Design, characterization and applications of polyester nanoparticles obtained by enzymatic polymerization in nano-emulsions prepared by low-energy methods

Abstract

The presence of volatile fragrant molecules is generally associated with a feeling of pleasantness or cleanness in consumer products. Because of their high volatilities, the perception of fragrances is limited over time. Therefore, various delivery systems have been developed by the industry to control and extend the release of volatile ingredients by diffusion and also to protect active molecules against degradation. A promising encapsulation technique is the formation of capsules with tunable permeability, so-called colloidosomes. These are generally formed by locking the nanoparticles at the interface of a Pickering emulsion. However, there are very few articles concerning colloidosomes stabilized by covalent cross-linking. In addition, although Pickering emulsions have been known for over a century, the range of nanoparticles used to stabilize emulsions has been limited to silica, clays, polystyrene, polyacrylate latexes or metallic nanoparticles. Within the aim to apply colloidosomes or Pickering emulsions as delivery systems, the use of biodegradable and biocompatible polymeric nanoparticles such as polyester nanoparticles is of major interest. Polyester nanoparticles are solid colloidal particles with sizes generally between 20 and 500 nm that can be prepared in colloidal systems by polymerization of monomers in nano-emulsions which act as template for the nanoparticle formation. Nano-emulsions are emulsions with extremely small droplet size, typically in the range of 20-500 nm. Due to their small droplet size, nano-emulsions are transparent and present a better stability against gravitational separation than conventional emulsions. Nano-emulsions can be prepared by low-energy methods taking advantage of the internal energy of the system components which is released during the emulsification process. Despite the large number of articles describing the polymerization of monomers in nano-emulsions, only few articles refer to this process in nano-emulsions obtained by low-energy methods (Spernath, 2007). Therefore, studies on the preparation of polymeric nanoparticles from nano-emulsion droplets obtained by low-energy methods are of great interest. Concerning in-situ polymerization of monomers in nano-emulsions, many types of reactions have been reported generally requiring harsh conditions (low pH, toxic initiator, high temperature) to obtain polymers with acceptable yield and molecular weight. A research effort is needed to obtain polyester nanoparticles by polymerization under mild conditions. The global objective of this research work was the design, characterization and applications of polyester nanoparticles obtained by polymerization in mild conditions of monomer-in-water nano-emulsions obtained by low-energy methods. In a first step, bluish transparent lactone-in-water nano-emulsions are formed using either the PIT or the PIC methods at various O/S ratios and high water content. Nano-emulsions presented a good stability at room temperature and at 45ºC to carry out the polymerization in the droplet of the nano-emulsions. The formation of polyester nanoparticles was achieved by ring-opening polymerization of a lactone at 45ºC in nano-emulsions obtained by low-energy methods. The properties of the obtained nanoparticles such as size or crystallinity were optimized by varying the composition of the nano-emulsions template and/or the reaction parameters. Furthermore, the functionalization of the nanoparticles was confirmed by synthesizing nanoparticles by ring-opening polymerization of a hydroxy functionalized monomer or by copolymerization of two lactone monomers thereby controlling the degree of functionalization. The stabilization of emulsions of perfume-in-water by polyesters nanoparticles was investigated. Parameters such as the wettability of the nanoparticles, the concentration of nanoparticles, the ratio of each phase as well as the addition of electrolytes were studied These emulsions have been used as controlled release systems for model fragrance, but could not effectively control the release. Furthermore, in order to avoid the rapid evaporation of the perfume, the nanoparticles at the oil/water Pickering emulsions interface were covalently cross-linked leading to the formation of core-shell microcapsules, allowing the controlled release of perfume.

En la presente tesis, se ha estudiado la formación de nanopartículas de poliésteres por polimerización por apertura de anillo de una lactona (catalizada por enzima y baja temperatura) usando nano-emulsiones obtenidas por métodos de baja energía. Se han optimizado las propiedades de las nanopartículas obtenidas tales como el tamaño o la cristalinidad mediante la variación de la composición de las nano-emulsiones plantilla y/o de los parámetros de reacción. Además, se ha confirmado la posibilidad de funcionalizar las nanopartículas mediante la adición de un grupo hidroxilo en su estructura o mediante la copolimerización de monómeros cíclicos controlando así el grado de funcionalización. Se ha estudiado también la estabilización de emulsiones de Pickering de un perfume-en-agua por las nanopartículas de poliésteres. Se han utilizado estas emulsiones como sistemas de liberación controlada de una fragancia modelo, pero no se pudo controlar eficientemente su liberación. Asimismo, con el objetivo de evitar la rápida evaporación del perfume, se han reticulado covalentemente las nanopartículas en la interfase aceite/agua de las emulsiones de Pickering conduciendo a la formación de microcápsulas con estructura núcleo-coraza, permitiendo un mayor control de la liberación de perfume.