Langmuir 2002, 18, 6031-6034
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Coupling of Chemical Cross-linking, Swelling, and Phase Separation in Microencapsulation Toshiaki Dobashi,*,† Toshiaki Furukawa,† Kimio Ichikawa,‡ and Takayuki Narita† Department of Biological and Chemical Engineering, Faculty of Engineering, Gunma University, Kiryu, Gunma 376-8515, Japan, and Fujinomiya Laboratory, Fuji Photo Film Company Ltd., Fujinomiya, Shizuoka 418-8666, Japan Received February 28, 2002. In Final Form: May 23, 2002 In the interfacial polymerization of poly(urea-urethane) membranes that result in the formation of microcapsules, the mechanism of microencapsulation can be complex. To clarify this process, a series of organic solvents with different degrees of compatibility between the solvent and the polymer, consisting of diethyl phthalate (DEP), dibutyl phthalate (DBP), and dioctyl phthalate (DOP), was used to study the polymerization of triisocyanate in both film and microcapsule formation. The physical properties of both the films and of the microcapsules were then determined by using a combination of differential scanning calorimetry, optical microscopy, and atomic force microscopy. The glass transition temperatures of films with limiting amounts of DEP, DBP, and DOP and without the organic solvent were, respectively, 98, 115, 123, and 124 °C. The triisocyanate + DOP system had a lower critical solution temperature of about 5 °C, with a two-phase domain structure for the membrane synthesized with DOP above the critical solution temperature. The shape of the microcapsules was found to be spherical, biconcave, and biconcave with a hump for compositions with weight fraction of DOP, ws, being 0, 0.3-0.5, and 0.78, respectively. By taking into account the compatibility between the solvent and the polymer, the combined results suggest that chemical cross-linking, swelling, and phase separation can be coupled to control the mechanism in microencapsulation of poly(urea-urethane) membranes to form microcapsules. A homogeneous elastic membrane is formed with organic solvents that are compatible with the wall-forming monomers, whereas the membrane becomes brittle and heterogeneous with less compatible solvents due to phase separations.
I. Introduction Poly(urea-urethane) (PUU) microcapsules are widely used as temperature-sensitive recording materials.1 They can be synthesized by means of an interfacial polymerization process; e.g., triisocyanate monomers in the organic phase react with water at the interface of oil-in-water suspensions to produce a microcapsule membrane. Based on the expected release rate of chemical agents such as pigments and dyes, an appropriate set of bulky stable organic solvents such as tricresyl phosphate, dioctyl phthalate, tris(2-chloroethyl) phosphate, and triisocyanate as monomer were chosen for such studies. According to viscoelasticity2 and scattering studies,3,4 a significant amount of the organic solvent is contained in the microcapsule membrane. The glass transition temperature of the microcapsule membrane containing tricresyl phosphate was considerably lower than that of the microspheres without tricresyl phosphate.2 In a previous paper,5 we determined the swelling ratio of microcapsules consisting of a dioctyl phthalate core and an outer PUU membrane by using single-particle light scattering and a freeze-fracture method in combination with electron microscopy. A swelling ratio of the membrane * Corresponding author. Fax: 81-277-30-1477. E-mail: dobashi@ bce.gunma-u.ac.jp. † Gunma University. ‡ Fuji Photo Film Company Ltd. (1) Kondo, T. J. Oleo Sci. 2001, 50, 143. (2) Ichikawa, K. J. Appl. Polym. Sci. 1994, 54, 1321. (3) Dobashi, T.; Yeh, F.-j.; Ying, Q.; Ichikawa, K.; Chu, B. Langmuir 1995, 11, 4278. (4) Dobashi, T.; Yeh, F.-j.; Takenaka, M.; Wu, G.; Ichikawa, K.; Chu, B. J. Colloid Interface Sci. 1996, 179, 640. (5) Dobashi, T.; Furukawa, T.; Narita, T.; Shimofure, S.; Ichikawa, K.; Chu, B. Langmuir 2001, 17, 4525.
as large as 1.2 was observed. According to the theory of swelling of a polymer network,6 the swelling ratio is related to the difference in the interaction parameter χ or the solubility parameter δ between the polymer and the solvent. According to the Fedors method,7 δ of triisocyanate monomer and dioctyl phthalate are calculated to be 12.0 and 9.6 (cal/cm3)1/2, respectively. From the large difference in δ, a swelling ratio of 1.2 as determined by the light scattering method seems to be too large. In a preliminary experiment, the microspheres prepared by using only triisocyanate monomers did not swell so much in dioctyl phthalate. This apparent inconsistency might be related to the mechanism of the interfacial polymerization. The present experimental findings suggest that the presence of organic solvents could affect the structure and physicochemical properties of microcapsule membranes, and the resultant effects could be generalized to explain the mechanism in the interfacial polymerization process. On the other hand, such effects might be utilized to control the microcapsule structure and then its function. Thus it is of great importance to clarify the correlation of chemical cross-linking and molecular interactions between the wallforming materials and the dispersing media in the mechanism of microencapsulation. In this study we prepared thin PUU films with and without a series of organic solvents of diethyl phthalate (DEP), dibutyl phthalate (DBP), and dioctyl phthalate (DOP), with higher membrane affinity and smaller size in the order DEP, DBP, and DOP. Thin films were used to provide a more straightforward determination of the swelling ratio. The amount of unreacted triisocyanate (6) Flory, P. J. Principles of polymer chemistry; Cornell University Press: Ithaca, NY, 1953. (7) Fedors, R. F. Polym. Eng. Sci. 1974, 14, 147.
10.1021/la020213x CCC: $22.00 © 2002 American Chemical Society Published on Web 07/09/2002
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monomers was measured by Fourier transform (FT-IR) spectroscopy. The organic solvent contained in the films was determined by measuring the absorbance in methanol as an elution medium. The glass transition temperature Tg of the films was measured by differential scanning calorimetry (DSC). Tg was also measured for PUU microcapsule membranes and compared with those of the films. The effect of DOP was further studied by cloud point curve determinations and by using light microscopic observation of the films and atomic force microscopic observation of the microcapsules. The results are combined to discuss the mechanism of microencapsulation. II. Experimental Section 1. Materials. a. Preparation of Thin Films. Takenate D110N (75% triisocyanate monomer in ethyl acetate) purchased from Takeda Chem. Co. was mixed with and without a series of phthalate compounds, DOP, DBP, and DEP, with varying weight fraction ws of the solvents in the range from ws ) 0 to 0.73. After addition of an appropriate amount of ethyl acetate, the solutions were stirred to ensure homogeneous mixing. A few drops of the solution were dipped onto slide glasses covered by polyethylene sheets. The solutions were then spread by using another slide glass. The slide glasses were incubated in pure water at 40 °C for 4 h for polycondensation (the same condition as microencapsulation) and dried in an oven at 50 °C for 8 h. The final samples were obtained as a PUU film by peeling it off from polyethylene sheets. The films prepared with DOP, DBP, and DEP and without any solvent were named as films A, B, C, and D, respectively. The solubility parameter δ is 9.56 (more hydrophobic), 10.07, and 10.54 (cal/cm3)1/2 (less hydrophobic),7 and the corresponding dielectric constant is 5.22, 6.58, and 7.86 C2/(J m),8 respectively, for DOP, DBP, and DEP. δ of the PUU network is estimated to be 12.03 (cal/cm3)1/2.7 Films C and D were transparent and elastic in the experimental range of ws, and films A and B were turbid and brittle for ws > 0.1. We note that a supernatant oil phase was observed when the slide glass was incubated, especially in the case with DEP. b. Preparation of Microcapsules. Microcapsules were prepared in the same solvent/monomer combination as the films, except that a protective colloid copoly(vinyl acetate-vinyl alcohol) was added into the aqueous phase, and emulsification was made before incubation at 40 °C. The details of the microencapsulation procedure have been described elsewhere.2 The average diameter of the microcapsules was around 0.2 µm. The weight fraction of the organic solvent ws was in the range from 0 to 0.73. 2. Measurements. a. FT-IR Measurement. The amount of residual triisocyanate in the film samples was measured by FTIR (Perkin-Elmer, FT-1600). b. Optical Absorbance Measurement. The organic solvents contained in the film samples were extracted by gently stirring in a dispersing medium of methanol at 40 °C for 8 h. The amount of the organic solvent was determined from the UV absorbance of the organic solvent in dispersing medium (methanol). A calibration curve was established by using known amounts of organic solvent in methanol. c. DSC Measurement. The film samples were cut into small pieces before use. Microcapsules were repeatedly dispersed in pure water and rinsed by a centrifuge to remove the protective colloid adsorbed on the microcapsule membrane. Then the microcapsules were freeze-dried. Differential scanning calorimetry (Perkin-Elmer, DSC-7) measurements were made for the samples at a temperature scanning rate of 10 °C/min from 25 to 150 °C, cooling to 25 °C, and waiting for 10 min before heating to 200 °C. From the endothermic peak in the last course, the glass transition temperature for each sample was determined. d. Cloud Point Curve Measurement. Triisocyanate was mixed with DOP in ethyl acetate at various weight fractions of DOP. Here, ethyl acetate was added to lower the viscosity and to form a homogeneous solution. The solutions were set in a water bath, the temperature of which was controlled to within (0.01 K. Then (8) Yagihara, S. Private communication.
Figure 1. FT-IR spectrum for film A with ws ) 0.2. The arrow shows the signal from unreacted triisocyanate monomer.
Figure 2. Weight fraction of the organic solvent in film wf as a function of that of the original solution used for the preparation ws for films A (3), B (]), and C (O). The dashed line is drawn for the case wf ) ws. the temperature of the water bath was raised from 4 °C at the rate of 1 K every 12 h. The cloud point temperature was determined by observing the transmitted laser spot from the sample at each temperature. This measurement was made for the solutions with different concentrations of ethyl acetate Ce. The cloud point curve for the binary mixture triisocyanate + DOP was determined by extrapolating the cloud point curve to Ce f 0. e. Light and Atomic Force Microscopy. Samples of film A with various weight fractions ws were observed by light microscopy (Olympus CK-40) and atomic force microscopy (Digital Nanoscope III) using the tapping mode. For the observation of atomic force microscopy, we used a gelatin-coated substrate.
III. Results and Discussion Figure 1 shows a typical example of the FT-IR profile for film A with ws ) 0.2. The arrow shows a signal from a trace amount of triisocyanate monomer at the wavelength n ) 2200-2400 cm-1. Significant signals of triisocyanate monomer were not observed for all the other samples. This result indicates that the triisocyanate monomers in the suspension reacted almost completely by incubation at 40 °C for 4 h. The wall membrane of microcapsules is much thinner than the films, so that no unreacted triisocyanate should be remaining in the membrane. Figure 2 shows the results for the methanol extraction, expressed in the relationship between the weight fraction of the organic solvent in the film, wf, and that of the organic
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Figure 3. Glass transition temperature as a function of weight fraction of organic solvent. Open symbols (3), (]), and (O) denote films A, B, and C, respectively, and closed symbols denote the corresponding microcapsules.
solvent in the original solution, ws, used to prepare the film. The dashed line is for ws ) wf. All the curves show saturation at high values of ws, with the saturated value of wf being 0.20, 0.14, and 0.07, respectively, for films A, B, and C. The value wf ) 0.20 is consistent with the swelling ratio determined from light scattering.5 It is no surprise to find that the amount of the organic solvent in the film increases with increasing affinity of the organic solvent with the membrane network. If we take into consideration the affinity of surrounding water with the organic solvent and the observed supernatant organic solvent phase in the preparation, i.e., a portion of DEP, the less bulky and less hydrophobic compound can be dissolved in water. Thus, DEP can get out through the membrane to form a supernatant phase. The glass transition temperature Tg decreased with increasing ws, as shown in Figure 3, for both the film samples and the microcapsule samples. No significant change in Tg was observed between films and corresponding microcapsules. The curves were saturated below ws ) 0.3 and could be fitted well to the equation
( )
Tg ) Tg∞ + ∆Tg exp
-c c0
Figure 4. Cloud point curve for the binary system triisocyanate monomer + DOP. The arrow shows the microencapsulation process.
Figure 5. Optical micrograph of film A with ws ) 0.27.
(1)
The value of Tg∞ agrees with that of PUU microspheres (no core materials). The glass transition temperature changes for each organic solvent and is expressed by ∆Tg with ∆Tg ) 1, 9, and 26 °C for films A, B, and C, respectively. From this result, it is suggested that DEP plays the role of a plasticizer, whereas DOP does not change the molecular conformation of the polymer network membrane. Figure 4 shows the cloud point curve for film A. A lower critical solution temperature is found around 5 °C. The arrow shows a part of the microencapsulation process from 0 °C for homogenization to 40 °C for interfacial polycondensation. Thus, the system passes across the cloud point curve in this process. Figure 5 shows a microscopic photograph for film A with ws ) 0.3. The observed pattern is similar to the spinodal decomposition. The atomic force microscopy (AFM) image of microcapsules shows spheres. No distinct difference was observed for microcapsules with various ws values in the dry stage. In contrast, we observed spherical, biconcave, and biconcave-with-a-hump images, respectively, for ws ) 0 (a), 0.27 (b), 0.53 (c), and 0.73, in
Figure 6. Atomic force microscopy image observed at wet stage with ws ) 0 (a), 0.27 (b), 0.53 (c), and 0.73 (d).
the wet stage, as shown in Figure 6. The biconcave shape of the AFM image was normally observed for materials which consist of a core and a soft shell.9 The hump at the center, newly observed for microcapsules with extremely large values of ws, may be related to the heterogeneous (9) Sanji, T.; Nakatsuka, Y.; Ohnishi, S.; Sakurai, H. Macromolecules 2000, 33, 8524.
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membrane structure observed in the film sample as shown in Figure 5. From the above experimental results, the following picture could be suggested: Membrane networks prepared by interfacial polymerization at the boundary of an aqueous phase and an organic solvent phase depend on the affinity of the membrane-forming materials with those phases, the concentration ws, and environmental conditions such as temperature. When DOP (the most bulky compound with the lowest affinity for the membrane) was used as an organic core phase, it remained in the membrane but phase separates in the course of the interfacial polycondensation (or microencapsulation) process. The resultant membrane had a two-phase domain structure, and it was mechanically brittle. When DEP (the least bulky compound with the highest affinity for the membrane) was used as an organic phase, it was partly lost in the supernatant phase (for film) or the aqueous phase (for microcapsules), and the remaining portion in the membrane could plasticize the membrane, resulting in a homogeneous elastic membrane. A schematic illustration is shown in Figure 7. We also tried to determine the microcapsule membrane structure by a combination of dynamic light scattering and small-angle X-ray scattering (SAXS). We first determined the size distribution of microcapsules by dynamic light scattering, and then the SAXS profiles were fitted to theoretical microcapsule models on the assumption of various relationships between the outer radius ro and the inner radius ri of the shell. The results seemed reasonable when ri was assumed to be a constant. However, for larger size microcapsules, even the two-phase domain structures had length scales in the submicron region, beyond the measurable range for conventional SAXS. Then, SAXS profiles could not be used to illustrate this point. In conclusion, chemical cross-linking, swelling, and phase separation are closely coupled in microencapsulation by means of the interfacial polymerization method. An important finding of this study is that the release rate for drugs, pigments, etc. can be varied by controlling the microstructure of the membrane. In a preliminary test, the release rate of a model drug varied more than 100 times by replacing the organic core of DOP by DEP. Clear understanding of the mechanism of the microencapsulation process and quantitative control of the release rate from the microcapsules is expected to bring microcapsule technology to a new height.
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Figure 7. Schematic illustration of membrane in the case where affinity of core organic solvent and membrane polymer network is high (a) and low (b).
Acknowledgment. T.D. expresses special thanks to Prof. Benjamin Chu for guiding him to this field, continuing valuable advice, and critical reading of the manuscript. This work was presented at the Stony Brook Symposium on Complex Matter in honor of Professor Benjamin Chu on his 70th birthday. We would like to thank Mr. Pieter VanVolkenburgh for important suggestions. It was partly supported by a Grant-in-Aid for Scientific Research (B) under Grant 11555170 and one for Exploratory Research under Grant 13875119 from Japan Society for the Promotion of Science. LA020213X
