6. Microencapsulation of pheromone using biodegradable polymers for mating
6.5. Microencapsulation of pheromone by emulsion / crosslinking method
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hydrophobic as compared to the pheromone which has a dodecyl group. Hence, ethyl caprate has a better affinity to poly(vinyl alcohol) in comparison to the pheromone.
6.5. Microencapsulation of pheromone by emulsion / crosslinking
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Figure 71. The image of PVA microcapsules obtained by the emulsion/crosslinking method as seen (A) under the digital microscope (B) by the scanning electron microscope (SEM).
The SEM image (Figure 71B) was unable to show the presence of microcapsules since they were found to be highly unstable under high vacuum used in the scanning electron microscopy.
Since benzaldehyde was used to crosslink PVA, two kinds of mechanisms took place:
intermolecular acetalization and intramolecular acetalization. The presence of the benzal group was indicated by the IR spectrum.
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Transmittance / %
Wavenumber / cm-1
Figure 72. Infra-red spectrum of PVA microcapsules prepared by emulsion / crosslinking method.
A B
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The IR spectrum of PVA microcapsules (Figure 72) showed the following absorption bands: the O-H stretching at 3302 cm-1, the C-O stretching at 1097 cm-1. The acetate groups of PVA showed an absorption band at 1736 cm-1, and the crosslinked PVA showed phenyl group absorption peak at 1454 cm-1. No carbonyl stretch at 1700 cm-1 was seen which showed that all the carbonyl groups of benzaldehyde were utilized in acetal bond formation with the hydroxyl groups of PVA.
In order to have an insight into the amount of pheromone contained in the PVA microcapsules, thermogravimetric analysis of the prepared microcapsules was performed.
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0 20 40 60 80 100
Weight %
Temperature / °C
A B
Figure 73. TGA of (A) pheromone (B) pheromone encapsulated PVA microcapsules prepared by the emulsion/crosslinking technique.
The TGA curve of the PVA microcapsules (Figure 73) from a temperature of 25 °C to 600 °C showed that the first degradation step at 170 °C corresponds to that of the pheromone and the second degradation step at 372 °C corresponds to that of crosslinked PVA. Again, as seen in the coacervation process, the amount of the pheromone encapsulated in the microcapsules was found to be less than 10 wt%.
The possible reasons for the low amount of the pheromone encapsulated in the PVA microcapsules could be: the presence of long chain dodecyl group in the pheromone decreases its affinity for PVA to a large extent.
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In order to increase the affinity of the pheromone for PVA, two methods were applied. In the first method, a different grade of PVA with low degree of hydrolysis was chosen for carrying out the microencapsulation process. This meant that fewer hydrophilic hydroxyl groups were present and more number of hydrophobic acetate groups were present in PVA. This would increase the hydrophobicity of PVA to some extent.
The microencapsulation process was repeated with this PVA (Mw = 14000 Da and degree of hydrolysis = 85%) keeping all other reaction conditions same. The microcapsules obtained by this method were again analyzed by TGA immediately after drying from a temperature of 25 °C to 600 °C at a heating rate of 10 °C/min.
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Weight %
Temperature / °C
A B
Figure 74. TGA of (A) pheromone and (B) pheromone-loaded PVA microcapsules using PVA with low degree of hydrolysis.
The TGA curve (Figure 74) of the microcapsules prepared by using PVA with low degree of hydrolysis showed that there was no change in the amount of pheromone encapsulated inside the microcapsules. The TGA thermogram showed similar degradation steps as in Figure 73 i.e. for the pheromone at 169 °C and for crosslinked PVA at 367 °C. The amount of encapsulated pheromone was also found to be the same – less than 10 wt%.
After the failure of the first method, the second method applied for increasing the affinity between PVA and the pheromone was the use of 1-decanol (Figure 75) as a solvent for dissolving the pheromone in the microencapsulation process.
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OH
Figure 75. Chemical structure of 1-decanol used as a solvent for dissolution of pheromone to improve its affinity for PVA.
PVA microcapsules were prepared by dissolving pheromone in 1-decanol which contains hydroxyl groups that might increase the affinity for PVA. The obtained microcapsules were again analyzed by thermogravimetric analysis to determine the amount of encapsulated pheromone. The TGA was performed from a temperature of 25 °C to 600 °C at a heating rate of 10 °C/min.
The thermogravimetric analysis (Figure 62) of the prepared microcapsules, however, did not show much change in the amount of encapsulated pheromone as evident from Figure 76.
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Weight %
Temperature / °C
A B
Figure 76. TGA of (A) pheromone and (B) pheromone-loaded PVA microcapsules with pheromone dissolved in 1–decanol.
Therefore none of the methods above succeeded in encapsulating sufficient amount of pheromone inside the PVA microcapsules.
The second reason for reduced quantity of encapsulated pheromone could be that the microparticles obtained were not actually microcapsules but microspheres. This would
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mean that the pheromone was not present in the form of a core material surrounded by a polymeric membrane, but instead it was dispersed in the polymer matrix or absorbed at the surface. This was proved by using Cryo SEM.
Cryo SEM is a low temperature scanning electron microscopy technique. In this, aqueous dispersion was rapidly cooled by plunging it into liquid nitrogen. The sample was then fractured to obtain a cross-sectional view.
Figure 77. Cryo SEM image of PVA microcapsules obtained by the emulsion/crosslinking method.
From the Cryo SEM image (Figure 77), the microparticles prepared by the emulsion/crosslinking method were found to be microspheres rather than microcapsules.
This was sighted as the reason for insufficient amount of pheromone present inside.
Hence, the limitation of this method was that it resulted in the formation of microspheres rather than microcapsules. In addition, the pheromone present inside the microcapsules leached out very fast on drying. This showed that the crosslinking of the polymer wall was not strong enough. This was attributed to the fact that in the reaction between PVA and benzaldehyde, it‘s the intramolecular acetalization that predominates over the intermolecular acetalization owing to the formation of a stable 6-membered ring structure. Hence, crosslinking between the polymer chains is not hundred percent.
Crosslinking of the microcapsule walls was also carried out by using water-soluble glutaraldehyde (25 wt%) but it resulted in the formation of gel instead of microcapsules.
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In this case, the crosslinking of PVA was accomplished by diffusion of the aldehyde from water.