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Production of core—shell composite polymer particles utilizing the stepwise heterocoagulation method
Colloids and Surfaces A: Physicochemical and Engineering Aspects 109 (1996) 49-53
A
SURFACES
Production of core-shell composite polymer particles utilizing the stepwise heterocoagulation method ’ M. Okubo *, Y. Lu Department of Chemical Science and Engineering, Faculty ofEngineering, 657 Japan
Kobe University, Rokko, Nada, Kobe,
Received 22 May 1995; accepted 16 August 1995
Abstract Core-shell composite polymer particles, in which the polymer composing the core was more hydrophilic than that composing the shell, were produced as follows. First, composite particles were formed by stepwise heterocoagulation of small cationic “hydrophobic” polymer particles (SP) onto a large anionic “hydrophilic” polymer particle which we suggested in 1990. These composite particles having uneven surfaces were then treated at a temperature above the glass transition temperature (T,) of SP. Keywords:
Composite polymer; Core-shell; Hydrophilic/hydrophobic;
1. Introduction
By utilizing a seeded emulsion polymerization technique, composite polymer particles in which two kinds of polymers form unique heterogeneous structures can be produced. Actually, we have produced some composite polymer particles having heterogeneous structures such as core-shell, polymeric oil-in-oil and hemispheres, and also clarified some factors which have a great influence on the particle morphology [ 11. Throughout the experiments, some anomalous composite polymer particles such as “raspberry-like” [ 21, “confetti-like” [ 31, “snowman-like” [ 41, “octopus ocellatus-like” [ 51, “void-containing” [ 61, and “golfball-like” [ 73 have been produced, which were derived from the corresponding unique morphologies. Cho and Lee * Corresponding author. ’Part CL Emulsion”.
of the
series
of “Studies
on
Suspension
and
0927-7757/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved SSDI 0927-7757(95)03473-O
Morphology; Stepwise heterocoagulation
also reported particles with “sandwich-like” and “halfmoon-like” arrangements [ 81. It is notable that in the majority of these cases, the morphologies obey kinetics control in the process of phase separation of polymers, because of a high viscosity inside the particle. In addition, in the seeded emulsion polymerization, the more hydrophilic polymer tends to distribute preferentially in the shell and the more hydrophobic polymer in the core, independent of the order in the production of polymers. In previous articles [ 9- 111, we suggested a novel technique to produce anomalous composite polymer particles having uneven surfaces by depositing stepwise small cationic particles onto a large anionic particle. We named it the stepwise heterocoagulation method. In this article, we developed it to produce spherical core-shell composite polymer particles in which core and shell should be, respectively, derived from large and small particles, regardless of the difference in hydrophilicities of the two kinds of polymers.
50
M. Okubo. Y. LulColloids
Surfaces A: Physicochem.
Table 2 Recipes for the preparation of cationic small S-BA-QDM terpolymer particles by emulsion polymerization”
2. Experimental 2.1. Materials All reagents were the same as those in a previous paper [ll]. 2.2. Preparation
Eng. Aspects 109 (1996) 49-53
of polymer particles
S/BA/QDM (Molar ratio)
SP-1 70.2/26.8/3.0
SP-2 57.3/39.1/3.0
SP-3 47.5/49.5/3.0
S (g) BA (g) AfBA (g) Tween 80 (g) Water (g)
16.1 1.55 1.37 0.125 1.25 125
12.8 10.9 1.34 0.125 1.25 125
10.4 13.3 1.34 0.125 1.25 100
D, (nm)b r, (“C)
100 50
100 30
QDM (g)
Three kinds of anionic large-size terpolymer particles (LPl-3) having different diameters (D,) were produced by emulsifier-free emulsion terpolymerization of methyl methacrylate (MMA), ethyl acrylate (EA), methacrylic acid (MAA) under the conditions listed in Table 1. Three kinds of cationic small-size terpolymer particles (SPl-3) having different glass transition temperatures (T,) were produced by emulsion terpolymerization of styrene (S), butyl acrylate (BA) and methacrylooxyethyl trimethyl ammonium chloride (QDM) under the conditions listed in Table 2.
92 17
a N,; 70°C; 24 h. b Measured by dynamic light scattering spectroscopy. ’ Measured by differential scanning calorimeter. Abbreviations: S, styrene; BA, butyl acrylate; QDM, methacryloyloxyethyl trimethyl ammonium chloride; AIBA, 2,2-azobis(2-amidinopropane)hydrochloride; Tween 80, polyoxyethylene sorbitan monooleate; D,, number-average diameter; Ts, glass transition temperature.
2.3. Stepwise heterocoagulation Heterocoagulation of LP and SP was carried out according to a previous article [ 111 as follows. ( 1) pH values of LP and SP emulsions were separately adjusted to pH 3 with 0.5 N HCl. The ratio of number of SP to number of LP in the Table 1 Recipes for the preparation of anionic large MMA-EA-MAA (66.5/30.5/3.0, molar ratio) terpolymer particles by emulsifierfree emulsion polymerization” LP-1 MMA (g) BA (g) MAA (g) KPS (g) Water (g) D, (nm)b r, (“C)
21.2 9.8 0.82 0.064 128 1000 70
LP-2
LP-3
10.6 4.9 0.41 0.032 128
10.6 4.9 0.41 0.064 256
576 70
343 70
a N,; 70°C; 24 h. b Measured by dynamic light scattering spectroscopy. ’ Measured by differential scanning calorimeter. Abbreviations: MMA, methyl methacrylate; EA, ethyl acrylate; MAA, methacrylic acid; KPS, potassium persulfate; D,, numberaverage diameter; Ts, glass transition temperature.
blend emulsion was 2N,,, (N,,, was the theoretical maximum number of SP needed to cover one LP completely in a single SP layer) [lo]. The polymer solid in the blend emulsion was 10%. Tween 80 (cloud point = 70°C) was added to the LP emulsion if the heat treatment temperature was greater than or equal to 70°C. If the treatment temperature was less than 70°C Emulgen 909 (cloud point = 40°C) was added. (2) The LP and SP emulsions were blended and kept at room temperature for 10 min. The pH of the blend emulsion was adjusted from 3 to 9 with 0.1 N KOH. These processes were carried out with gentle stirring. The temperature was then raised and kept at 40°C or 70°C for 30 min. 2.4. Heat treatment After unheterocoagulated SPs were removed by centrifugation, the dispersion of heterocoagulated composite particles (HPs) in water was treated at various heat treatment conditions. When the heat treatment temperature was lOO”C, Emulgen 950 (cloud point> 100°C) was pre-added to the dispersion.
M. Ok&o, Y. LulColloids Surfaces A: Physicochem. Eng. Aspects 109 ( 1996) 49-53
51
2.5. Particle morphology The morphology of HPs was identified by transmission electron microscopic (TEM) observation of ultrathin cross sections as follows. Dried particles were embedded in an epoxy matrix, cured at room temperature for three days and then microtomed. The ultrathin cross sections were exposed to RuO, vapor at room temperature for 30 min in the presence of 1% RuO, solution.
3. Results and discussion Fig. 1 shows TEM photographs of HPs produced using LP-1 (Tg = 70°C) and three kinds of SPs with different TBvalues, after heat treatment for 48 h at 70°C. The surface of HP tended to change to the smooth state with a decrease of the Tg of SP. These results were in accord with the data shown in a previous article [ 111. In the case of HPs produced using SP-3 (T,= 17”(Z), the particles became spherical. When Tg of SP was higher than the treatment temperature, the uneven surface of HP remained (the data were omitted). Fig. 2 shows TEM photographs of HPs produced using LP-1 and SP-2, after heat treatment at 70°C for 12 h (a), 1 day (b), 2 days (c), and 1 week (d). The surface of HP gradually changed to the smooth state with the treatment time. After one week, the spherical particles were obtained. Fig. 3 shows TEM photographs of HPs produced using LP-1 and SP-2, after the heat treat-
Fig. 2. TEM the stepwise (T,=3O”C), (a) 12 h; (b)
photographs of composite particles produced by heterocoagulation of LP-1 (T,= 70°C) and SP-2 after heat treatment at 7O’C for different times: 1 day; (c) 2 days; (d) 1 week.
ments for 48 h at different temperatures. The surface of HP markedly changed to the smooth state with an increase in the temperature above Tg of SP-2. When the treatment temperature was lOO”C, spherical particles were obtained. Fig. 4 shows TEM photographs of ultrathin cross sections of HPs produced using LP-1 and SP-2, after the heat treatments for 48 h at 40 (a), 70 (b), and 1OO’C (c). The ultrathin cross sections were exposed to RuO, vapor. This vapor stains PS which is the main component of SP, but does not stain PMMA, which is that of LP [ 123. Therefore, SPs in HP should be selectively stained by RuO,. As will be seen, in all cases no stained region was
(cl
-
L, 0.6pm Fig. 1. TEM photographs of composite particles produced by the stepwise heterocoagulations of LP-1 (T,=7O’C) and three kinds of SPs with different r, values after heat treatment for 48 h at 70°C. T8: (a) 50°C (SP-1); (b) 30°C (SP-2); (c) 17°C (SP-3).
52
h4. Okubo, Y. Lujcolloids Surfaces A: Physicochem. Eng. Aspects 109 (I 996) 49-53
Fig. 3. TEM photographs of composite particles produced by the stepwise heterocoagulation 3O”C), after heat treatment for 48 h at different temperatures (“C): (a) 40; (b) 70; (c) 100.
of LP-1 (T,=7O”C) and SP-2 (T,=
Fig. 4. TEM photographs of ultrathin cross sections of composite particles produced by the stepwise heterocoagulation of LP-1 (<=7O”C) and SP-2 (i= 3O”C), after the heat treatments for 48 h at different temperatures (“C): (a) 40; (b) 70; (c) 100. The cross sections were stained by RuO,.
Fig. 5. TEM photographs of ultrathin cross sections of composite particles produced by the stepwise heterocoagulation of three kinds of LP having different D, values and SP-2 (D, = 100 nm), after the heat treatments at 70°C for 48 h. D, of LPs (nm): (a) 1000 (LP-1); (b) 576 (LP-2); (c) 343 (LP-3). The cross sections were stained by RuO,.
M. Okubo, Y. Lu/Colloids Surfaces A: Physicochem. Eng. Aspects 109 (1996) 49-53
observed inside of HP, but was observed at the surface layers. This indicates that SPs did not diffuse into the inside of LP, but remained at the surface of LP. At 70 and lOO”C, SPs almost melted at the surface of LP, resulting in the complete shell layer, though at 40°C they partially melted, resulting in the incomplete shell layer. Fig. 5 shows TEM photographs of ultrathin cross sections of HPs produced using three kinds of LPs having different D, values and SP-2 (D,= 100 nm), after the heat treatment at 70°C for 48 h. The sections were also exposed to RuO, vapor. In all cases, SPs existed at the surface of LP, resulting in shell layer of HP. Therefore, as D, of LP was decreased, the ratio of the shell thickness to the diameter of the core was relatively increased. The number of SPs to deposit on one LP was decreased with the decrease in D, of LP. This may be based on that the number of the anionic surface charges of LP would be decreased with the decrease in D, (i.e. surface area) of LP. From these results, it is concluded that the spherical core-shell composite polymer particles, in which the core consists of a hydrophilic component and the shell consists of a hydrophobic one, can be produced by the stepwise heterocoagulation method with heat treatment process at higher temperature than Tg of SP.
53
References [II M. Okubo, Makromol. Chem., Macromol. Symp., 35/36 (1990) 307. c21 M. Okubo, Y. Katsuta and T. Matsumoto, J. Polym. Sci., Polym. Chem. Ed., 18 (1981) 481. II31T. Matsumoto, M. Okubo and M. Shibao, Koubunshi Roununshu, 33 (1976) 575. c41 M. Okubo, Y. Katsuta and T. Matsumoto, J. Polym. Sci., Polym. Lett. Ed., 20 (1982) 45. [51 M. Okubo, K. Kanaida and T. Matsumoto, Colloid Polym. Sci., 265 (1987) 876. [61 M. Okubo, M. Ando, A. Yamada, Y. Katsuta and T. Matsumoto, J. Polym. Sci., Polym. Lett. Ed., 19 (1981) 143. [71 M. Okubo, Y. Murakami and Y. Tsukuda, Chem. Express, 4 (1993) 253. ES1I. Cho and K.W. Lee, J. Appl. Polym. Sci., 30 (1985) 1903. c91 M. Okubo, K. Ichikawa, M. Tsujihiro and Y. He, Colloid Polym. Sci., 268 (1990) 791. Cl01 M. Okubo, Y. He and K. Ichikawa, Colloid Polym. Sci., 269 (1991) 125. [Ill M. Okubo, N. Miyachi and Y. Lu, Colloid Polym. Sci., 272 (1994) 270. El21 J.S. Trent, J.I. Scheinbeim and P.R. Couchman, Macromolecules, 16 (1983) 589.
A
SURFACES
Production of core-shell composite polymer particles utilizing the stepwise heterocoagulation method ’ M. Okubo *, Y. Lu Department of Chemical Science and Engineering, Faculty ofEngineering, 657 Japan
Kobe University, Rokko, Nada, Kobe,
Received 22 May 1995; accepted 16 August 1995
Abstract Core-shell composite polymer particles, in which the polymer composing the core was more hydrophilic than that composing the shell, were produced as follows. First, composite particles were formed by stepwise heterocoagulation of small cationic “hydrophobic” polymer particles (SP) onto a large anionic “hydrophilic” polymer particle which we suggested in 1990. These composite particles having uneven surfaces were then treated at a temperature above the glass transition temperature (T,) of SP. Keywords:
Composite polymer; Core-shell; Hydrophilic/hydrophobic;
1. Introduction
By utilizing a seeded emulsion polymerization technique, composite polymer particles in which two kinds of polymers form unique heterogeneous structures can be produced. Actually, we have produced some composite polymer particles having heterogeneous structures such as core-shell, polymeric oil-in-oil and hemispheres, and also clarified some factors which have a great influence on the particle morphology [ 11. Throughout the experiments, some anomalous composite polymer particles such as “raspberry-like” [ 21, “confetti-like” [ 31, “snowman-like” [ 41, “octopus ocellatus-like” [ 51, “void-containing” [ 61, and “golfball-like” [ 73 have been produced, which were derived from the corresponding unique morphologies. Cho and Lee * Corresponding author. ’Part CL Emulsion”.
of the
series
of “Studies
on
Suspension
and
0927-7757/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved SSDI 0927-7757(95)03473-O
Morphology; Stepwise heterocoagulation
also reported particles with “sandwich-like” and “halfmoon-like” arrangements [ 81. It is notable that in the majority of these cases, the morphologies obey kinetics control in the process of phase separation of polymers, because of a high viscosity inside the particle. In addition, in the seeded emulsion polymerization, the more hydrophilic polymer tends to distribute preferentially in the shell and the more hydrophobic polymer in the core, independent of the order in the production of polymers. In previous articles [ 9- 111, we suggested a novel technique to produce anomalous composite polymer particles having uneven surfaces by depositing stepwise small cationic particles onto a large anionic particle. We named it the stepwise heterocoagulation method. In this article, we developed it to produce spherical core-shell composite polymer particles in which core and shell should be, respectively, derived from large and small particles, regardless of the difference in hydrophilicities of the two kinds of polymers.
50
M. Okubo. Y. LulColloids
Surfaces A: Physicochem.
Table 2 Recipes for the preparation of cationic small S-BA-QDM terpolymer particles by emulsion polymerization”
2. Experimental 2.1. Materials All reagents were the same as those in a previous paper [ll]. 2.2. Preparation
Eng. Aspects 109 (1996) 49-53
of polymer particles
S/BA/QDM (Molar ratio)
SP-1 70.2/26.8/3.0
SP-2 57.3/39.1/3.0
SP-3 47.5/49.5/3.0
S (g) BA (g) AfBA (g) Tween 80 (g) Water (g)
16.1 1.55 1.37 0.125 1.25 125
12.8 10.9 1.34 0.125 1.25 125
10.4 13.3 1.34 0.125 1.25 100
D, (nm)b r, (“C)
100 50
100 30
QDM (g)
Three kinds of anionic large-size terpolymer particles (LPl-3) having different diameters (D,) were produced by emulsifier-free emulsion terpolymerization of methyl methacrylate (MMA), ethyl acrylate (EA), methacrylic acid (MAA) under the conditions listed in Table 1. Three kinds of cationic small-size terpolymer particles (SPl-3) having different glass transition temperatures (T,) were produced by emulsion terpolymerization of styrene (S), butyl acrylate (BA) and methacrylooxyethyl trimethyl ammonium chloride (QDM) under the conditions listed in Table 2.
92 17
a N,; 70°C; 24 h. b Measured by dynamic light scattering spectroscopy. ’ Measured by differential scanning calorimeter. Abbreviations: S, styrene; BA, butyl acrylate; QDM, methacryloyloxyethyl trimethyl ammonium chloride; AIBA, 2,2-azobis(2-amidinopropane)hydrochloride; Tween 80, polyoxyethylene sorbitan monooleate; D,, number-average diameter; Ts, glass transition temperature.
2.3. Stepwise heterocoagulation Heterocoagulation of LP and SP was carried out according to a previous article [ 111 as follows. ( 1) pH values of LP and SP emulsions were separately adjusted to pH 3 with 0.5 N HCl. The ratio of number of SP to number of LP in the Table 1 Recipes for the preparation of anionic large MMA-EA-MAA (66.5/30.5/3.0, molar ratio) terpolymer particles by emulsifierfree emulsion polymerization” LP-1 MMA (g) BA (g) MAA (g) KPS (g) Water (g) D, (nm)b r, (“C)
21.2 9.8 0.82 0.064 128 1000 70
LP-2
LP-3
10.6 4.9 0.41 0.032 128
10.6 4.9 0.41 0.064 256
576 70
343 70
a N,; 70°C; 24 h. b Measured by dynamic light scattering spectroscopy. ’ Measured by differential scanning calorimeter. Abbreviations: MMA, methyl methacrylate; EA, ethyl acrylate; MAA, methacrylic acid; KPS, potassium persulfate; D,, numberaverage diameter; Ts, glass transition temperature.
blend emulsion was 2N,,, (N,,, was the theoretical maximum number of SP needed to cover one LP completely in a single SP layer) [lo]. The polymer solid in the blend emulsion was 10%. Tween 80 (cloud point = 70°C) was added to the LP emulsion if the heat treatment temperature was greater than or equal to 70°C. If the treatment temperature was less than 70°C Emulgen 909 (cloud point = 40°C) was added. (2) The LP and SP emulsions were blended and kept at room temperature for 10 min. The pH of the blend emulsion was adjusted from 3 to 9 with 0.1 N KOH. These processes were carried out with gentle stirring. The temperature was then raised and kept at 40°C or 70°C for 30 min. 2.4. Heat treatment After unheterocoagulated SPs were removed by centrifugation, the dispersion of heterocoagulated composite particles (HPs) in water was treated at various heat treatment conditions. When the heat treatment temperature was lOO”C, Emulgen 950 (cloud point> 100°C) was pre-added to the dispersion.
M. Ok&o, Y. LulColloids Surfaces A: Physicochem. Eng. Aspects 109 ( 1996) 49-53
51
2.5. Particle morphology The morphology of HPs was identified by transmission electron microscopic (TEM) observation of ultrathin cross sections as follows. Dried particles were embedded in an epoxy matrix, cured at room temperature for three days and then microtomed. The ultrathin cross sections were exposed to RuO, vapor at room temperature for 30 min in the presence of 1% RuO, solution.
3. Results and discussion Fig. 1 shows TEM photographs of HPs produced using LP-1 (Tg = 70°C) and three kinds of SPs with different TBvalues, after heat treatment for 48 h at 70°C. The surface of HP tended to change to the smooth state with a decrease of the Tg of SP. These results were in accord with the data shown in a previous article [ 111. In the case of HPs produced using SP-3 (T,= 17”(Z), the particles became spherical. When Tg of SP was higher than the treatment temperature, the uneven surface of HP remained (the data were omitted). Fig. 2 shows TEM photographs of HPs produced using LP-1 and SP-2, after heat treatment at 70°C for 12 h (a), 1 day (b), 2 days (c), and 1 week (d). The surface of HP gradually changed to the smooth state with the treatment time. After one week, the spherical particles were obtained. Fig. 3 shows TEM photographs of HPs produced using LP-1 and SP-2, after the heat treat-
Fig. 2. TEM the stepwise (T,=3O”C), (a) 12 h; (b)
photographs of composite particles produced by heterocoagulation of LP-1 (T,= 70°C) and SP-2 after heat treatment at 7O’C for different times: 1 day; (c) 2 days; (d) 1 week.
ments for 48 h at different temperatures. The surface of HP markedly changed to the smooth state with an increase in the temperature above Tg of SP-2. When the treatment temperature was lOO”C, spherical particles were obtained. Fig. 4 shows TEM photographs of ultrathin cross sections of HPs produced using LP-1 and SP-2, after the heat treatments for 48 h at 40 (a), 70 (b), and 1OO’C (c). The ultrathin cross sections were exposed to RuO, vapor. This vapor stains PS which is the main component of SP, but does not stain PMMA, which is that of LP [ 123. Therefore, SPs in HP should be selectively stained by RuO,. As will be seen, in all cases no stained region was
(cl
-
L, 0.6pm Fig. 1. TEM photographs of composite particles produced by the stepwise heterocoagulations of LP-1 (T,=7O’C) and three kinds of SPs with different r, values after heat treatment for 48 h at 70°C. T8: (a) 50°C (SP-1); (b) 30°C (SP-2); (c) 17°C (SP-3).
52
h4. Okubo, Y. Lujcolloids Surfaces A: Physicochem. Eng. Aspects 109 (I 996) 49-53
Fig. 3. TEM photographs of composite particles produced by the stepwise heterocoagulation 3O”C), after heat treatment for 48 h at different temperatures (“C): (a) 40; (b) 70; (c) 100.
of LP-1 (T,=7O”C) and SP-2 (T,=
Fig. 4. TEM photographs of ultrathin cross sections of composite particles produced by the stepwise heterocoagulation of LP-1 (<=7O”C) and SP-2 (i= 3O”C), after the heat treatments for 48 h at different temperatures (“C): (a) 40; (b) 70; (c) 100. The cross sections were stained by RuO,.
Fig. 5. TEM photographs of ultrathin cross sections of composite particles produced by the stepwise heterocoagulation of three kinds of LP having different D, values and SP-2 (D, = 100 nm), after the heat treatments at 70°C for 48 h. D, of LPs (nm): (a) 1000 (LP-1); (b) 576 (LP-2); (c) 343 (LP-3). The cross sections were stained by RuO,.
M. Okubo, Y. Lu/Colloids Surfaces A: Physicochem. Eng. Aspects 109 (1996) 49-53
observed inside of HP, but was observed at the surface layers. This indicates that SPs did not diffuse into the inside of LP, but remained at the surface of LP. At 70 and lOO”C, SPs almost melted at the surface of LP, resulting in the complete shell layer, though at 40°C they partially melted, resulting in the incomplete shell layer. Fig. 5 shows TEM photographs of ultrathin cross sections of HPs produced using three kinds of LPs having different D, values and SP-2 (D,= 100 nm), after the heat treatment at 70°C for 48 h. The sections were also exposed to RuO, vapor. In all cases, SPs existed at the surface of LP, resulting in shell layer of HP. Therefore, as D, of LP was decreased, the ratio of the shell thickness to the diameter of the core was relatively increased. The number of SPs to deposit on one LP was decreased with the decrease in D, of LP. This may be based on that the number of the anionic surface charges of LP would be decreased with the decrease in D, (i.e. surface area) of LP. From these results, it is concluded that the spherical core-shell composite polymer particles, in which the core consists of a hydrophilic component and the shell consists of a hydrophobic one, can be produced by the stepwise heterocoagulation method with heat treatment process at higher temperature than Tg of SP.
53
References [II M. Okubo, Makromol. Chem., Macromol. Symp., 35/36 (1990) 307. c21 M. Okubo, Y. Katsuta and T. Matsumoto, J. Polym. Sci., Polym. Chem. Ed., 18 (1981) 481. II31T. Matsumoto, M. Okubo and M. Shibao, Koubunshi Roununshu, 33 (1976) 575. c41 M. Okubo, Y. Katsuta and T. Matsumoto, J. Polym. Sci., Polym. Lett. Ed., 20 (1982) 45. [51 M. Okubo, K. Kanaida and T. Matsumoto, Colloid Polym. Sci., 265 (1987) 876. [61 M. Okubo, M. Ando, A. Yamada, Y. Katsuta and T. Matsumoto, J. Polym. Sci., Polym. Lett. Ed., 19 (1981) 143. [71 M. Okubo, Y. Murakami and Y. Tsukuda, Chem. Express, 4 (1993) 253. ES1I. Cho and K.W. Lee, J. Appl. Polym. Sci., 30 (1985) 1903. c91 M. Okubo, K. Ichikawa, M. Tsujihiro and Y. He, Colloid Polym. Sci., 268 (1990) 791. Cl01 M. Okubo, Y. He and K. Ichikawa, Colloid Polym. Sci., 269 (1991) 125. [Ill M. Okubo, N. Miyachi and Y. Lu, Colloid Polym. Sci., 272 (1994) 270. El21 J.S. Trent, J.I. Scheinbeim and P.R. Couchman, Macromolecules, 16 (1983) 589.