- Email: [email protected]
Characterization of ovalbumin-containing polyurethane microcapsules with different structures
Polymer Testing 19 (2000) 975–984
Material Characterisation
Characterization of ovalbumin-containing polyurethane microcapsules with different structures Kijeong Hong, Soomin Park
*
Department of Textile Engineering, Faculty of Applied Chemical Engineering, Pusan National University, #30, Changeon-dong, Kumjeong-ku, Pusan 609-735, South Korea Received 9 July 1999; accepted 27 October 1999
Abstract Ovalbumin (OVA)-containing polyurethane microcapsules were successfully prepared by a reaction between toluene diisocyanate (TDI) and different polyols such as glycerol, ethane diol, and propylene glycol. The structural and thermal properties of the resultant microcapsules and the release profile of the OVA from the wall membranes were studied. In conclusion, the microcapsules from the glycerol showed the highest thermal stability, with the formation of many hydrogen bonds. From the data of release profiles, it was confirmed that the particle size distribution and morphologies of microcapsules determined the release profiles of the OVA from the wall membranes. 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction Polyurethanes are attractive polymer groups with widespread characteristics of molecular composition [1–3]. In particular, segmented-polyether polyurethanes have been studied extensively due to their excellent physical properties, resistance against infections, and superior blood compatibility [4,5]. An increase in hard segments in polyurethane structures can result in polymers with higher elastic modulus, and polyurethanes with modified properties can be produced by molecular weight, chemical structures of soft segments and the structure differences of chain extenders [6]. However, applications of microcapsules need general polyurethanes composed of diisocyanate and polyol that has a low molecular weight more than they need segmented polyurethanes from polyol with a high molecular weight, due to the formation of hard segments * Corresponding author. Tel.: +82-51-510-2412; fax: +82-51-512-8175. E-mail address: [email protected] (S. Park)
0142-9418/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 1 4 2 - 9 4 1 8 ( 9 9 ) 0 0 0 7 0 - 7
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only. As it were, a microcapsule membrane with only hard segments can have superior monodispersiveness and is less sticky than a membrane that simultaneously has hard and soft segments, which makes a shorter methylene chain of polyol in the resultant wall membrane. In a previous study, we reported on the preparation and the characterization of the segmentedpolyurethane, polyurea, and melamine resin microcapsules, produced by encapsulation of fragrant migrin oil as the hydrophobic material [7–9]. Studies on the preparation and characterization of hydrophilic core material-containing microcapsules should be conducted to widen the applications of microcapsules to areas such as pesticides, fragrances, dyestuff, etc. In this study, polyurethane microcapsules containing ovalbumin as a hydrophilic-model material are prepared, then the structures, thermal properties, and morphologies of the microcapsules produced by changing polyol types are investigated.
2. Experimental 2.1. Materials Toluene diisocyanate (TDI) purchased from Merck and three types of alcohol—glycerol (Glyc.), ethane diol (ED), and propylene glycol (PG)—purchased Wako Pure Chemical Co. Japan were used as wall-forming materials after being dried in a vacuum drier for 3 h. OVA as a hydrophilic protein, which was encapsulated in a small vessel, was purchased from Sigma Chemical Co. USA. Sorbitan monooleate (Span 80) and poly(vinyl alcohol) (PVA, Mw=72,000) were obtained from Wako Pure Chemical Co. Japan and used without further purification. Dibutyl tin dilaurate (DBTDL, Sigma) was used as the catalyst. All the purchased chemicals were of reagent grade. 2.2. Microcapsule preparation The first w/o emulsion was formed by mixing 0.05 M TDI and 3 ml of 10% ovalbumin aqueous solution under 0.3 ml of Span 80, which was used as an emulsifier. An emulsification was performed at 25,000 rpm for 1 min with a homogenizer (Physcotron NS-60, Ichioniri Kikai, Japan). Subsequently, the emulsion was added to 150 ml of 0.5% PVA aqueous solution for the production of (w/o)/w second emulsion, which was stirred with a homomixer (T.K. Homo Mixer Mark II, Tokushu Kika, Japan). The stirring rate was reduced to 10,000 rpm, and the second emulsion was maintained for 5 min. 0.05 M three polyols of glycerol, ethane diol and propylene glycol, and a catalytic content of DBTDL was added to the (w/o)/w solution to produce a polyurethane wall membrane on the emulsion globules. Interfacial polymerization was performed at 50°C. The stirring rate was decreased to 7000 rpm after the reaction for 5 min. 150 ml of distilled water was added to the solution to disperse the resulting particles after the reaction for 5 min. The stirring rate in the solution was decreased to 5000 rpm to accelerate wall-formability on the emulsion globules, and polymerization onto the particles was performed for 5 h. The obtained microcapsules were washed with distilled water and recentrifuged three times at 10,000 rpm for 20 min and then dried for at least 24 h under reduced pressure at ambient temperature.
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2.3. Microcapsule characterization IR spectra were obtained by Nicolet Impact 400D Fourier transform infrared spectrometer (Seiko Co., Japan). Differential scanning calorimetry (DSC) was performed by using a SSC 5200H (Seiko Co., Japan). Samples of approximately 5 mg each were heated at the rate of 10°C/min up to 450°C under a constant N2 flow. Thermal gravimetric (TG) analysis for microcapsules were performed by using a TG-DTA 2000S, Mac Science, Japan. Samples of approximately 6.0 mg each were heated to 470°C at a rate of 10°C/min. The mean number–diameter and the particle size distribution were determined by a Galai CIS-I particle sizer (Galai Production Ltd, Israel). A test using a few drops of microcapsule slurry was performed after a sonication for 5 min. Scanning electron microscopy (SEM) was performed by using a Hitachi S-4200, Japan. Microcapsules were sprinkled onto a double-sided tape, sputter-coated with platinum, and examined with a microscope. 2.4. Release behavior of OVA from microcapsules Release profiles of OVA as a hydrophilic protein from 200 mg of the microcapsules were examined in 100 ml of distilled water as an extraction solvent with light stirring and kept at 20°C. pH of all the samples in this study reached continuous values after 24 h. Release amount of the OVA was determined by using an expandable ionAnalyzer EA 920, Orion Research Co. USA. 3. Results and discussion 3.1. Structure of microcapsules Fig. 1 shows the FT–IR spectra of polyurethane microcapsules from different polyols of glycerol, ethane diol, and propylene glycol. All the spectra showed absorption bands at 1740–1700
Fig. 1. FT–IR spectra of polyurethane microcapsules from different polyols.
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cm⫺1 for the C=O stretching of urethane, and at 1690–1650 cm⫺1 for urethane–urea formation. The N–H stretching vibrations, which indicate the formation of strong hydrogen bonds on the wall membranes, were observed at 3400–3300 cm⫺1. All the spectra indicated the completion of the reaction between diisocyanate and polyols from the disappearance of a NCO absorption band at 2270 cm⫺1 and appearance of the N–H and C=O absorption bands. C–H stretching vibrations of aliphatic diamine are shown at 2950 and 2850 cm⫺1. From these characteristic peaks, it was confirmed that wall membranes of the microcapsules are polyurethanes. A glycerol has three –OH groups, while ethane diol and propylene glycol have two –OH groups. The main difference in chemical structures between ethane diol and propylene glycol is in the existence or absence of a side methyl group. Fig. 1 indicates that peaks of –NH and –C=O were stronger for the sample prepared from glycerol as a polyol than for the samples from ethane diol and propylene glycol. This shows that the more rapid polymerization occurred with a three –OH group of glycerol, which show the formation of strong hydrogen bonds. Furthermore, polyurethane microcapsules from ethane diol show a higher intensity of –NH to –C=O peaks than those from propylene glycol, due to the absence of a side methyl group, which inhibits the formation of strong hydrogen bonds. 3.2. Thermal properties Fig. 2 presents DSC diagrams of polyurethane microcapsules from TDI and various polyols. As shown in Fig. 2, all the samples were confirmed of having high thermal stability with the formation of polymer wall membranes. The melting temperatures of these samples were all above 320°C, and these temperatures increased in the order of the microcapsules from propylene glycol, ethane diol, and glycerol as a polyol. This corresponds with the results of structural analyses for the samples with different intensities of hydrogen bonding caused by different chemical properties
Fig. 2. DSC diagrams of polyurethane microcapsules from different polyols.
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of polyols. The differences in chemical structures of glycerol with three hydroxyl groups, ethane diol without any methylene group, and propylene glycol with methylene group in a side chain caused the formation of polymer wall membranes that have different physical properties. However, the melting temperatures of these samples did not correspond to the degree of adsorption energy because the samples from ethane diol and propylene glycol had a higher degree of adsorption energy than that from glycerol. These adsorption areas are not very significant because, in general, they depend on the degree of exotherm by the crystallization of a polymer just before the melting state. Therefore, the microcapsules from glycerol with the narrowest adsorptive transition are concluded to have the highest thermal properties. TG diagrams of the microcapsules from various polyols are shown in Fig. 3. The residual weight of each sample in this study also verifies the formation of the microcapsules with different level of resistance to heat, which is related with the difference of polyol structures. The residual weight of each sample became greater in the order of propylene glycol, ethane diol, and glycerol, which indicated that the sample from glycerol with more –OH groups than other types of polyol samples consist of a stronger wall membrane. This is due to a stronger hydrogen bond, which in turn resulted in higher thermal resistance. The initial weight loss starts below 280°C, and the weight loss at a melt state approached to approximately 80% of the original weight at 320°C. It was confirmed that the microcapsules were formed with good thermal stability because the thermal transition of polymers in Fig. 3 corresponded to that in Fig. 2. From the results of thermal analyses, it has also been confirmed that a monomer’s chemical condition such as number of hydroxyl groups, linearity, and existence of a hydrophobic group in a side chain can control the physical properties of the resultant microcapsules. 3.3. Morphologies In the present study, the first emulsification between the OVA as a core material and TDI as a monomer was performed at 25,000 rpm for 1 min, and the second emulsification was created
Fig. 3. TG diagrams of polyurethane microcapsules from different polyols.
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by adding a catalyst and polyol as a monomer at 10,000 rpm for 5 min. The diameter of the emulsion globules may be in the range of a few nanometers, owing to vigorous agitation. However, to heighten the reactivity between monomers on the emulsion globules and to avoid the membrane breakage by agitation, the interfacial polymerization was performed at a reduced stirring speed, 5000 rpm, for 5 h. Ni et al. [10] reported that membranes may break due to agitation if the membranes’ initial strength was not strong enough and, in some of their experiments, membranes fragmented. They assumed that if the membranes were initially strong enough to prevent emulsion droplets from coalescing, the microcapsule size and its distribution would remain unchanged. They also showed that the membrane of the polyurea microcapsule was fairly strong when agitated at 600–1800 rpm. In our study, we chose a more rapid stirring speed, in the range of 5000–25,000 rpm. Surface morphologies of the polyurethane microcapsules are shown in Fig. 4. As shown in the photographs, the size distribution of the resultant particles was in the range of 0.1–3.0 µm, and the range of distribution was broader than expected. It is suggested that during polymerization, coalescence might be partially produced among the second emulsified globules with a few nanometers, which also contradicts with Ni et al.’s reports [10]. The number of emulsion globules is one of the most important factors in assigning the limited space of a reactor. In other words, by raising the stirring speed, the number of resultant emulsions would be increased, and consequently the possibility of collisions among the globules would also be increased. Therefore, the potential for coalescence among the emulsions in this study is higher than that in the experiments by Ni et al. [10]. From the results of this study, it can be concluded that the size stability of final particles depends on the number of emulsion as well as the initial strength of membranes under different conditions of agitation. Fig. 4 illustrates that the spherical formation of the sample was determined as being the worst for the microcapsules from propylene glycol. This indicated that coalescence occurred most easily among the emulsion globules by adding propylene glycol, which seemed to be due to the difference of wall-formability with TDI as another monomer. Fig. 5 shows the surface morphologies of single microcapsules from different polyols with the diameter of 600 nm. Even in the differences of polyols used in this study, the differences in surface morphologies are not significant for all of the samples. However, the microcapsules with different mean particle sizes and morphologies could influence the release profiles of the core material, OVA, from the wall membranes. 3.4. Particle size distribution Fig. 6 shows the particle size distribution of polyurethane microcapsules prepared from different polyols. Over 80% of all the microcapsules were in the size range of 0.1–3.0 µum. The significantly small particles were prepared by stirring emulsions vigorously. The small particles could form much larger specific surface areas than the larger ones could, and the small particles could also produce highly controlled and sustained release profiles of the core material. Therefore, the particle size distribution can be a very important factor when creating release profiles of the OVA, because a narrow distribution indicates the formation of many small particles, which create larger surface areas than large particles do. The particle monodispersity of all the samples was relatively good in that over 80% of the particles were below 3 µm. However, the agglomerate formation was also seen in all the samples and increased in the order of the samples prepared from glycerol,
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Fig. 4. SEM photographs of polyurethane microcapsules from different polyols (×10,000); (a) glycerol, (b) butane diol, (c) propylene glycol.
ethane diol, and propylene glycol. Therefore, it could be assumed that there was a difference among the release profiles of OVA from the various polyurethane microcapsules. 3.5. Release profiles Fig. 7 shows the release profiles of the OVA from the wall membranes prepared from TDI and different polyols. The release profiles were determined from pH level changes in the aqueous extraction medium. As assumed from the results of the morphologies and the particle size distribution of the samples, the initial values of pH increased in the order of the samples from glycerol,
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Fig. 5. SEM photographs of polyurethane microcapsules from different polyols (×100,000); (a) glycerol, (b) butane dial, (c) propylene glycol.
ethand diol, and propylene glycol. These values indicate the content of the OVA on the microcapsule surface. However, the sustained release behavior increased in the reverse order of that of the initial values. As shown in the profiles, the sample from glycerol indicates the most controlled and sustained release of the OVA from the wall membrane. This can be explained by the size distribution of the samples. In the samples from ethane diol and glycerol, the OVA could be released from the microcapsules with a good and high monodispersity of the particles, while in the sample from propylene glycol, the OVA could not be easily released from the multi-nucleus capsule, due to the formation of many agglomerated particles in the lower specific surface area. Therefore, the release rate of the OVA from the propylene glycol sample sharply decreases after
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Fig. 6. Particle size distribution of polyurethane microcapsules from different polyols.
Fig. 7. Release profiles of polyurethane microcapsules from different polyols.
the initial release on the capsule surface. Therefore, particle size distribution was determined to be an especially important factor in controlling the release rate of core material in the microcapsules. 4. Conclusion Polyurethane microcapsules containing a hydrophilic protein, OVA, were prepared by interfacial polycondensation between TDI and different polyols, glycerol, ethane diol, and propylene
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glycol on the surface of emulsion globules. It was confirmed that the polyurethane microcapsule could be successfully prepared by the completion of a reaction between diisocyanate and polyols from the disappearance of an NCO absorption band and the appearance of the N–H and C=O absorption bands. From the results of thermal analysis, it was determined that differences of chemical structures such as glycerol with three hydroxyl groups, ethane diol without any methylene group, and propylene glycol with methylene group in a side chain caused the formation of polymer wall membranes that had different levels of thermal stability. Furthermore, the differences in particle size distribution and morphologies influenced the release profiles of the core material, OVA, from the membranes. References [1] Yui N, Kataoka K, Yamada A, Sakurai Y, Sanui K, Ogata N. Drug release from monolithic devices of segmented polyether-poly(urethane)s having both hydrophobic and hydrophilic soft segments. Macromol Chem Rapid Commun 1988;7:197. [2] Kohjiya S, Ikeda Y, Takesako S, Yamashita S. Drug release behavior from polyurethane gel. React Funct Polym 1991;15:165. [3] Goldberg EP, Nakajima A. Biomedical polymers: polymeric materials and pharmaceuticals for biomedical use. New York: Academic Press, 1984. [4] Kohjiya K, Ikeda Y. Polyurethane elastomers for biomedical applications. Nihon Gomu Kyokaishi 1989;62:357. [5] Lelah MD, Lambrecht LL, Young BR, Cooper SL. Physiochemical characterization and in vivo blood tolerability of cast and extruded biomer. J Biomed Mater Res 1983;17:1. [6] Spathis G, Niaounakis M, Kontou E, Apekis L, Pissis P, Christodoulides C. Morphological changes in segmented polyurethane elastomers by varying the NCO/OH ratio. J Appl Polym Sci 1994;54:831. [7] Hong K, Park S. Preparation of polyurethane microcapsules with different soft segments and their characteristics React Funct Polym, in press. [8] Hong K, Park S. Preparation of polyurea microcapsules with different diamine and their characteristics. Mater Res Bull 1999;34:963. [9] Hong K, Park S. Melamine resin microcapsules containing fragrant oil: synthesis and characterization. Mater Chem Phys 1999;58:128. [10] Ni P, Zhang M, Yan N. Effect of operating variables and monomers on the formation of polyurea microcapsules. J Membr Sci 1995;103:51.
Material Characterisation
Characterization of ovalbumin-containing polyurethane microcapsules with different structures Kijeong Hong, Soomin Park
*
Department of Textile Engineering, Faculty of Applied Chemical Engineering, Pusan National University, #30, Changeon-dong, Kumjeong-ku, Pusan 609-735, South Korea Received 9 July 1999; accepted 27 October 1999
Abstract Ovalbumin (OVA)-containing polyurethane microcapsules were successfully prepared by a reaction between toluene diisocyanate (TDI) and different polyols such as glycerol, ethane diol, and propylene glycol. The structural and thermal properties of the resultant microcapsules and the release profile of the OVA from the wall membranes were studied. In conclusion, the microcapsules from the glycerol showed the highest thermal stability, with the formation of many hydrogen bonds. From the data of release profiles, it was confirmed that the particle size distribution and morphologies of microcapsules determined the release profiles of the OVA from the wall membranes. 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction Polyurethanes are attractive polymer groups with widespread characteristics of molecular composition [1–3]. In particular, segmented-polyether polyurethanes have been studied extensively due to their excellent physical properties, resistance against infections, and superior blood compatibility [4,5]. An increase in hard segments in polyurethane structures can result in polymers with higher elastic modulus, and polyurethanes with modified properties can be produced by molecular weight, chemical structures of soft segments and the structure differences of chain extenders [6]. However, applications of microcapsules need general polyurethanes composed of diisocyanate and polyol that has a low molecular weight more than they need segmented polyurethanes from polyol with a high molecular weight, due to the formation of hard segments * Corresponding author. Tel.: +82-51-510-2412; fax: +82-51-512-8175. E-mail address: [email protected] (S. Park)
0142-9418/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 1 4 2 - 9 4 1 8 ( 9 9 ) 0 0 0 7 0 - 7
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only. As it were, a microcapsule membrane with only hard segments can have superior monodispersiveness and is less sticky than a membrane that simultaneously has hard and soft segments, which makes a shorter methylene chain of polyol in the resultant wall membrane. In a previous study, we reported on the preparation and the characterization of the segmentedpolyurethane, polyurea, and melamine resin microcapsules, produced by encapsulation of fragrant migrin oil as the hydrophobic material [7–9]. Studies on the preparation and characterization of hydrophilic core material-containing microcapsules should be conducted to widen the applications of microcapsules to areas such as pesticides, fragrances, dyestuff, etc. In this study, polyurethane microcapsules containing ovalbumin as a hydrophilic-model material are prepared, then the structures, thermal properties, and morphologies of the microcapsules produced by changing polyol types are investigated.
2. Experimental 2.1. Materials Toluene diisocyanate (TDI) purchased from Merck and three types of alcohol—glycerol (Glyc.), ethane diol (ED), and propylene glycol (PG)—purchased Wako Pure Chemical Co. Japan were used as wall-forming materials after being dried in a vacuum drier for 3 h. OVA as a hydrophilic protein, which was encapsulated in a small vessel, was purchased from Sigma Chemical Co. USA. Sorbitan monooleate (Span 80) and poly(vinyl alcohol) (PVA, Mw=72,000) were obtained from Wako Pure Chemical Co. Japan and used without further purification. Dibutyl tin dilaurate (DBTDL, Sigma) was used as the catalyst. All the purchased chemicals were of reagent grade. 2.2. Microcapsule preparation The first w/o emulsion was formed by mixing 0.05 M TDI and 3 ml of 10% ovalbumin aqueous solution under 0.3 ml of Span 80, which was used as an emulsifier. An emulsification was performed at 25,000 rpm for 1 min with a homogenizer (Physcotron NS-60, Ichioniri Kikai, Japan). Subsequently, the emulsion was added to 150 ml of 0.5% PVA aqueous solution for the production of (w/o)/w second emulsion, which was stirred with a homomixer (T.K. Homo Mixer Mark II, Tokushu Kika, Japan). The stirring rate was reduced to 10,000 rpm, and the second emulsion was maintained for 5 min. 0.05 M three polyols of glycerol, ethane diol and propylene glycol, and a catalytic content of DBTDL was added to the (w/o)/w solution to produce a polyurethane wall membrane on the emulsion globules. Interfacial polymerization was performed at 50°C. The stirring rate was decreased to 7000 rpm after the reaction for 5 min. 150 ml of distilled water was added to the solution to disperse the resulting particles after the reaction for 5 min. The stirring rate in the solution was decreased to 5000 rpm to accelerate wall-formability on the emulsion globules, and polymerization onto the particles was performed for 5 h. The obtained microcapsules were washed with distilled water and recentrifuged three times at 10,000 rpm for 20 min and then dried for at least 24 h under reduced pressure at ambient temperature.
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2.3. Microcapsule characterization IR spectra were obtained by Nicolet Impact 400D Fourier transform infrared spectrometer (Seiko Co., Japan). Differential scanning calorimetry (DSC) was performed by using a SSC 5200H (Seiko Co., Japan). Samples of approximately 5 mg each were heated at the rate of 10°C/min up to 450°C under a constant N2 flow. Thermal gravimetric (TG) analysis for microcapsules were performed by using a TG-DTA 2000S, Mac Science, Japan. Samples of approximately 6.0 mg each were heated to 470°C at a rate of 10°C/min. The mean number–diameter and the particle size distribution were determined by a Galai CIS-I particle sizer (Galai Production Ltd, Israel). A test using a few drops of microcapsule slurry was performed after a sonication for 5 min. Scanning electron microscopy (SEM) was performed by using a Hitachi S-4200, Japan. Microcapsules were sprinkled onto a double-sided tape, sputter-coated with platinum, and examined with a microscope. 2.4. Release behavior of OVA from microcapsules Release profiles of OVA as a hydrophilic protein from 200 mg of the microcapsules were examined in 100 ml of distilled water as an extraction solvent with light stirring and kept at 20°C. pH of all the samples in this study reached continuous values after 24 h. Release amount of the OVA was determined by using an expandable ionAnalyzer EA 920, Orion Research Co. USA. 3. Results and discussion 3.1. Structure of microcapsules Fig. 1 shows the FT–IR spectra of polyurethane microcapsules from different polyols of glycerol, ethane diol, and propylene glycol. All the spectra showed absorption bands at 1740–1700
Fig. 1. FT–IR spectra of polyurethane microcapsules from different polyols.
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cm⫺1 for the C=O stretching of urethane, and at 1690–1650 cm⫺1 for urethane–urea formation. The N–H stretching vibrations, which indicate the formation of strong hydrogen bonds on the wall membranes, were observed at 3400–3300 cm⫺1. All the spectra indicated the completion of the reaction between diisocyanate and polyols from the disappearance of a NCO absorption band at 2270 cm⫺1 and appearance of the N–H and C=O absorption bands. C–H stretching vibrations of aliphatic diamine are shown at 2950 and 2850 cm⫺1. From these characteristic peaks, it was confirmed that wall membranes of the microcapsules are polyurethanes. A glycerol has three –OH groups, while ethane diol and propylene glycol have two –OH groups. The main difference in chemical structures between ethane diol and propylene glycol is in the existence or absence of a side methyl group. Fig. 1 indicates that peaks of –NH and –C=O were stronger for the sample prepared from glycerol as a polyol than for the samples from ethane diol and propylene glycol. This shows that the more rapid polymerization occurred with a three –OH group of glycerol, which show the formation of strong hydrogen bonds. Furthermore, polyurethane microcapsules from ethane diol show a higher intensity of –NH to –C=O peaks than those from propylene glycol, due to the absence of a side methyl group, which inhibits the formation of strong hydrogen bonds. 3.2. Thermal properties Fig. 2 presents DSC diagrams of polyurethane microcapsules from TDI and various polyols. As shown in Fig. 2, all the samples were confirmed of having high thermal stability with the formation of polymer wall membranes. The melting temperatures of these samples were all above 320°C, and these temperatures increased in the order of the microcapsules from propylene glycol, ethane diol, and glycerol as a polyol. This corresponds with the results of structural analyses for the samples with different intensities of hydrogen bonding caused by different chemical properties
Fig. 2. DSC diagrams of polyurethane microcapsules from different polyols.
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of polyols. The differences in chemical structures of glycerol with three hydroxyl groups, ethane diol without any methylene group, and propylene glycol with methylene group in a side chain caused the formation of polymer wall membranes that have different physical properties. However, the melting temperatures of these samples did not correspond to the degree of adsorption energy because the samples from ethane diol and propylene glycol had a higher degree of adsorption energy than that from glycerol. These adsorption areas are not very significant because, in general, they depend on the degree of exotherm by the crystallization of a polymer just before the melting state. Therefore, the microcapsules from glycerol with the narrowest adsorptive transition are concluded to have the highest thermal properties. TG diagrams of the microcapsules from various polyols are shown in Fig. 3. The residual weight of each sample in this study also verifies the formation of the microcapsules with different level of resistance to heat, which is related with the difference of polyol structures. The residual weight of each sample became greater in the order of propylene glycol, ethane diol, and glycerol, which indicated that the sample from glycerol with more –OH groups than other types of polyol samples consist of a stronger wall membrane. This is due to a stronger hydrogen bond, which in turn resulted in higher thermal resistance. The initial weight loss starts below 280°C, and the weight loss at a melt state approached to approximately 80% of the original weight at 320°C. It was confirmed that the microcapsules were formed with good thermal stability because the thermal transition of polymers in Fig. 3 corresponded to that in Fig. 2. From the results of thermal analyses, it has also been confirmed that a monomer’s chemical condition such as number of hydroxyl groups, linearity, and existence of a hydrophobic group in a side chain can control the physical properties of the resultant microcapsules. 3.3. Morphologies In the present study, the first emulsification between the OVA as a core material and TDI as a monomer was performed at 25,000 rpm for 1 min, and the second emulsification was created
Fig. 3. TG diagrams of polyurethane microcapsules from different polyols.
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by adding a catalyst and polyol as a monomer at 10,000 rpm for 5 min. The diameter of the emulsion globules may be in the range of a few nanometers, owing to vigorous agitation. However, to heighten the reactivity between monomers on the emulsion globules and to avoid the membrane breakage by agitation, the interfacial polymerization was performed at a reduced stirring speed, 5000 rpm, for 5 h. Ni et al. [10] reported that membranes may break due to agitation if the membranes’ initial strength was not strong enough and, in some of their experiments, membranes fragmented. They assumed that if the membranes were initially strong enough to prevent emulsion droplets from coalescing, the microcapsule size and its distribution would remain unchanged. They also showed that the membrane of the polyurea microcapsule was fairly strong when agitated at 600–1800 rpm. In our study, we chose a more rapid stirring speed, in the range of 5000–25,000 rpm. Surface morphologies of the polyurethane microcapsules are shown in Fig. 4. As shown in the photographs, the size distribution of the resultant particles was in the range of 0.1–3.0 µm, and the range of distribution was broader than expected. It is suggested that during polymerization, coalescence might be partially produced among the second emulsified globules with a few nanometers, which also contradicts with Ni et al.’s reports [10]. The number of emulsion globules is one of the most important factors in assigning the limited space of a reactor. In other words, by raising the stirring speed, the number of resultant emulsions would be increased, and consequently the possibility of collisions among the globules would also be increased. Therefore, the potential for coalescence among the emulsions in this study is higher than that in the experiments by Ni et al. [10]. From the results of this study, it can be concluded that the size stability of final particles depends on the number of emulsion as well as the initial strength of membranes under different conditions of agitation. Fig. 4 illustrates that the spherical formation of the sample was determined as being the worst for the microcapsules from propylene glycol. This indicated that coalescence occurred most easily among the emulsion globules by adding propylene glycol, which seemed to be due to the difference of wall-formability with TDI as another monomer. Fig. 5 shows the surface morphologies of single microcapsules from different polyols with the diameter of 600 nm. Even in the differences of polyols used in this study, the differences in surface morphologies are not significant for all of the samples. However, the microcapsules with different mean particle sizes and morphologies could influence the release profiles of the core material, OVA, from the wall membranes. 3.4. Particle size distribution Fig. 6 shows the particle size distribution of polyurethane microcapsules prepared from different polyols. Over 80% of all the microcapsules were in the size range of 0.1–3.0 µum. The significantly small particles were prepared by stirring emulsions vigorously. The small particles could form much larger specific surface areas than the larger ones could, and the small particles could also produce highly controlled and sustained release profiles of the core material. Therefore, the particle size distribution can be a very important factor when creating release profiles of the OVA, because a narrow distribution indicates the formation of many small particles, which create larger surface areas than large particles do. The particle monodispersity of all the samples was relatively good in that over 80% of the particles were below 3 µm. However, the agglomerate formation was also seen in all the samples and increased in the order of the samples prepared from glycerol,
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Fig. 4. SEM photographs of polyurethane microcapsules from different polyols (×10,000); (a) glycerol, (b) butane diol, (c) propylene glycol.
ethane diol, and propylene glycol. Therefore, it could be assumed that there was a difference among the release profiles of OVA from the various polyurethane microcapsules. 3.5. Release profiles Fig. 7 shows the release profiles of the OVA from the wall membranes prepared from TDI and different polyols. The release profiles were determined from pH level changes in the aqueous extraction medium. As assumed from the results of the morphologies and the particle size distribution of the samples, the initial values of pH increased in the order of the samples from glycerol,
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Fig. 5. SEM photographs of polyurethane microcapsules from different polyols (×100,000); (a) glycerol, (b) butane dial, (c) propylene glycol.
ethand diol, and propylene glycol. These values indicate the content of the OVA on the microcapsule surface. However, the sustained release behavior increased in the reverse order of that of the initial values. As shown in the profiles, the sample from glycerol indicates the most controlled and sustained release of the OVA from the wall membrane. This can be explained by the size distribution of the samples. In the samples from ethane diol and glycerol, the OVA could be released from the microcapsules with a good and high monodispersity of the particles, while in the sample from propylene glycol, the OVA could not be easily released from the multi-nucleus capsule, due to the formation of many agglomerated particles in the lower specific surface area. Therefore, the release rate of the OVA from the propylene glycol sample sharply decreases after
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Fig. 6. Particle size distribution of polyurethane microcapsules from different polyols.
Fig. 7. Release profiles of polyurethane microcapsules from different polyols.
the initial release on the capsule surface. Therefore, particle size distribution was determined to be an especially important factor in controlling the release rate of core material in the microcapsules. 4. Conclusion Polyurethane microcapsules containing a hydrophilic protein, OVA, were prepared by interfacial polycondensation between TDI and different polyols, glycerol, ethane diol, and propylene
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glycol on the surface of emulsion globules. It was confirmed that the polyurethane microcapsule could be successfully prepared by the completion of a reaction between diisocyanate and polyols from the disappearance of an NCO absorption band and the appearance of the N–H and C=O absorption bands. From the results of thermal analysis, it was determined that differences of chemical structures such as glycerol with three hydroxyl groups, ethane diol without any methylene group, and propylene glycol with methylene group in a side chain caused the formation of polymer wall membranes that had different levels of thermal stability. Furthermore, the differences in particle size distribution and morphologies influenced the release profiles of the core material, OVA, from the membranes. References [1] Yui N, Kataoka K, Yamada A, Sakurai Y, Sanui K, Ogata N. Drug release from monolithic devices of segmented polyether-poly(urethane)s having both hydrophobic and hydrophilic soft segments. Macromol Chem Rapid Commun 1988;7:197. [2] Kohjiya S, Ikeda Y, Takesako S, Yamashita S. Drug release behavior from polyurethane gel. React Funct Polym 1991;15:165. [3] Goldberg EP, Nakajima A. Biomedical polymers: polymeric materials and pharmaceuticals for biomedical use. New York: Academic Press, 1984. [4] Kohjiya K, Ikeda Y. Polyurethane elastomers for biomedical applications. Nihon Gomu Kyokaishi 1989;62:357. [5] Lelah MD, Lambrecht LL, Young BR, Cooper SL. Physiochemical characterization and in vivo blood tolerability of cast and extruded biomer. J Biomed Mater Res 1983;17:1. [6] Spathis G, Niaounakis M, Kontou E, Apekis L, Pissis P, Christodoulides C. Morphological changes in segmented polyurethane elastomers by varying the NCO/OH ratio. J Appl Polym Sci 1994;54:831. [7] Hong K, Park S. Preparation of polyurethane microcapsules with different soft segments and their characteristics React Funct Polym, in press. [8] Hong K, Park S. Preparation of polyurea microcapsules with different diamine and their characteristics. Mater Res Bull 1999;34:963. [9] Hong K, Park S. Melamine resin microcapsules containing fragrant oil: synthesis and characterization. Mater Chem Phys 1999;58:128. [10] Ni P, Zhang M, Yan N. Effect of operating variables and monomers on the formation of polyurea microcapsules. J Membr Sci 1995;103:51.