Biodegradable hollow poly-lactic acid (PLA) microcapsules were synthesized by directly adsorbing the polymer to N2 (air) microbubbles using the bubble template method. In this method microbubbles nucleate inside droplets, made of a solution of PLA in dichloromethane, as a result of solvent diffusion into a continuous phase of either water or a PVA aqueous solution. Subsequently PLA adsorbs to the microbubble surface, covers it and then the microbubble covered with PLA is spontaneously released from the drople’s interior yielding to hollow PLA microcapsules. For special fabrication conditions, it was found that the final capsules were uniform in size. It was found that the high uniformity is directly correlated to the type of release. This was classified into either single or multiple bubble release. When the predominant type of release is single bubble release, the resultant microcapsules were uniform. In this study we also aim to elucidate the conditions required to attain single bubble release. It is believed that this type of release is attained when the energy barrier at the liquid-liquid interface is lowered, thus microcapsules can be smoothly released. From experiments, it was understood that the use of PVA, a low molecular weight PLA and an initially low PLA concentration is the requirement in order to attain single bubble release. Furthermore, based on rheological and the surface tension measurements at the liquid-liquid interface and inner flow circulation inside the droplet we proposed two plausible mechanisms for single bubble release: (a) mechanism based on the repulsive force between two adjacent microbubbles and (b) mechanism based on the imbalance of interfacial tensions around a bubble.
SEM images revealed that microcapsules obtained from ‘multiple bubble’ type can display a non-spherical cores. In addition, these capsules can have briged shells or bridged cores (share cores). Microcapsule bridging can happen inside the droplet (when cores are shared) or outside (when only the shell is bridged). Increasing the initial concentration of PLA or using a PLA of high molecular weight yields to this phenomenon. The zeta potential of these capsules at pH = 7.0 was above 20mV, and thus the colloidal suspension of microcapsules in water was stable.