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poly(butylene succinate) microcapsules
European Polymer Journal 38 (2002) 305±311
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Eects of protective colloids on the preparation of poly(l-lactide)/poly(butylene succinate) microcapsules K. Hong a, K. Nakayama b, S. Park a,* a
Department of Textile Engineering, Faculty of Applied Chemical Engineering, Pusan National University, #30 Changeon-Dong, Kumjeong-Ku, Pusan 609-735, South Korea b National Institute of Materials and Chemical Research, Higashi 1-1, Tsukuba, Ibaraki 305-8565, Japan Received 10 October 2000; received in revised form 3 April 2001; accepted 11 April 2001
Abstract Poly(l-lactide)/poly(butylene succinate) microcapsules containing an aqueous solution of sodium -tartrate dihydrate were prepared by the interfacial precipitation method through solvent evaporation from (w/o)/w emulsion. The eects of poly(vinyl alcohol) used as a protective colloid in the microencapsulation were investigated regarding thermal properties, particle size distributions, surface morphologies, and release behaviors of the biodegradable microcapsules. It was concluded that encapsulation eciency, surface morphologies, thermal properties, and releasing speed were closely related to the particle size distributions of microcapsules under dierent conditions of the protective colloid. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Biodegradable microcapsules; Interfacial precipitation; Protective colloid
1. Introduction Currently, much attention is focused on environmental problems and, as a result, studies on biodegradable plastics as highly functional materials have been carried out by numerous researchers [1±4]. We have previously reported about the miscibility and physical properties of poly(l-lactide) (PLA)/poly(butylene succinate) (PBS) ®bers as biodegradable blends [5]. However, compared to poly(d- or dl-lactides) with their higher degradable properties, PLA and PBS as aliphatic polyesters have rarely been considered as a shape for microcapsules in industrial or medical applications, due to their high transition temperatures [1,2]. Moreover, unlike in other synthetic polymers, the processing con-
* Corresponding author. Tel.: +82-51-510-2412; fax: +82-51512-8175. E-mail address: [email protected] (S. Park).
ditions in the preparation of blend microcapsules have not been established to prepare biodegradable microcapsules containing functional core materials such as drugs, fragrances, pesticides, and dyestus [3,4]. Also, compared to conventional microcapsules, the need for biodegradable microcapsules will become greater, due to environment-friendly movements [6±8]. In a previous study, we reported on the preparation of PLA microcapsules as a biodegradable material for sustained release behavior [9], but studies on microcapsules made from biodegradable blends for agricultural and industrial applications have never been done. PBS can be expected to accelerate the releasing speed of core material through blends with PLA because the former has lower transition temperatures than the latter. Therefore, in this study, PLA/PBS microcapsules containing sodium -tartrate dihydrate as a core material were prepared and the eects of the concentration and molecular weight of protective colloids were investigated regarding the diverse properties of the resultant microcapsules.
0014-3057/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 4 - 3 0 5 7 ( 0 1 ) 0 0 1 1 0 - 0
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K. Hong et al. / European Polymer Journal 38 (2002) 305±311
2. Experimental 2.1. Microcapsule preparation PLA and PBS, as wall-forming materials, were respectively obtained from Shimazu Co., Japan and Showa Highpolymer Co. Japan. Sodium -tartrate dihydrate (Kanto Chemicals Co., Japan) was used as a core material. Fig. 1 illustrates the chemical structures of the wall and core materials used in this study. Sorbitan monooleate, as an emulsifying agent, poly(vinyl alcohol) (PVA, Mw 10K, 30K), as a protective colloid, and chloroform, as a solvent, were purchased as a reagent grade from Wako Pure Chemicals Co., Japan and used without any further puri®cation. A 40 ml aqueous solution containing 5 wt.% of sodium -tartrate dihydrate was prepared as a core material. A w/o emulsion was formed by adding the resultant aqueous solution to 200 ml of chloroform with 2 wt.% of PLA/PBS blend at the same ratio as the wall material. After adding 1.0 wt.% of sorbitan monooleate as an emulsifying agent, the w/o emulsion was vigorously stirred at 2500 rpm. The resulting w/o emulsion was added to a 400 ml aqueous solution with PVA as a protective colloid for a (w/o)/w second emulsion. The (w/o)/w solution was heated to about 38°C, which corresponds to the boiling point of the solvent. Chloroform, as the solvent, was evaporated thoroughly from the surface of w/o emulsion globules for 6 h to induce the interfacial precipitation of PLA/PBS onto the surface of the aqueous core materials. The solution was
stirred for one more hour after heating to prevent the occurrence of any temperature-related eects between the prepared microglobules. The resulting PLA/PBS microcapsules containing the aqueous solutions of sodium -tartrate dihydrate were washed with distilled water, ®ltered, then dried in a vacuum at 40°C for 24 h. 2.2. Characterization and release test Infrared spectra were obtained using a Nicolet Impact 400D Fourier transform infrared spectrophotometer. The mean particle size and size distribution of the microcapsules were determined by using a laser scattering particle size distribution analyzer (Horiba LA-920, Japan). A particle-analyzing test using several drops of microcapsule suspension was conducted with simultaneous sonication. Scanning electron microscopy (SEM) was performed using a Dual-stag SEM DS-720 (Topcon, Japan). Microcapsules were sprinkled onto a double-sided tape, sputter coated with gold, then examined under a microscope. The diagrams from differential scanning calorimetry (DSC) and thermogravimetry (TG) were obtained using DSC 8320 (Rigaku, Japan) and TG/DTA30 (Seiko Elec. Co., Japan), respectively. Samples approximately 10 mg were heated to 200°C and 400°C at the rate of 10°C/min under a constant N2 ¯ow. To ®nd the release pro®les, 0.5 g of the microcapsules containing sodium -tartrate dihydrate as a core material, which were obtained under dierent conditions of PVA, were placed in 50 ml of distilled water and stirred mildly at 25°C. The concentration of core material released in accordance with the stirring time was assayed with a conductivity meter (Horiba, Japan). The pH change in the microcapsule suspension caused by alkaline degradation were determined with a Piccolo2 ATC pH meter (Hanna Instruments, Japan).
3. Results and discussion 3.1. Structure of microcapsules
Fig. 1. The chemical structures of the core and wall materials used in this study.
Fig. 2 shows the FT-Infrared spectra of sodium tartrate dihydrate as a core material, PLA/PBS blend, and PLA/PBS microcapsules containing core material. In case of STD, the adsorption peaks at 3400 cm 1 and 1621 cm 1 were respectively assigned to O±H and ±C@O in an ester bond. In the polymer blend, 1715, 2970, 1455, 2945, and 1185 cm 1 were respectively assigned to an aliphatic ester, CH3 in PLA, C±H in PLA, CH2 in PBS, and OC@O. Based on the presence of the adsorption peaks in STD and the PLA/PBS blend in the spectrum of microcapsules, it was con®rmed that STD is encapsulated in the PLA/PBS wall membrane.
K. Hong et al. / European Polymer Journal 38 (2002) 305±311
Fig. 2. The FT-IR spectra of sodium -tartrate dihydrate, PLA/PBS blend, and PLA/PBS microcapsules.
3.2. Particle size distribution To investigate the eects of the protective colloid on the various properties of microcapsules, this study introduced PVA with molecular weights of 10,000 and 30,000, and concentrations of 0.5, 1.0, and 2.0 wt.% as the protective colloid in the (w/o)/w second emulsion. Fig. 3 shows the particle size distributions of PLA/PBS microcapsules under dierent conditions of the PVA. With an increase in PVA concentration, the mean particle sizes of microcapsules with the molecular weight of
Fig. 3. The particle size distribution of PLA/PBS microcapsules prepared at dierent molecular weights and concentrations of protective colloid.
307
10,000 were respectively 12.83, 0.94, and 0.93 lm. When the PVA molecular weight was 30,000, the sizes of microcapsules were 2.91, 1.37, and 5.83 lm with an increase in the PVA concentration of 0.5, 1.0, and 2.0 wt.%, respectively. With the increase in the PVA concentration, even though the sizes and numbers of initially prepared emulsion globules of w/o were the same, the mean diameter became smaller and the globules also became more stable. However, when the PVA molecular weight was 30,000, the sizes increased somewhat and became more unstable. Such change is related to an increase of viscosity in the (w/o)/w second emulsion followed by the formation of agglomerates between emulsion globules. It was con®rmed that high molecular weight and high concentration of PVA causes agglomerates between globules, whereas low molecular weight and low concentration cause of the stabilizer to leakage in the second emulsion. As a result, the microcapsules at the PVA molecular weight of 10,000 and concentration of 1.0 wt.% had the smallest mean diameter and the narrowest particle size distribution with superior symmetry, while those at the PVA molecular weight of 10,000 and concentration of 0.5 wt.%, as well as those at the PVA of 30,000 and 2.0 wt.%, had larger mean diameters and broader distributions with inferior symmetry. This dierence in the ®nal particle size distribution will aect other properties of microcapsules. 3.3. Encapsulation eciency Fig. 4 shows the conductivity of the residual solution after microencapsulation at dierent molecular weights and concentrations of PVA. Because STD is a conductive sodium, relative encapsulation eciency can be
Fig. 4. The conductivity of a residual solution after microencapsulation at dierent conditions of the protective colloid.
308
K. Hong et al. / European Polymer Journal 38 (2002) 305±311
determined as a relative value. The STD concentration in the residual solution under dierent conditions of PVA is much dierent for a supposition that an encapsulation eciency of STD into a polymer±solvent organic solution could be the same under the same
condition of initial emulsi®cation. With an increase in the PVA molecular weight and concentration, the conductive value of STD in the residual solution decreases, and a decrease shows an increase in the encapsulation eciency. However the shortage of a protective colloid
Fig. 5. The SEM photographs of PLA/PBS microcapsules prepared at dierent molecular weights and concentrations of the protective colloid.
K. Hong et al. / European Polymer Journal 38 (2002) 305±311
309
as a stabilizer for samples at the PVA concentration of 0.5 wt.% was considered to cause destruction of w/o emulsion globules. As a result, the encapsulation eciency decreased due to the partial release of STD to an outer solution before the ®nal encapsulation. 3.4. Morphologies Fig. 5 shows the SEM photos of PLA/PBS microcapsules containing STD under dierent conditions of the protective colloid. Encapsulations were successfully carried out under all of the conditions of PVA in this study. However the eects of the PVA were signi®cant, and especially for microcapsules under PVA concentration of 0.5 wt.%, the agglomeration was caused by inadequate stabilizing eciency. Also the agglomeration among microcapsules under PVA concentration of 2.0 wt.% was con®rmed due to an increase of viscosity in the solution, even in the same formation conditions of w/o emulsion globules. As shown in the diagrams, the microcapsules at the PVA molecular weight of 10,000 and concentration of 1.0 wt.% had the smoothest surface and the narrowest size distribution. This corresponds with the analyses of particle size distribution and encapsulation eciency. 3.5. Thermal properties Fig. 6 shows the DSC thermograms of PLA/PBS microcapsules under dierent conditions of PVA. Compared with a pure PLA/PBS blend, the microcapsules have more or less changes in the melting temperatures of PBS and PLA as wall materials prepared under dierent conditions of the PVA. However, no signi®cant decrease in the melting temperature appeared, which corresponds with the characteristics of high crystalline polymers. The thermal properties found by the results of DSC and TG are shown in Table 1. The glass transition temperature of PLA as the wall material decreases for the microcapsules at the PVA molecular weight of 10,000, while that of PLA increases for the microcapsules at the
Fig. 6. The DSC thermograms of PLA/PBS microcapsules prepared at dierent molecular weights and concentrations of the protective colloid.
PVA molecular weight of 30,000. As for the melting temperature, no signi®cant decrease appeared, even in the plasticization eects of STD as a low molecule on the PLA/PBS wall membrane as high molecules. However, degradation temperatures, (I) and (II) of PBS and PLA, greatly decreased in the range of 37±100°C, compared with the reference sample. Furthermore, it was con®rmed that the degree of decrease was signi®cantly related to the results of particle size distribution. As it were, the microcapsules with the greatest mean diameter and the widest size distribution showed the most critical decrease in the thermal degradation temperatures. 3.6. Release behavior Fig. 7 shows the release pro®les of STD as an indicator as well as the core material drawn out from the
Table 1 Thermal properties of microcapsules at dierent conditions of protective colloid Polymer
Tg of PLA °C
Tm of PBS °C
Tm of PLA °C
Degradation point (I) °C
Degradation point (II) °C
Blend
63.1
112.9
173.1
330.1
382.1
59.6 58.6 58.4 63.8 64.8 65.7
112.0 111.8 112.1 112.3 112.3 112.1
169.6 170.2 170.7 170.8 170.8 169.8
237.6 262.3 266.0 252.5 270.3 258.1
326.3 339.3 345.0 336.5 340.5 335.1
M/Cs at PVA at PVA at PVA at PVA at PVA at PVA
[Mw [Mw [Mw [Mw [Mw [Mw
10K 10K 10K 30K 30K 30K
(0.5%)] (1.0%)] (2.0%)] (0.5%)] (1.0%)] (2.0%)]
310
K. Hong et al. / European Polymer Journal 38 (2002) 305±311
Fig. 7. The release pro®les of sodium -tartrate dihydrate from PLA/PBS microcapsules prepared at dierent molecular weights and concentrations of the protective colloid.
microcapsules prepared under dierent conditions of PVA. Based on release tests for 500 min, it was determined that microcapsules at the PVA molecular weight of 10,000 and concentration of 1.0 wt.% showed the greatest burst eect and the best sustained release of STD, which is consistent with particle uniformity. In contrast, the microcapsules with the worst sustained release of STD were prepared at the PVA molecular weight of 10,000 and concentration of 0.5 wt.%, PVA molecular weight of 30,000 and concentration of 0.5 wt.%, and concentration of 2.0 wt.%. This indicates a strong relationship between the particle size distribution and the release pro®le. The pH changes in the microcapsule suspension under dierent conditions of PVA caused by alkaline degradation are shown in Fig. 8. It was con®rmed that a scission of the ester linkage caused by alkaline degradation occurs at the PLA/PBS wall membrane, followed by the formation of carboxylic acid and thereafter a decrease in the pH. Furthermore, the release of the STD solution as a core material with pH of 7.16 accelerates the acidity of the total microcapsule suspension. However, when sodium hydroxide was used as a strong base, no signi®cant changes in the pH value were found among the samples under dierent conditions of PVA.
4. Summary Biodegradable PLA/PBS microcapsules containing sodium -tartrate dihydrate were prepared by interfacial precipitation through solvent evaporation. With an
Fig. 8. The pH values of an alkaline solution with PLA/PBS microcapsules prepared at dierent molecular weights and concentrations of the protective colloid.
increase in the concentration of PVA and a decrease in the molecular weight of PVA, the mean diameter and particle size distribution became smaller and narrower. Moreover, the microcapsules with the smallest mean diameter and the narrowest distribution showed a high eciency in encapsulation. The thermal properties of microcapsules with better uniformity in particle size were superior to those with less uniformity, due to the plasticization eects of the core material on the polymer membrane. Regarding the release behavior, the burst eects of the microcapsules became greater and the wall permeability more rapid with an increase in the uniformity of the particle size distribution.
Acknowledgements This work was ®nancially supported by the Small Manpower Plan of Brain Korea 21, 2000.
References [1] Blumm E, Owen AJ. Miscibility, crystallization and melting of poly(3-hydroxybutyrate)/poly(L-lactide) blends. Polymer 1995;36(21):4077±81. [2] Vert M, Schwarch G, Coudane J. Present and future of PLA polymers. JMS Pure Appl Chem A 1995;32(4):787± 96. [3] Yoo ES, Im SS. Melting behavior of poly(butylene succinate) during heating scan by dsc. J Polym Sci: Part B: Polym Phys 1999;37:1357±66.
K. Hong et al. / European Polymer Journal 38 (2002) 305±311 [4] Kenley RA, Lee MO, Mahoney TR, Sanders LM. Poly(lactide-co-glycolide) decomposition kinetics in vivo and in vitro. Macromolecules 1987;20(10):2398±402. [5] Hong K, Qi K, Nakayama K. Poly(l-lactide)-poly(butylene succinate) blend ®bers: a miscibility study. Polym Degrad Stab, submitted for publication. [6] Hong K, Park S. Melamine resin microcapsules containing fragrant oil: synthesis and characterization. Mater Chem Phys 1999;58:128±31.
311
[7] Hong K, Park S. Preparation and characterization of polyurea microcapsules with dierent diamines. Mater Res Bull 1999;34:963±9. [8] Hong K, Park S. Preparation of polyurethane microcapsules with dierent soft segments and their characteristics. React Funct Polym 1999;42:193±200. [9] Hong K, Park S. Preparation of poly(l-lactide) microcapsules for fragrant ®ber and their characteristics. Polymer 2000;41:4567±72.
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Eects of protective colloids on the preparation of poly(l-lactide)/poly(butylene succinate) microcapsules K. Hong a, K. Nakayama b, S. Park a,* a
Department of Textile Engineering, Faculty of Applied Chemical Engineering, Pusan National University, #30 Changeon-Dong, Kumjeong-Ku, Pusan 609-735, South Korea b National Institute of Materials and Chemical Research, Higashi 1-1, Tsukuba, Ibaraki 305-8565, Japan Received 10 October 2000; received in revised form 3 April 2001; accepted 11 April 2001
Abstract Poly(l-lactide)/poly(butylene succinate) microcapsules containing an aqueous solution of sodium -tartrate dihydrate were prepared by the interfacial precipitation method through solvent evaporation from (w/o)/w emulsion. The eects of poly(vinyl alcohol) used as a protective colloid in the microencapsulation were investigated regarding thermal properties, particle size distributions, surface morphologies, and release behaviors of the biodegradable microcapsules. It was concluded that encapsulation eciency, surface morphologies, thermal properties, and releasing speed were closely related to the particle size distributions of microcapsules under dierent conditions of the protective colloid. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Biodegradable microcapsules; Interfacial precipitation; Protective colloid
1. Introduction Currently, much attention is focused on environmental problems and, as a result, studies on biodegradable plastics as highly functional materials have been carried out by numerous researchers [1±4]. We have previously reported about the miscibility and physical properties of poly(l-lactide) (PLA)/poly(butylene succinate) (PBS) ®bers as biodegradable blends [5]. However, compared to poly(d- or dl-lactides) with their higher degradable properties, PLA and PBS as aliphatic polyesters have rarely been considered as a shape for microcapsules in industrial or medical applications, due to their high transition temperatures [1,2]. Moreover, unlike in other synthetic polymers, the processing con-
* Corresponding author. Tel.: +82-51-510-2412; fax: +82-51512-8175. E-mail address: [email protected] (S. Park).
ditions in the preparation of blend microcapsules have not been established to prepare biodegradable microcapsules containing functional core materials such as drugs, fragrances, pesticides, and dyestus [3,4]. Also, compared to conventional microcapsules, the need for biodegradable microcapsules will become greater, due to environment-friendly movements [6±8]. In a previous study, we reported on the preparation of PLA microcapsules as a biodegradable material for sustained release behavior [9], but studies on microcapsules made from biodegradable blends for agricultural and industrial applications have never been done. PBS can be expected to accelerate the releasing speed of core material through blends with PLA because the former has lower transition temperatures than the latter. Therefore, in this study, PLA/PBS microcapsules containing sodium -tartrate dihydrate as a core material were prepared and the eects of the concentration and molecular weight of protective colloids were investigated regarding the diverse properties of the resultant microcapsules.
0014-3057/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 4 - 3 0 5 7 ( 0 1 ) 0 0 1 1 0 - 0
306
K. Hong et al. / European Polymer Journal 38 (2002) 305±311
2. Experimental 2.1. Microcapsule preparation PLA and PBS, as wall-forming materials, were respectively obtained from Shimazu Co., Japan and Showa Highpolymer Co. Japan. Sodium -tartrate dihydrate (Kanto Chemicals Co., Japan) was used as a core material. Fig. 1 illustrates the chemical structures of the wall and core materials used in this study. Sorbitan monooleate, as an emulsifying agent, poly(vinyl alcohol) (PVA, Mw 10K, 30K), as a protective colloid, and chloroform, as a solvent, were purchased as a reagent grade from Wako Pure Chemicals Co., Japan and used without any further puri®cation. A 40 ml aqueous solution containing 5 wt.% of sodium -tartrate dihydrate was prepared as a core material. A w/o emulsion was formed by adding the resultant aqueous solution to 200 ml of chloroform with 2 wt.% of PLA/PBS blend at the same ratio as the wall material. After adding 1.0 wt.% of sorbitan monooleate as an emulsifying agent, the w/o emulsion was vigorously stirred at 2500 rpm. The resulting w/o emulsion was added to a 400 ml aqueous solution with PVA as a protective colloid for a (w/o)/w second emulsion. The (w/o)/w solution was heated to about 38°C, which corresponds to the boiling point of the solvent. Chloroform, as the solvent, was evaporated thoroughly from the surface of w/o emulsion globules for 6 h to induce the interfacial precipitation of PLA/PBS onto the surface of the aqueous core materials. The solution was
stirred for one more hour after heating to prevent the occurrence of any temperature-related eects between the prepared microglobules. The resulting PLA/PBS microcapsules containing the aqueous solutions of sodium -tartrate dihydrate were washed with distilled water, ®ltered, then dried in a vacuum at 40°C for 24 h. 2.2. Characterization and release test Infrared spectra were obtained using a Nicolet Impact 400D Fourier transform infrared spectrophotometer. The mean particle size and size distribution of the microcapsules were determined by using a laser scattering particle size distribution analyzer (Horiba LA-920, Japan). A particle-analyzing test using several drops of microcapsule suspension was conducted with simultaneous sonication. Scanning electron microscopy (SEM) was performed using a Dual-stag SEM DS-720 (Topcon, Japan). Microcapsules were sprinkled onto a double-sided tape, sputter coated with gold, then examined under a microscope. The diagrams from differential scanning calorimetry (DSC) and thermogravimetry (TG) were obtained using DSC 8320 (Rigaku, Japan) and TG/DTA30 (Seiko Elec. Co., Japan), respectively. Samples approximately 10 mg were heated to 200°C and 400°C at the rate of 10°C/min under a constant N2 ¯ow. To ®nd the release pro®les, 0.5 g of the microcapsules containing sodium -tartrate dihydrate as a core material, which were obtained under dierent conditions of PVA, were placed in 50 ml of distilled water and stirred mildly at 25°C. The concentration of core material released in accordance with the stirring time was assayed with a conductivity meter (Horiba, Japan). The pH change in the microcapsule suspension caused by alkaline degradation were determined with a Piccolo2 ATC pH meter (Hanna Instruments, Japan).
3. Results and discussion 3.1. Structure of microcapsules
Fig. 1. The chemical structures of the core and wall materials used in this study.
Fig. 2 shows the FT-Infrared spectra of sodium tartrate dihydrate as a core material, PLA/PBS blend, and PLA/PBS microcapsules containing core material. In case of STD, the adsorption peaks at 3400 cm 1 and 1621 cm 1 were respectively assigned to O±H and ±C@O in an ester bond. In the polymer blend, 1715, 2970, 1455, 2945, and 1185 cm 1 were respectively assigned to an aliphatic ester, CH3 in PLA, C±H in PLA, CH2 in PBS, and OC@O. Based on the presence of the adsorption peaks in STD and the PLA/PBS blend in the spectrum of microcapsules, it was con®rmed that STD is encapsulated in the PLA/PBS wall membrane.
K. Hong et al. / European Polymer Journal 38 (2002) 305±311
Fig. 2. The FT-IR spectra of sodium -tartrate dihydrate, PLA/PBS blend, and PLA/PBS microcapsules.
3.2. Particle size distribution To investigate the eects of the protective colloid on the various properties of microcapsules, this study introduced PVA with molecular weights of 10,000 and 30,000, and concentrations of 0.5, 1.0, and 2.0 wt.% as the protective colloid in the (w/o)/w second emulsion. Fig. 3 shows the particle size distributions of PLA/PBS microcapsules under dierent conditions of the PVA. With an increase in PVA concentration, the mean particle sizes of microcapsules with the molecular weight of
Fig. 3. The particle size distribution of PLA/PBS microcapsules prepared at dierent molecular weights and concentrations of protective colloid.
307
10,000 were respectively 12.83, 0.94, and 0.93 lm. When the PVA molecular weight was 30,000, the sizes of microcapsules were 2.91, 1.37, and 5.83 lm with an increase in the PVA concentration of 0.5, 1.0, and 2.0 wt.%, respectively. With the increase in the PVA concentration, even though the sizes and numbers of initially prepared emulsion globules of w/o were the same, the mean diameter became smaller and the globules also became more stable. However, when the PVA molecular weight was 30,000, the sizes increased somewhat and became more unstable. Such change is related to an increase of viscosity in the (w/o)/w second emulsion followed by the formation of agglomerates between emulsion globules. It was con®rmed that high molecular weight and high concentration of PVA causes agglomerates between globules, whereas low molecular weight and low concentration cause of the stabilizer to leakage in the second emulsion. As a result, the microcapsules at the PVA molecular weight of 10,000 and concentration of 1.0 wt.% had the smallest mean diameter and the narrowest particle size distribution with superior symmetry, while those at the PVA molecular weight of 10,000 and concentration of 0.5 wt.%, as well as those at the PVA of 30,000 and 2.0 wt.%, had larger mean diameters and broader distributions with inferior symmetry. This dierence in the ®nal particle size distribution will aect other properties of microcapsules. 3.3. Encapsulation eciency Fig. 4 shows the conductivity of the residual solution after microencapsulation at dierent molecular weights and concentrations of PVA. Because STD is a conductive sodium, relative encapsulation eciency can be
Fig. 4. The conductivity of a residual solution after microencapsulation at dierent conditions of the protective colloid.
308
K. Hong et al. / European Polymer Journal 38 (2002) 305±311
determined as a relative value. The STD concentration in the residual solution under dierent conditions of PVA is much dierent for a supposition that an encapsulation eciency of STD into a polymer±solvent organic solution could be the same under the same
condition of initial emulsi®cation. With an increase in the PVA molecular weight and concentration, the conductive value of STD in the residual solution decreases, and a decrease shows an increase in the encapsulation eciency. However the shortage of a protective colloid
Fig. 5. The SEM photographs of PLA/PBS microcapsules prepared at dierent molecular weights and concentrations of the protective colloid.
K. Hong et al. / European Polymer Journal 38 (2002) 305±311
309
as a stabilizer for samples at the PVA concentration of 0.5 wt.% was considered to cause destruction of w/o emulsion globules. As a result, the encapsulation eciency decreased due to the partial release of STD to an outer solution before the ®nal encapsulation. 3.4. Morphologies Fig. 5 shows the SEM photos of PLA/PBS microcapsules containing STD under dierent conditions of the protective colloid. Encapsulations were successfully carried out under all of the conditions of PVA in this study. However the eects of the PVA were signi®cant, and especially for microcapsules under PVA concentration of 0.5 wt.%, the agglomeration was caused by inadequate stabilizing eciency. Also the agglomeration among microcapsules under PVA concentration of 2.0 wt.% was con®rmed due to an increase of viscosity in the solution, even in the same formation conditions of w/o emulsion globules. As shown in the diagrams, the microcapsules at the PVA molecular weight of 10,000 and concentration of 1.0 wt.% had the smoothest surface and the narrowest size distribution. This corresponds with the analyses of particle size distribution and encapsulation eciency. 3.5. Thermal properties Fig. 6 shows the DSC thermograms of PLA/PBS microcapsules under dierent conditions of PVA. Compared with a pure PLA/PBS blend, the microcapsules have more or less changes in the melting temperatures of PBS and PLA as wall materials prepared under dierent conditions of the PVA. However, no signi®cant decrease in the melting temperature appeared, which corresponds with the characteristics of high crystalline polymers. The thermal properties found by the results of DSC and TG are shown in Table 1. The glass transition temperature of PLA as the wall material decreases for the microcapsules at the PVA molecular weight of 10,000, while that of PLA increases for the microcapsules at the
Fig. 6. The DSC thermograms of PLA/PBS microcapsules prepared at dierent molecular weights and concentrations of the protective colloid.
PVA molecular weight of 30,000. As for the melting temperature, no signi®cant decrease appeared, even in the plasticization eects of STD as a low molecule on the PLA/PBS wall membrane as high molecules. However, degradation temperatures, (I) and (II) of PBS and PLA, greatly decreased in the range of 37±100°C, compared with the reference sample. Furthermore, it was con®rmed that the degree of decrease was signi®cantly related to the results of particle size distribution. As it were, the microcapsules with the greatest mean diameter and the widest size distribution showed the most critical decrease in the thermal degradation temperatures. 3.6. Release behavior Fig. 7 shows the release pro®les of STD as an indicator as well as the core material drawn out from the
Table 1 Thermal properties of microcapsules at dierent conditions of protective colloid Polymer
Tg of PLA °C
Tm of PBS °C
Tm of PLA °C
Degradation point (I) °C
Degradation point (II) °C
Blend
63.1
112.9
173.1
330.1
382.1
59.6 58.6 58.4 63.8 64.8 65.7
112.0 111.8 112.1 112.3 112.3 112.1
169.6 170.2 170.7 170.8 170.8 169.8
237.6 262.3 266.0 252.5 270.3 258.1
326.3 339.3 345.0 336.5 340.5 335.1
M/Cs at PVA at PVA at PVA at PVA at PVA at PVA
[Mw [Mw [Mw [Mw [Mw [Mw
10K 10K 10K 30K 30K 30K
(0.5%)] (1.0%)] (2.0%)] (0.5%)] (1.0%)] (2.0%)]
310
K. Hong et al. / European Polymer Journal 38 (2002) 305±311
Fig. 7. The release pro®les of sodium -tartrate dihydrate from PLA/PBS microcapsules prepared at dierent molecular weights and concentrations of the protective colloid.
microcapsules prepared under dierent conditions of PVA. Based on release tests for 500 min, it was determined that microcapsules at the PVA molecular weight of 10,000 and concentration of 1.0 wt.% showed the greatest burst eect and the best sustained release of STD, which is consistent with particle uniformity. In contrast, the microcapsules with the worst sustained release of STD were prepared at the PVA molecular weight of 10,000 and concentration of 0.5 wt.%, PVA molecular weight of 30,000 and concentration of 0.5 wt.%, and concentration of 2.0 wt.%. This indicates a strong relationship between the particle size distribution and the release pro®le. The pH changes in the microcapsule suspension under dierent conditions of PVA caused by alkaline degradation are shown in Fig. 8. It was con®rmed that a scission of the ester linkage caused by alkaline degradation occurs at the PLA/PBS wall membrane, followed by the formation of carboxylic acid and thereafter a decrease in the pH. Furthermore, the release of the STD solution as a core material with pH of 7.16 accelerates the acidity of the total microcapsule suspension. However, when sodium hydroxide was used as a strong base, no signi®cant changes in the pH value were found among the samples under dierent conditions of PVA.
4. Summary Biodegradable PLA/PBS microcapsules containing sodium -tartrate dihydrate were prepared by interfacial precipitation through solvent evaporation. With an
Fig. 8. The pH values of an alkaline solution with PLA/PBS microcapsules prepared at dierent molecular weights and concentrations of the protective colloid.
increase in the concentration of PVA and a decrease in the molecular weight of PVA, the mean diameter and particle size distribution became smaller and narrower. Moreover, the microcapsules with the smallest mean diameter and the narrowest distribution showed a high eciency in encapsulation. The thermal properties of microcapsules with better uniformity in particle size were superior to those with less uniformity, due to the plasticization eects of the core material on the polymer membrane. Regarding the release behavior, the burst eects of the microcapsules became greater and the wall permeability more rapid with an increase in the uniformity of the particle size distribution.
Acknowledgements This work was ®nancially supported by the Small Manpower Plan of Brain Korea 21, 2000.
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