such as polymers and inorganic materials. When * Corresponding author. E-mail:
[email protected]. Tel: +81-47136-4658. Fax: +81-4-7136-4659. † The University of Tokyo. ‡ AIST. § Current address: Department of Mechanical Systems Engineering, Yamagata University, 4-3-16 Jyonan, Yonezawa 992-8510, Japan.
a polymer is required as the capsule-shell component material, the in situ polymerization method can be used in the encapsulation of a dispersed liquid or solid core in a continuous phase, and polymerization occurs exclusively in the continuous phase. The only difference between these two methods (microbubble template technique vs in situ polymerization method) is preparation of the cores themselves, namely, of the microbubbles, or of the liquid or solid cores, respectively. However, the residence time of microbubbles in liquid is significantly shorter than that for dispersed liquid or solid cores. Some microbubbles coalesce into large bubbles and float into the air, and others disappear due to the dissolution of the gases into the liquid. Thus, for successful fabrication using the microbubble template technique, the encapsulation rate should be faster than the dissolution rate of microbubbles. Harris et al.10 reported the structure of hollow n-butyl-2-cyanoacrylate (NBCA) microcapsules fabricated using the microbubble template technique. A gas-water emulsion was created by stirring surfactant in water, while adding the fluid, NBCA monomer. Harris et al. reported that the NBCA monomer polymerized rapidly in the presence of water and that the polymer shell of the hollow microcapsules possessed a globular outer surface and a smooth inner surface. Though the encapsulation process of microbubbles has been carefully studied, the conditions to control the surface morphology, shell thickness and diameter of hollow microcapsules have not yet been clarified. The objectives of this study were to demonstrate the fabrication of hollow melamine-formaldehyde microcapsules from microbubble templates and to clarify the conditions for controlling the surface morphology, shell thickness, and diameter of hollow melamine-formaldehyde microcapsules. Furthermore, the second objective is to clarify not only the conditions for controlling three parameters of hollow melamine-formaldehyde microcapsules but also the effect of each condition on the microbubble encapsulation process, that is, transport and reaction
10.1021/jp072131t CCC: $37.00 © 2007 American Chemical Society Published on Web 06/28/2007
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phenomena on the bubble surface for versatile application of this microbubble template technique to other capsule-shell component materials. The reason for choosing melamineformaldehyde resin as the capsule shell component is that due to their high reactivity, melamine-formaldehyde pre-polymers offer advantages over the in situ polymerization method in the production of microcapsules, such as relatively short reaction times and efficient utilization of polymer in the microcapsule shells. Furthermore, melamine-formaldehyde microcapsules have been widely used in carbonless copy paper and other pressure-sensitive products such as carriers for fragrant oils11 and pest repellents.12 Also, studies have reported the various properties of these microcapsules, including mechanical strength13-15 as well as the chemical structure of melamineformaldehyde resin in various states.16,17 In this study, for controlling the surface morphology of hollow microcapsules, the effects of (i) reaction time, (ii) glycine concentration, and (iii) pre-polymer concentration were investigated. For controlling the capsule shell thickness, the effect of aniline concentration was investigated. The effect of aniline on the capsule shell formation could be clarified in more detail by using microbubbles instead of liquid cores as templates because prepolymer on the microbubble surface reacts only with aniline, whereas pre-polymer on the liquid core surface reacts either with aniline or the core liquid, and aniline is normally located more stably at the liquid-gas interface rather than at the liquidliquid interface. For controlling the diameter of hollow microcapsules, two types of gases with dissimilar dissolution rates were used, namely, air and sulfur hexafluoride (SF6). Prepolymers are not normally a dispersion-stabilizing agent, but actually might accelerate coalescence of dispersed liquid cores. In contrast, pre-polymers might not accelerate coalescence of microbubbles. Because compared with liquid cores, the diameter of microbubbles is less dependent on the condition of the aqueous continuous phase, the conditions for controlling the diameter of hollow microcapsules could also be clarified in more detail by using microbubbles instead of liquid cores as templates.
2.3. Preparation of Melamine-Formaldehyde Prepolymers. Melamine-formaldehyde pre-polymers were prepared as follows based on the formulation procedure recorded in Japanese patents18,19 and a U.K. patent.20 First, 10 g of 37% w/w formaldehyde aqueous solution with 3.3 g of melamine and 13.3 g distilled water was poured into a 100-mL conical flask that had a mechanical stirrer. After the pH of the reaction mixture was adjusted to 10.0 by using 10% w/w Na2CO3 aqueous solution, the mixture was stirred at 338 K for 15 min. During stirring, the methylolmelamine formation reaction occurred:21-24
where M-NH2 represents melamine. 2.4. Microencapsulation. The fabrication process described in this section is hence called the “default process”. The melamine-formaldehyde pre-polymer was added to the microbubble solution via a burette. Encapsulation of microbubbles via in situ polymerization was executed by stirring this mixture at 338 K for 15 min. During stirring, melamine-formaldehyde oligomeric derivatives were produced by the formation of two different types of bridges between triaging rings, namely, a methylene ether bridge, represented by reaction 2a below25-27 and a methylene bridge represented by reaction 2b:28,29
2. Experimental 2.1. Materials. The shell material was melamine (Wako, Japan) and formaldehyde (37% w/w aqueous solution, Alfa Aesar) and was used without purification. The continuum phase was 4% w/w polyvinyl alcohol aqueous solution made from anionic polyvinyl alcohol (Gohsenal T-350, Nippon Gohsei, Japan). The pH was adjusted by adding 10% w/w Na2CO3 aqueous solution obtained from anhydrous sodium carbonate (Wako, Japan). The chemical reactions of melamine and formaldehyde were controlled by using aniline (Wako, Japan) and glycine (Wako, Japan). All chemicals used in this study were reagent grade. Distilled water was used for all experiments. 2.2. Preparation of Microbubbles. In the fabrication of the microbubbles, first, 100 g of 4% w/w polyvinyl alcohol aqueous solution with 1.3 g of aniline was poured into a 300 mL pressure vessel. The vessel was then pressurized to 4 atm, and the vessel valve was closed. The vessel was maintained at 4 atm and 338 K for 15 min, and part of the air then dissolved into the solution. The valve was then opened and the pressure was decreased to atmospheric pressure, thus generating bubbles in the solution. Large bubbles rose into the air due to buoyancy force, whereas microbubbles remained in the solution. The solution with microbubbles was poured into a 300 mL beaker and maintained at 338 K. Finally, 0.33 g of glycine was added to the solution as an anionization agent of melamineformaldehyde polymer.18
3.1. Fabricated Hollow Melamine-Formaldehyde Microcapsules. Panels a and b of Figure 1 show SEM images of the hollow melamine-formaldehyde microcapsules fabricated by the microbubble template technique and located on the surface and in the sediment of the microbubble solution, respectively. The microcapsule shown in Figure 1b had a hole in its shell, indicating that the microcapsule was hollow and that the shell is on the order of 100 nm thick. Other evidence that the spheres were not solid but hollow is that the spheres floated and accumulated on the surface of the microbubble solution. If the spheres were solid, they would sink because the density of melamine-formaldehyde resin is higher than that of 4% w/w polyvinyl alcohol aqueous solution. Most sediments in the solution were needle- and plane-shaped polymer particles as shown in Figure 1b. In the microbubble template technique, hollow capsules were easily fabricated and purified because they accumulated on the liquid surface. This technique can therefore be used in mass fabrication of hollow microcapsules. 3.2. Surface Morphology. 3.2.1. Effect of Reaction Time. Figure 2 shows SEM images of hollow melamine-formaldehyde microcapsules for different reaction times, tr ) 1.0, 5.0, 15.0, and 60.0 min. The reaction was quenched by adjusting the pH of the solution to 9.5 by adding 10% w/w Na2CO3 aqueous solution after each tr. In all SEM images, hollow melamine-formaldehyde microcapsules floating on the solution surface were sampled and observed. If hollow melamineformaldehyde microcapsules have any hole in their shell, they could not float on the solution surface. Therefore, the hole in
M-NH2 + CH2O a M-NHCH2OH
(1)
M-NHCH2OH + HOCH2HN-M a M-NHCH2OCH2NH-M + H2O (2a) M-NHCH2OH + H2N-M a M-NHCH2NH-M + H2O (2b) A sequentially crosslinked network was formed by the polycondensation reaction. After completion of the encapsulation reaction, the reaction was quenched by adjusting the pH of the solution to 9.5 by adding 10% w/w Na2CO3 aqueous solution. 3. Results and Discussion
Melamine-Formaldehyde Microcapsules
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Figure 3. SEM images of hollow melamine-formaldehyde microcapsules for different glycine concentrations, mg/mg0 ) 0.2, 1.0, 2.0, and 10.0, where mg0 is the mass of glycine in the default process. Except for mg/mg0, the fabrication conditions were the same as those for the default process.
Figure 1. SEM images of hollow melamine-formaldehyde microcapsules (a) on the surface and (b) in the sediment of the microbubble solution.
Figure 2. SEM images of melamine-formaldehyde hollow microcapsules for different reaction times, tr ) 1, 5, 15, and 60 min. In the default process, tr ) 15. Except for tr, the fabrication conditions were the same as those for the default process. The initial pH of the solution was 7.60.
the capsule shell shown in Figure 2 (top image) would be produced during evacuation for SEM measurement. Hollow melamine-formaldehyde microcapsules with holes in their shell would be more fragile than those without holes though their mechanical properties were not evaluated in detail. The pH of the polyvinyl aqueous solution immediately after adding the prepolymer was 7.60, and at tr ) 1.0, 5.0, 15.0, and 60.0 min was 6.85, 6.49, 6.45, and 6.46, respectively. The pH of the solution decreased rapidly during the first minute (tr < 1.0 min) after the pre-polymer was added, and reached a constant value around tr ) 5 min. When tr < 1.0 min, no hollow capsules were
observed, suggesting that the time needed to form a capsule shell is more than 1.0 min. This point is discussed in Sec. 3.4 in more detail. The SEM images suggested that with increasing tr, the surface roughness of the capsule shells decreased, and when tr > 5.0 min, microbubbles were encapsulated without any holes in their shells. Further evidence that microbubbles were encapsulated without holes at larger tr is that most hollow microcapsules at tr ) 5.0 and 15.0 min floated on the solution surface more than a few days and then sank, and those at tr ) 60.0 min floated more than a week. Furthermore, the capsule shell thicknesses at tr ) 5.0, 15.0, and 60.0 min were about 400 nm, suggesting that when tr > 5 min, the capsule shell thickness was independent of tr. The main factor to control the capsule shell thickness is discussed in Sec. 3.3. 3.2.2. Effect of Glycine Concentration. Figure 3 shows SEM images of hollow melamine-formaldehyde microcapsules for different concentrations of glycine, mg/mg0 ) 0.2, 1.0, 2.0, and 10.0, respectively, where mg0 is the mass of glycine in the default process. Except for mg/mg0, the fabrication conditions were the same as those in the default process. Figure 3 shows that the surface roughness decreased with increasing mg/mg0. The pH of the polyvinyl aqueous solution at mg/mg0 ) 0.2, 1.0, 2.0, and 10.0 immediately after addition of the pre-polymer was 7.60, 6.90, 6.55, and 5.75, respectively, and 15 min after the addition, the pH decreased to 7.31, 6.45, 6.19, and 4.91, respectively. When mg/mg0 was increased, the pH of the solution decreased, which in turn promoted the poly-condensation reaction.30 A Japanese patent18 reported that when glycine is added to the pre-polymer of melamine-formaldehyde as an anionization agent, uniform-diameter microcapsules are produced by the in situpolymerizationmethod.Becauseglycine-modifiedmelamineformaldehyde microcapsules do not coalesce into large capsules due to the electrostatic repulsion force, uniform-diameter microcapsules could be produced. But in our current experiment, the value of mg/mg0 did not affect the size of microcapsules, but did affect the surface morphology. 3.2.3. Effect of Pre-polymer Concentration. Figure 4 shows SEM images of hollow melamine-formaldehyde microcapsules for different concentrations of pre-polymer, mp/mp0 ) 0.2, 0.5, 1.0, and 2.0, respectively, where mp0 is the mass of the pre-
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Figure 4. SEM images of melamine-formaldehyde hollow microcapsules for different pre-polymer concentrations, mp/mp0 ) 0.2, 0.5, 1.0, and 2.0, where mp0 is the mass of the pre-polymer in the default process. Except for mp/mp0, the fabrication conditions were the same as those for the default process.
polymer in the default process. Except for mp/mp0, the fabrication conditions were the same as those in the default process. At mp/mp0 ) 0.2, only aggregates of large plane-shaped polymer particles were evident, whereas at mp/mp0 ) 0.5, hollow capsules composed of small plane-shaped polymer particles were evident. At mp/mp0 ) 1.0, the capsule shell surface was much smoother than that at mp/mp0 ) 0.5, and at mp/mp0 ) 2.0, the capsule shells had many small holes, and aggregates of round-shaped polymer particles were absorbed on the shell surface. The capsule shell was formed either by (i) poly-condensation reaction on the bubble surface or (ii) adsorption of polymer particles that grew in the bulk liquid onto the bubble surface. These SEM images suggest that at mp/mp0 ) 0.2 and 0.5, poly-condensation reaction did not occur after formation of the capsule shell, thus plane-shaped polymer particles were evident. Whereas at mp/ mp0 ) 1.0, after formation of the capsule shell, poly-condensation reaction occurred on the shell surface consecutively, and the boundary between the capsule shell and an adsorbed particle disappeared. At mp/mp0 ) 2.0, poly-condensation reaction could occur on the shell surface as well, but pre-polymer particles grew into large aggregates in the bulk liquid, and then diffused onto the shell surface. Consequently, the capsule shell consisted of large aggregates of polymer particles, and some of the boundaries between the aggregates disappeared due to further poly-condensation reaction. In the SEM image for mp/mp0 ) 2.0 (Figure 4), these boundaries appeared as small holes in the capsule shell. Aggregates of polymer particles that did not become part of a capsule shell were simply absorbed on the shell surface. With increasing mp/mp0, the equilibrium reactions 2a and 2b advance to the right, that is, the poly-condensation reaction is promoted. Furthermore, the pH of solution with mp/ mp0 ) 0.2, 0.5, 1.0, or 2.0 immediately after addition of the pre-polymer was 7.07, 7.04, 6.90, and 6.74, respectively, and 15 min after the addition, the pH was 7.00, 6.72, 6.45, and 6.83, respectively. Immediately after addition of the pre-polymer, the pH of the solution slightly decreased with increasing mp/mp0, but about 5 min after the addition, the pH of the solution reached a minimum when mp/ mp0 ) 1.0. Because the decrease in pH promoted the poly-condensation reaction,30 the capsule shells at mp/mp0 ) 1.0 had the smoothest surface.
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Figure 5. SEM images of melamine-formaldehyde hollow microcapsules (left) and their cross sections (right) for different aniline concentrations, ma/ma0 ) 0.2, 1.0, 2.0, and 4.0, where ma0 is the mass of aniline in the default process. Except for ma/ma0, the fabrication conditions were the same as those for the default process.
3.3. Capsule Shell Thickness. A Japanese patent19 reported that when an aldehyde condensation polymerization resin is used as capsule shell material and if the core material contains oilsoluble aromatic mono-amino chemical compounds such as aniline, then such compounds cover and protect the core materials while simultaneously reacting with the shell material. The result is formation of a spherical capsule shell with uniform thickness. The effect of aromatic mono-amino chemical compounds on the encapsulation process has not yet been clarified, however. The same encapsulation process of Figure 1 but without aniline could not produce hollow microcapsules. If aniline was added to the solution, however, the microbubbles remained in the solution longer, suggesting that aniline played the role of a surfactant in that it stabilized the microbubbles. Furthermore, if aniline was added to the solution, the solution became cloudy immediately after addition of pre-polymer, suggesting that the pre-polymer reacted with the aniline adsorbed on the surface of the microbubbles. Figure 5 shows SEM images of melamine-formaldehyde hollow microcapsules (left) and their cross sections (right) for different concentrations of aniline, ma/ma0 ) 0.2, 1.0, 2.0, and 4.0, where ma0 is the mass of aniline in the default process. Except for ma/ ma0, the fabrication conditions were the same as those in the default process. The pH of solution with ma/ma0 ) 0.2, 1.0, 2.0, or 4.0 immediately after addition of the pre-polymer was 6.78, 6.90, 7.42, and 8.30, respectively, and 15 min after the addition, the pH was 6.68, 6.45, 6.88, and 7.60, respectively. With increasing ma/ ma0, the pH of solution immediately after addition of the pre-polymer increased and the difference in pH immediately and 15 min after addition of the pre-polymer also increased. Because the increase in pH reduced the polycondensation reaction, the surface roughness of capsule shells increased with increasing ma/ma0, and at ma/ma0 ) 4.0, microbubbles were no longer encapsulated completely (Figure 5, left SEM images). Furthermore, the capsule shell thickness increased with increasing ma/ma0 (Figure 5, right SEM images). Figure 6 shows the capsule shell thickness versus aniline mass (d-ma) curve. The average d and its standard deviation were calculated by using the measured values of d from several SEM images. The d-ma curve could be fitted by the following
Melamine-Formaldehyde Microcapsules
Figure 6. Capsule shell thickness vs mass of aniline (d-ma) curve. Circles and error bars are average values and standard deviations of d, respectively.
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Figure 8. Time evolution of the diameter, Db, of air and SF6 gas microbubbles dissolving into 4% w/w polyvinyl alcohol aqueous solution.
Figure 7. Schematics of melamine-formaldehyde polymer reacted with aniline adsorbed on the bubble surface.
equation: d ) Rma/(1 + Rma), where R is a fitting parameter. The d-ma curve reveals that (i) adsorption of aniline on the bubble surface follows the Langmuir type of adsorption and (ii) d is proportional to the mass of aniline adsorbed on the bubble surface. Figure 7 shows a schematic of melamineformaldehyde polymer reacted with aniline adsorbed on the bubble surface. Because the number of aniline molecules adsorbed on the bubble surface could be regarded as the number of initial reaction sites, the amount of reacted pre-polymer around a microbubble would be determined by the number of initial reaction sites. Though the mechanism to encapsulate microbubbles has not yet been clarified, the capsule shell thickness could be controlled by adjusting the aniline concentration. The three parameters found to control the surface morphology (Sec. 3.2) that is, reaction time, glycine concentration, and pre-polymer concentration, could not be used to control the capsule shell thickness. 3.4. Diameter Distribution of Hollow Microcapsules. In the microbubble template technique, the obtained microcapsules should retain the size of the original microbubbles if the microbubbles do not dissolve into the surrounding liquid during the encapsulation process. Here, the dissolution rate of a single microbubble was investigated for two different gases, air and SF6. Figure 8 shows the time evolution of the microbubble diameter Db for air and SF6 gas bubbles dissolving into a 4% w/w polyvinyl alcohol aqueous solution. The Db was measured (using an optical light microscope and CCD camera) based on consecutive images of the dissolution process of a single bubble, and the dissolution rate was estimated from the Db versus time plot. When Db < 20 µm, the dissolution rate of an SF6 bubble
Figure 9. Probability density function (PDF) of the microcapsule diameter, Dc, for air and SF6 microbubble templates in 4% w/w polyvinyl alcohol aqueous solution.
was much slower than that of an air bubble, indicating that compared with air microbubbles, SF6 microbubbles resided longer in the aqueous solution. Figure 8 shows that the dissolution time for a 20 µm-diameter SF6 microbubble was about 100 s, compared with about 10 s for a 20 µm-diameter air microbubble. Figure 9 shows the probability density function (PDF) of the microcapsule diameter Dc for air and SF6 microbubble templates, after large capsules were removed by filtration with a wire mesh with a grid size of 45 µm. The horizontal axis in Figure 9 is the equivalent diameter of a black circle in the binarized image of microcapsules just below the water surface determined using an optical light microscope. Figure 9 shows that the PDF for the SF6 microbubble template was larger than that for the air microbubble template at Dc < 20 µm. The difference in PDF between the two gases for Dc < 20 µm was due to neither the filtration nor the chemical reaction of gases because Dc was much smaller than the grid size of the filter and neither SF6 nor air reacted with the shell material, respectively. The difference in PDF between the two gases for Dc < 20 µm might be due to the difference in dissolution rate shown in Figure 8. Considering that the estimated time needed to form a capsule shell was more than 1.0 min from the tr experiments discussed
8884 J. Phys. Chem. B, Vol. 111, No. 30, 2007 in Sec. 3.2.1, the microcapsules of Dc < 20 µm could be fabricated more successfully around SF6 microbubbles of Db < 20 µm rather than air microbubbles of Db < 20 µm because the change in Db during encapsulation of SF6 microbubbles is smaller than that of air microbubbles. The difference in PDF between the two gases suggests that the bubbles were not encapsulated instantaneously but when tr > 1.0 min. 4. Conclusions A fabrication method for hollow melamine-formaldehyde microcapsules from microbubble templates was developed. Fabrication of these hollow microcapsules by using this method was demonstrated, and the conditions to control the surface morphology, shell thickness, and diameter of hollow microcapsules were determined. The following conclusions could be drawn from this study: 1. The surface morphology of the microcapsules could be controlled by adjusting either the (i) reaction time, (ii) glycine concentration (pH of the solution), or (iii) pre-polymer concentration. 2. The capsule shell thickness could be controlled by adjusting the aniline concentration. Hollow microcapsules could not be produced without aniline. The capsule shell thickness versus mass of aniline (d-ma) curve revealed that the adsorption of aniline on the bubble surface follows the Langmuir type of adsorption and that d is proportional to the mass of aniline adsorbed on the bubble surface. The three parameters capable of controlling the surface morphology (i.e., reaction time, glycine concentration, and pre-polymer concentration) could not be used to control the capsule shell thickness. 3. The diameter of hollow microcapsules fabricated from air microbubble templates could be controlled in the range from 5 to 200 µm, and the diameter distribution depends on the dissolution rate of gases.
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