Factors Affecting the Size and Uniformity of Hollow Poly(lactic acid) Microcapsules Fabricated from Microbubble Templates

Abstract

Hollow microcapsules of poly(lactic acid) (PLA) were fabricated using the bubble template method, which involves the nucleation of bubbles inside droplets of a dichloromethane solution of PLA prepared in a continuum medium of an aqueous solution of poly(vinyl alcohol) (PVA). PLA-covered microbubbles are formed by PLA adsorbing onto the microbubble surface and then spontaneously released from the dichloromethane droplets into the surrounding aqueous solution. Thereby, hollow PLA microcapsules are formed. In the present study, we focus on the effects of the initial PLA concentration, addition of PVA to the aqueous medium, and PLA molecular weight on the radius distribution of the hollow PLA microcapsules. Results show that a low initial concentration of PLA, the addition of PVA, and low molecular weight PLA are required to form uniform hollow microcapsules. It is suggested that these factors reduce the energy barrier at the liquid-liquid interface, which allows the PLA-covered microbubbles to pass smoothly through the interface. Furthermore, we observed the release of the microbubbles from the dichloromethane droplets through a microscope and clarified the relationship between the uniformity of the hollow PLA microcapsules and the bubble-release mechanism.

Mass Production of Uniformly Sized Air-Filled Poly-Lactic Acid Microcapsules Made from Microbubble Templates

“…It is worth to note that the final dried hollow microcapsule had a smaller size compared to the microbubble template. This result was observed in our previous work as well (Molino Cornejo et al, 2011).…”

“…Immediately, methylene chloride started to diffuse into the continuous phase which allowed microbubbles to nucleate inside the droplet. As it was found in our previous (Daiguji et al, 2009;Molino Cornejo et al, 2011), since the solubility of air in the aqueous solution is smaller than in the oil phase, the air did not diffuse to the outside of the droplet but instead nucleated in the inside forming thermodynamically stable microbubbles. Afterward, PLA adsorbed to the microbubble surface which resulted in PLA covered microbubbles being released from the droplet interior into the PVAaq solution.…”

“…4. This reveals that the energy required for capsule release (Molino Cornejo et al, 2011) is not affected by the initial c PLA . The experiments were performed at room temperature.…”

“…In Method A, the largest and the most uniformly sized capsules could be obtained at c PLA =2 g L −1 and the capsule size decreased and the uniformity deteriorated with increasing c PLA (Molino Cornejo et al, 2011). When c PLA was higher than 30 g L −1 , most fabricated capsules formed aggregates and thus it was difficult to obtain independent capsules.…”

“…Since the frequency of the employed ultrasound was 38 kHz, this wave cannot collapse, nor cavitate a microbubble by itself. Thus the ultrasound only aided the nucleation and furthermore, it cannot produce the calculated large pressure required for releasing the microbubbles from the droplet's interior (Molino Cornejo et al, 2011). Nonetheless, if more bubbles are nucleated and distributed at the liquid-liquid interface, the frequency of bubble release increases due to microbubble collision and thus a specific microbubble would have enough kinetic energy to overcome the energy barrier at the interface.…”