Absorption and Scattering in Concentrated Monomer Miniemulsions: Static and Dynamic Investigations

• Published in: Macromolecular chemistry and physics 2014-06, Vol.215 (12), p.1201-1211
• Main Authors: Lobry, Emeline, Jasinski, Florent, Penconi, Marta, Chemtob, Abraham, Ley, Christian, Croutxé-Barghorn, Céline, Oliveros, Esther, Braun, André M., Criqui, Adrien
• Format: Article
• Language: English
• Subjects: Applied sciences; Chemical Sciences; Droplets; Dynamic tests; Dynamics; Exact sciences and technology; Irradiation; Kubelka-Munk theory; Material chemistry; miniemulsions; Monomers; Nucleation; Optical properties; Photopolymerization; Physicochemistry of polymers; Polymerization; Polymers and radiations; reflectance; Scattering
• ISSN: 1022-1352; 1521-3935
• DOI: 10.1002/macp.201400072

Low‐scattering monomer miniemulsions have recently emerged as a suitable system for the development of efficient photopolymerization processes in dispersed media. Since their reactivity under UV exposure is strongly dependent on their optical properties, there is a strong interest to find simple analytical methods for studying absorption and scattering with ready‐to‐polymerize concentrated miniemulsions, while avoiding dilution. This paper focuses on three types of concentrated miniemulsions without photoinitiator (PI) and containing either a hydrophilic or hydrophobic PI. The application of the two‐flux theory of Kubelka–Munk on these multiple scattering miniemulsions enables the determination of the scattering and absorption coefficients for different initial droplet sizes. These results show that reaction kinetics are strongly correlated with scattering efficiency. Below a threshold average diameter of ca. 150 nm, any decrease of droplet size diminishes the extent of scattering significantly, thus improving light penetration and the reaction rate. Additionally, a complementary dynamic study proves that the smallest miniemulsions are subjected to a decrease of scattering during irradiation. This result opens up interesting perspectives on the elucidation of the nucleation mechanism operating in a miniemulsion photopolymerization. Studying the optical properties of concentrated miniemulsions prior to and during UV irradiation provides a wealth of information on their photopolymerization efficiency and nucleation process. For example, after a short UV exposure (3 min) causing a complete polymerization, an acrylate miniemulsion with an average droplet diameter of 40 nm causes much less scattering.