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

Abstract

Low-scattering monomer miniemulsions have recently emerged as a suitable system for the development of effi cient photopolymerization processes in dispersed media. Since their reactivity under UV exposure is strongly dependent on their optical properties, there is a strong interest to fi nd simple analytical methods for studying absorption and scattering with readyto-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-fl ux theory of Kubelka-Munk on these multiple scattering miniemulsions enables the determination of the scattering and absorption coeffi cients for different initial droplet sizes. These results show that reaction kinetics are strongly correlated with scattering effi ciency. Below a threshold average diameter of ca. 150 nm, any decrease of droplet size diminishes the extent of scattering signifi cantly, 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.

Flash Latex Production in a Continuous Helical Photoreactor: Releasing the Brake Pedal on Acrylate Chain Radical Polymerization

“…[ 22 ] Scattering and absorption coeffi cients of acrylate miniemulsions were also determined in diluted [ 22 ] and concentrated [ 23 ] media using an integration sphere or the Kubelka-Munk theory, respectively. In this work, we employed a simple spectroscopic set-up to assess light penetration.…”

Reversible Destabilization of UV‐Responsive Polymer Particles (Latex) using a Photoresponsive Surfactant

“…According to the Beer–Lambert law, the absorbance ( A ) at wavelength λ can be expressed by where E is the attenuation coefficient, l the optical path length. The attenuation coefficient can be separated into three components; the absorption coefficient from C4AzoTAB ( K C 4 AzoTAB ), the absorption coefficient from PSt particles (swollen with monomer in case of incomplete conversion) ( K PSt ) and the scattering coefficient from PSt particles ( S PSt ): …”

“…Previous work reported that the main factor responsible for the high attenuation of waterborne dispersion was scattering rather than absorption . It can thus be presumed that the colloidal stability despite the UV irradiation is caused by the high scattering ( S λ ) of UV light at 365 nm by PSt particles, thus reducing the efficiency of the C4AzoTAB trans to cis photoisomerization even under vigorous stirring conditions…”

“…Previous work reported that the main factor responsible for the high attenuation of waterborne dispersion was scattering rather than absorption. [29,30] It can thus be presumed that the colloidal stability despite the UV irradiation is caused by the high scattering (S λ ) of UV light at 365 nm by PSt particles, thus reducing the efficiency of the C4AzoTAB trans to cis photoisomerization even under vigorous stirring conditions. [31] The Rayleigh scattering coefficient S λ can be used to highlight the parameters influencing scattering: N is the number of particles per unit volume, V is the particle volume, λ is the irradiation wavelength, n polymer is the refractive index of the polymer at 365 nm and n water is the refractive index of water at 365 nm.…”

Dispersion Photopolymerization of Acrylated Oligomers Using a Flexible Continuous Reactor

“…Photo‐initiated polymerization of monomer droplets or particles dispersed in a liquid medium is an appealing approach to prepare dispersions of polymers by avoiding any source of heat. The principle has been documented for over two decades where, for instance, the radical polymerization of (meth)acrylates in conventional emulsion polymerization, in microemulsion and more recently in miniemulsion has been described in several papers and patents . Lately, miniemulsion photopolymerization has enjoyed particular attention due to its versatility for the preparation of polymer particles with virtually endless options in terms of composition and structure .…”