Photopolymerization of acrylate monomer miniemulsions: from fundamentals to implementation in photoreactors

Over the last 40 years, miniemulsion polymerization has resulted in the generation of many polymer colloidal structures, currently unattainable by other heterophase processes. One of its salient features is the virtual lack of monomer transport across the aqueous phase. As a result, the nucleation takes place preferentially in monomer droplets, which makes miniemulsion amenable to many unconventional applications extending to water-sensitive processes (ionic, catalytic, step polymerizations), controlled radical polymerizations, encapsulation of liquids, preformed polymer or inorganic particles, and many others. One attractive area, recently explored, has been the development of photo-initiated miniemulsion polymerizations. Extensive literature reports on UV-driven cross-linking of bulk monomer films, whilst monomer emulsions have been rarely photo-polymerized because of their propensity for scattering light. In contrast to macroemulsions (500 nm – 50 μm), the smaller size (50 nm- 500 nm) of miniemulsion droplets enables the ratio of light absorption vs. scattering to be optimized in order to work under the best conditions as far as light penetration is concerned. Another key feature is the possibility to encapsulate a range of commercial radical photoinitiators, which are frequently water-insoluble.

The concept of photopolymerization in miniemulsion, or in aqueous dispersed media in general, suggests tremendous potentialities and innovative applications. A photochemical initiation promotes unique capabilities related to spatial and temporal control of the polymerization, as well as the implementation of continuous processes. Ambient temperature reaction conditions reduce the energy consumption, the risks of colloidal destabilization, and are safer for highly exothermic polymerization reactions. Very few studies have been published on photopolymerizations in miniemulsion systems so far [1-3].

In this paper, we underline the relation between the optical properties and the kinetics for free-initiator system and with photoinitiators through a study in spectrophotometric cells. This step is crucial to understand the relationship between droplet size, optical properties and kinetics and then identify optimal conditions for the implementation in reactors. The miniemulsion photopolymerization reaction is then investigated in different type of reactors. In this study, we present the annular batch photo-reactor and a 1 mm depth channel microreactor (LTF-V) provided by Little Thing Factory.