Langmuir 2001, 17, 4525-4528
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Determination of the Swelling Ratio of Poly(Urea-urethane) Microcapsules by Single-Particle Light Scattering Toshiaki Dobashi,* Toshiaki Furukawa, Takayuki Narita, and Shinsuke Shimofure Department of Biological and Chemical Engineering, Faculty of Engineering, Gunma University, Kiryu, Gunma 376-8515, Japan
Kimio Ichikawa Fujinomiya Laboratory, Fuji Photo Film Company Ltd., Fujinomiya, Shizuoka 418, Japan
Benjamin Chu Chemistry Department, State University of New York, Stony Brook, New York 11794-3400 Received January 10, 2001. In Final Form: April 12, 2001 The thickness δ of microcapsules consisting of a dioctyl phthalate core and an outer poly(urea-urethane) membrane has been determined by means of single-particle light scattering and the freeze-fracture method in combination with electron microscopy as a function of the dioctyl phthalate/wall-forming triisocyanate monomer volume ratio φ. The thickness δ increased nonlinearly with decreasing φ. The swelling ratio of the membrane was determined to be 1.2 in the range φ < 3, which was explained by the relatively low plasticizing effect of dioctyl phthalate.
Introduction Emulsion polymerization is one of the most useful techniques for synthesizing microcapsules.1 By emulsification, wall-forming materials collected at the interface between the inside core and the outside dispersing medium are polymerized by a stimulation, such as by an increase in temperature, by use of ultraviolet radiation, etc., resulting in achieving the microencapsulation process. The size of the microcapsules obtained after the microencapsulation is not always the same as that of the emulsion droplets, and the wall formed by the polymer network swells with the core and/or the dispersing phase. Thus, we cannot estimate the thickness of the membrane simply by using the outer diameter of the microcapsule and the overall volume ratio of the core and the wall-forming material φ. For microcapsules commercially used on facsimile papers, poly(urea-urethane) microcapsules, a combination of small-angle X-ray scattering and light scattering measurements suggested that when phosphoric acid, bis(2,3-dibromopropyl)-2,3-dichloropropyl ester or tris(2-chloroethyl)phosphate with φ ) 1.1 was used as the core liquid, the core liquid could penetrate into the membrane.2,3 Scanning electron microscopy and a dynamic viscoelasticity study showed that the microcapsules containing tricresyl phosphate did not form a core-shell structure but formed a swollen microsphere when the overall weight ratio of the core and the wall-forming material w < 0.5.4 It should be noted that the cross-linking reaction at the surface of the emulsion could be coupled (1) Microencapsulation; Kondo, T., Ed.; CRC: Boca Raton, FL (in Japanese). (2) Dobashi, T.; Yeh, F.-j.; Ying, Q.; Ichikawa K.; Chu, B. Langmuir 1995, 11, 4278. (3) Dobashi, T.; Yeh, F.-j.; Takenaka, M.; Wu, G.; Ichikawa, K.; Chu, B. J. Colloid Interface Sci. 1996, 179, 640. (4) Ichikawa, K. J. Appl. Polym. Sci. 1994, 54, 1321.
by the swelling of the polymer network membrane and the swelling ratio could not be experimentally determined by dipping a membrane prepared in the bulk state into the core liquid. Therefore, it is worthwhile to develop an experimental method to determine the microcapsule thickness δ in situ. Torza and Manson5 have introduced an equilibrium theory for the configurations of liquid compounds to predict the particle shape of the emulsion droplets, assuming the spreading coefficients obtained from the interfacial tension of each oil and dispersing phase. The theory was successfully applied to the poly(methylmethacryrate) microcapsules prepared by the phase separation method,6 several kinds of microcapsules prepared by the in situ vinyl polymerization method,7 and polyurethane microcapsules prepared by the interfacial polycondensation method, when the volume ratio (φ) of the core material and wall-forming material was constant.8 However, it should be mentioned that the structure of microcapsules could also depend on φ and the swelling of the microcapsule membrane could be one of the major factors affecting the formation of the core-shell structure. The single-particle light scattering technique9 is appropriate for determining both the thickness and the outer diameter of the microcapsule. The single-particle study also simplifies the complications that come from a polydisperse system, as is often the case for microcapsules.10 In this study we measured the swelling ratio of the poly(5) Torza, S.; Manson, G. J. Colloid Interface Sci. 1970, 33, 67. (6) Loxley, A.; Vincent, B. J. Colloid Interface Sci. 1998, 208, 49. (7) Berg, J.; Sundberg, D.; Kronberg, B. J. Microencapsulation 1989, 6, 327. (8) Frere, Y.; Danicher, L.; Gramain, P. Eur. Polym. J. 1998, 34, 193. (9) Dobashi, T.; Narita, T.; Masuda, J.; Makino, K.; Mogi, T.; Ohshima, H.; Takenaka, M.; Chu, B. Langmuir 1998, 14, 745. (10) Dobashi, T.; Chu, B. In Surface Characterization Method; Milling, M. J., Ed.; Marcel Dekker: New York, 1999; Chapter 8.
10.1021/la010063a CCC: $20.00 © 2001 American Chemical Society Published on Web 06/22/2001
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(urea-urethane) membrane by dioctyl phthalate in the microencapsulation process at different values of φ and discussed the factors determining the membrane structure. Experimental Section Materials. Takenate D-110N (Triisocyanate monomer (75%) in ethyl acetate) was a gift from Takeda Chemical Co. Ltd. and used as the wall-forming material. Dioctyl phthalate (DOP) was purchased from Wako Chemical Co. and used as the core material. Copoly(vinyl alcohol-vinyl acetate) was used as a protective colloid for the microcapsules. The degree of polymerization and hydrolysis were 1.7 × 103 and 85 mol %, respectively. An appropriate amount (0.3-1.4 g) of triisocyanate monomer solution was added to 3 g of ethyl acetate with 1.25 g of DOP to obtain an organic phase (conventionally called “oil”). The oil was poured into 10 g of 5 wt % protective colloid aqueous solution and immersed vigorously at 5000 rpm by using an emulsifier (Excel Auto, Nihon Seiki Co. Ltd.). The resultant emulsion was stirred under atmospheric pressure at 40 °C for 4 h. The volumes of oil and wall-forming material could be calculated by using the respective densities 0.981 and 1.30.11 The ratio φ of the volume of oil vo and that of the wall-forming material vw ranged from 1 to 8, and coded system a (φ ) 1.64), system b (1.75), system c (2.70), and system d (7.81). Single-Particle Light Scattering. A laboratory-made light scattering apparatus operating at 632.8 nm9 was used for determining both the thickness and the outer diameter of an arbitrarily chosen microcapsule of systems a-d. A capillary sample cell with an inner diameter of 50 µm, which contained a dilute microcapsule suspension, was set on the light scattering stage. Scattered light intensity from a single microcapsule in the suspension was measured with a photodiode array detector. The magnitude of the scattering vector q ()(4πn/λ) sin(θ/2)) covered a range between 1 and 10 µm-1, with n, λ, and θ being the refractive index of the dispersing medium, the wavelength of light in vacuo, and the scattering angle, respectively. The details of the apparatus are described in ref 9. Dynamic Mechanical Measurement. The microcapsule suspension was coated on a plain paper substrate with a thickness of 60 µm and dried in an oven at 50 °C. Dynamic mechanical measurements were carried out using an apparatus (DMTA mkII, Polymer Laboratories Co. Ltd.) for the microcapsule-coated paper with a ramp of 5 °C/min at 1 Hz. The details have been published elsewhere.4 Surface Tension. The surface tensions were measured by the DuNuoy ring method using a surface and interfacial tensiometer (Shimadzu Co. Ltd.) at 20 °C ((0.5 °C). The surface tension for ultrapure water was measured before and after the measurements of the samples. Particle Scattering Factor. The particle scattering factor P(q) of a spherical shell has the form12
P(q) ) (9π/2)[J3/2(roq)/(roq)3/2 - f(nm)(ri/ro)3J3/2[(ri)q]/ [(ri)q]3/2]2 (1) 3 3
1/2
J3/2(rq) ) [2/(πr q )] [sin(rq) - rq cos(rq)]
(2)
f(nm) ) (no - nm)/(nm - ni)
(3)
with
where ro and ri are the outer and inner radii of the shell, respectively. ni, nm, and no denote the refractive index of the inside core oil, membrane, and outside aqueous solution, respectively. When the interfacial thickness between the membrane and the dispersing medium is finite, a term proportional to exp(-s2q2) is multiplied by P(q) in eqs 1-3 at large q, where (11) Ichikawa, K.; Ishimaru, H.; Sakanishi, A.; Dobashi, T. To be published. (12) Kerker, M.; Kratohvil, J. P.; Matijevic, E. J. Opt. Soc. Am. 1962, 52, 551.
Figure 1. Observed scattering curves for system b and calculated ones using δ and ro from Table 1 (solid curve). s denotes an index of the surface thickness.13 The refractive indices of DOP, the poly(urea-urethane) network, and aqueous solution are 1.49, 1.58,14 and 1.33, respectively. If the polymer network wall does not swell, these values correspond to ni, nm, and no, respectively. When the polymer network wall swells with the core oil and/or outside aqueous solution, nm must be estimated. Linear poly(urea-urethane) is compatible with DOP. According to the dielectric constant measurement by means of time domain reflectometry, no hydration of the poly(urea-urethane) microcapsule membrane was detected.15 Thus, we can assume that the membrane should swell only with the core oil. For simplicity, we assume that the refractive index of the wall membrane nm depends on the volume fraction of oil in the membrane ψ as
nm ) ψno + (1 - ψ)nw
(4)
ψ is related to ri, ro, and φ as
ψ ) [(ro3 - ri3)φ - ri3]/[(ro3 - ri3)(1 + φ)]
(5)
If we substitute eqs 4 and 5 into eq 3, we find f(nm) decreases gradually with increasing thickness of the membrane at constant φ. The swelling ratio σ is defined by the volume of the swollen polymer network vm divided by the volume of the original polymer network vw. We can determine the unknown parameters of the inner and outer radii of the shell, ri and ro, by fitting the observed P(q) to the combined eqs 1-5.
Results and Discussion The solid curves in Figure 1 show typical scattering curves by a single microcapsule at different φ values. Characteristic higher order peaks for the single microcapsule were observed up to q ) 6 µm-1. The data points for each sample were fitted to eqs 1-5 by a least-squares method, and the fitting parameters ri and ro were obtained. The observed (dotted) and calculated (solid) curves using the values of ro and δ from Table 1 together with a contrast factor are in reasonable agreement, as shown typically for system b in Figure 1. The introduction of the fourth parameter, the index of the surface roughness s ≈ 0.1 µm, slightly improved the fitting. In Figure 2, [(1 - ri/ro)3]-1 is plotted against 1 + φ. The dashed line represents the (13) Hashimoto, H.; Fujimura, M.; Hashimoto, T.; Kawai, H. Macromolecules 1981, 14, 844. (14) Usami, T.; Shimoura, A. J. Imag. Technol. 1990, 16, 234. (15) Private communications from Prof. S. Yagihara at Tokai University.
Swelling Ratio of Poly(urea-urethane) Microcapsules
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Figure 2. Relationship between the dimensions of a microcapsule and of the oil/wall-forming material volume ratio determined by single-particle light scattering (O) and scanning electron microscopy with the freeze-fracture method (]). Solid and dashed lines correspond to σ ) 1 and 1.2, respectively. The chain line was drawn to guide the eyes. Table 1. Oil/Wall-Forming Material Volume Ratio O, Membrane Thickness δ, and Outer Diameter ro of a Poly(urea-urethane) Microcapsule system
φ
ro (µm)
δ (µm)
a b c d
1.64 1.75 2.70 7.81
0.75 1.81 1.68 1.72
0.14 0.34 0.25 0.14
calculated curve without swelling of the polymer network membrane, as given by
[1 - (ri/ro)3]-1 ) 1 + φ
(6)
The disagreement between the observed curve and the calculated one by eq 6 indicates a significant swelling of the microcapsule membrane. Now, if the swelling equilibrium has been reached, the volumes of the core and the membrane could be approximated by vo - (σ - 1)vw and σvw, respectively. Then we have 3
3
3
[vo - (σ - 1)vw]/σvw ) vc/vm ) ri /(ro - ri )
(7)
[1 - (ri/ro)3]-1 ) (1 + φ)/σ
(8)
or
in the range of φ > σ - 1. The inverse of the swelling ratio σ could then be determined from the slope in a plot of [1 - (ri/ro)3]-1 vs 1 + φ. The volume fraction of oil in the membrane ψ is related to σ as
ψ ) 1 - 1/σ
(9)
In deriving eqs 4-9, we implicitly assumed that the swelling of the membrane was homogeneous, the swelling ratio did not depend on the size of the microcapsule, and the refractive index decreased in proportion to the volume fraction of oil in the membrane. Using three data points at φ < 3 in Figure 2, σ and ψ were estimated to be 1.2 and 0.17, respectively. The solid line in Figure 2 corresponds to σ ) 1.2. The critical value for the core-shell structure was obtained as φc ) σ - 1 ) 0.2. On the other hand, the data point for φ + 1 ) 8.81 (system d) was much lower than the solid line, indicating a very large amount of
Figure 3. Cross-sectional views of microcapsules for systems a (a) and d (b) as observed by scanning electron microscopy with the freeze-fracture method.
swelling. This result is compared with the scanning electron microscopy study using the freeze-fracture method. The surface of the microcapsule membrane is clear for system a (low φ) as shown in Figure 3a, and we can estimate the swelling ratio from the micrograph as shown by tilted squares in Figure 2. On the other hand, the membrane seems to be melted and very soft for system d as shown in Figure 3b, and we cannot determine the swelling ratio. In Figure 2, we could not find a clear difference among data points obtained by the two different methods, light scattering and electron microscopy. It is interesting to determine the swelling ratio of a microcapsule membrane prepared in the bulk state and to compare it with the present result. We prepared the microsphere consisting of only the wall-forming material without any protective colloids, dried the outside water, and immersed the microsphere in DOP. The swelling ratio determined from the volume change was 1.1. This value is a little smaller than that determined for microcapsules determined by light scattering, suggesting a coupling of swelling and cross-linking in the microencapsulation resulting from a loose network structure. For poly(urea-urethane) microcapsules containing tricresyl phosphate (TCP), φc could be estimated to be close to 0.37 from both dynamic mechanical measurements and
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scanning electron microscopy.4,11 It is interesting to compare the plasticizing effect of DOP and TCP. The glass transition temperatures (Tg) of the microcapsules containing DOP with different volume ratios (φ) obtained by dynamic mechanical measurements were 147 °C (φ ) 0.10), 147 °C (φ ) 0.20), 146 °C (φ ) 0.50), and 144 °C (φ ) 0.66). The Tg values of microcapsules containing TCP (φ ) 0.56) and no core materials were reported to be 137 and 147 °C, respectively,4 indicating a smaller plasticizing effect of DOP to the wall membrane than TCP. According to Torza and Manson, the microcapsule structure is assumed to be determined mainly by the difference in the interfacial tensions of the core and wall-forming materials. The surface tensions of DOP and TCP are 34.7 × 10-3 and 46.2 × 10-3 J‚m-2, respectively, at 25 °C, and the surface tensions obtained for the polyamide series such as nylon 6 and nylon 66 range from 32 × 10-3 to 48.5 × 10-3 J‚m-2. The interfacial tensions between the core and wall materials may be estimated from these surface tension values, and it could be reasonable to assume that there is no significant difference in DOP and TCP in the miscibility with the wall polymer. From the present experiment and ref 4 the swelling ratio of poly(ureaurethane) microcapsules containing TCP (φc ≈ 0.37) was larger than those containing DOP (φc ≈ 0.2). Thus, the interfacial tension is not the only parameter to determine
Dobashi et al.
the microcapsule structure. The solubility parameters calculated from Fedor’s equation16 are 40.0 J1/2‚m-3/2 (DOP) and 43.7 J1/2‚m-3/2 (TCP), also indicating no significant difference. The poly(urea-urethane) network has hydrogen-bonding structures between amide groups; an interaction of the core materials to the hydrogen bondings is thought to be an important factor for determining the extent of the swelling of the wall membrane to form the core-shell structures. Dipole moments of the core materials are 4.37 D for DOP and 7.33 D for TCP.15 This indicates that the interaction of DOP with hydrogen bondings should be weaker than that of TCP. Therefore, the difference in the critical values for the core-shell structure of DOP system and TCP system could be reasonably understood on the basis of the specific interaction mentioned above. Acknowledgment. This work was partly supported by a Grant-in-Aid for Scientific Research (B) from the Japan Society for the Promotion of Science under Grant 11555170 and by the U.S. National Science Foundation. LA010063A (16) Properties of Polymers; Van Krevelen, D. W., Ed.; Elsevier Publishing: Amsterdam, 1972; p 135.
