Absorption and Scattering in Concentrated Monomer Miniemulsions: Static and Dynamic Investigations (original) (raw)
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Photochem. Photobiol. Sci., 2015
Oil-in-water miniemulsions containing a mixture of monomers as the dispersed organic phase have been shown recently to be promising media for the development of photoinitiated polymerization processes. Albeit a crucial factor for a successful application, the efficiency of light absorption by the photoinitiator in these highly scattering systems is difficult to evaluate. In this work, a well-characterized water insoluble chemical actinometer (DFIS) replaced the oil-soluble photoinitiator, and was used as a probe and a model for UV light absorption in miniemulsions of variable droplet sizes and organic phase compositions (i.e. at different levels of scattered light). In the first step, the photon flux absorbed by the actinometer was determined in model miniemulsions based on an inert solvent (ethyl acetate), at a low oil phase content (3.0-6.0 wt%). For these low to moderately scattering systems, the photon flux absorbed by the actinometer in the miniemulsions was comparable to that in a homogeneous solution of ethyl acetate. In the second step, the absorbed photon flux was investigated in photopolymerizable miniemulsions (a mixture of acrylate monomers as oil phase). Surprisingly, in spite of much higher scattering coefficients than those found for ethyl acetate based miniemulsions of otherwise the same composition, the photon flux absorbed by the actinometer in photopolymerizable miniemulsions showed only a small decreasing trend. Such a result may be considered favorable for the further development of applications of photopolymerizations in miniemulsions.
Photopolymerizable Monomer Miniemulsions: Why Does Droplet Size Matter?
Macromolecular Chemistry and Physics, 2013
In monomer miniemulsions, droplet size is a key parameter impacting the optical properties, and consequently their photopolymerizability under UV light. Three simple spectrophotometric methodologies are developed, based on an integrating sphere, to evaluate precisely the effect of droplet size on radiation absorption and scattering, which both contribute to light attenuation. Using a series of diluted acrylate monomer miniemulsions ranging from 40 to 300 nm, all the methods converge toward a constant absorption coeffi cient, comparable with that in solution. In contrast, a droplet-size decrease causes a signifi cant and progressive reduction of the scattering coeffi cient, with a positive effect on light penetration.
Radical Photopolymerization in Miniemulsions. Fundamental Investigations and Technical Development
Macromolecules, 2011
A photochemical means to convert acrylate monomer emulsions into polymer nanolatexes has been explored using radical miniemulsion photopolymerization. Our aim is to offer a complete overview through a stepwise mechanistic investigation, addressing first the key aspect of the electronic excitation of the photoinitiator in monomer miniemulsions and ending with the implementation of a photochemical reactor. In a first step, the photon flux absorbed by different photochemical initiators was investigated as a function of the miniemulsion droplet size. A series of miniemulsion polymerizations were also performed and the effects of the primary experimental parameters on the reactions kinetics, molar masses, and colloidal properties were assessed. As expected, droplet size, incident photon flux, and the nature of the photochemical initiator were found to be the primary reaction parameters. Finally, miniemulsion polymerizations were performed in an annular photochemical reactor in batch and semibatch processes.
Macromolecular Chemistry and Physics, 2011
ABSTRACT Two novel photoinitiator-free approaches to photopolymerize acrylic monomers with a conventional Hg lamp starting from an acrylates monomer miniemulsion are investigated. In one system the acrylate nanodroplet reaction is self-initiated and in the other the use of a photoactive di-Ph ether surfactant yields Ph and phenoxyl initiating radicals upon UV irradn. Photopolymn. kinetics are monitored in situ by real-time Fourier transform near IR spectroscopy (RT-FTNIR) and the colloidal properties are systematically investigated by dynamic light scattering (DLS). The up-scaling of these PI-free miniemulsion photopolymns. is carried out in an annular photoreactor. [on SciFinder(R)]
Monomer compartmentalisation in miniemulsion polymerisation studied by infrared spectroscopy
Comptes Rendus Chimie, 2003
Miniemulsion polymerisations of styrene and butyl methacrylate were carried out in order to study mass transfer during the reaction. An in-line Attenuated Total Reflectance-Fourier Transform Infrared (ATR-FTIR) probe was used to follow the reactions, and the collected spectra were used to identify the different species that appeared and disappeared during polymerisation. The results show that the droplets were completely compartmentalized during the reaction, and that even when blends of droplets with different composition were polymerised together, no detectable levels of copolymer were formed. To cite this article: K.
Elucidation of the miniemulsion stabilization mechanism and polymerization kinetics
Journal of Applied Polymer Science, 2003
Styrene/hexadecane miniemulsions were polymerized at 50°C using a redox initiator. The miniemulsions and their corresponding latexes were characterized in terms of size, polymerization rate, and surface properties. The resulting data were analyzed to elucidate the miniemulsion stabilization and polymerization mechanisms. It was found that the free surfactant concentration exceeded the critical micelle concentration when large amounts of surfactant (60 mM sodium lauryl sulfate) were used, resulting in simultaneous micellar and droplet nucleation. Most surfactant was on the surface of the droplets (85%) or particles (95%). The fractional surface coverage was proportional to the surfactant concentration to the 0.55 power. Using a particle diameter equation, the number of particles was calculated to be proportional to the surfactant concentration to the 1.35 power. Through direct particle size measurements, a power of 1.38 was confirmed. The rate of polymerization was determined by reaction calorimetry to be proportional to the number of particles to the 0.59 power, in contrast to classical Smith-Ewart kinetics for conventional emulsions (1.0 power). The average number of radicals per particle was estimated from the rate and number data, and varied with the particle diameter to the 0.97 power. The observed kinetic dependencies were validated through an extension of Smith-Ewart theory.
Langmuir, 2008
Small-angle and ultrasmall-angle neutron scattering (SANS/USANS) measurements were used to determine the structural changes induced by photopolymerization of AOT/D 2 O/(dodecyl acrylate) inverse microemulsion systems. Scattering profiles were collected for the initial microemulsions and the films resulting from photopolymerization of the oil phase. The SANS data for the microemulsions were modeled as spherical, core-shell droplets. Upon polymerization, the clear mircoemulsions formed opaque films. From the SANS/USANS data of the films, it was apparent that this morphology was not preserved upon polymerization; however, it was clearly observed that the formulation of the microemulsion had a large impact on the structure within the films. The Guinier region in the USANS data (2.5 × 10 -5 Å -1 e Q e 5.3 × 10 -3 Å -1 ) from the films indicates that very large structures are formed. Simultaneously, a well-defined peak (0.15 Å -1 e Q e 0.25 Å -1 ) in the SANS data indicates that there are also much smaller structures formed. It is proposed that the low-Q scattering arises from aggregation of the nanometer-size water droplets in the microemulsion to form droplets large enough to scatter visible light, while the peak in the high-Q region results from bilayered structures formed by the surfactant.
Macromolecules, 2001
We investigate the polymerization kinetics of microemulsions prepared with the cationic surfactant dodecyltrimethylammonium bromide and the hydrophobic monomers n-butyl methacrylate, tert-butyl methacrylate, n-hexyl methacrylate, and styrene. Our previous model for microemulsion polymerization kinetics cannot account for the kinetics of these systems. Using the results of smallangle neutron scattering monomer partitioning studies and an extended kinetic model to analyze the data, the failure of the original kinetic model is shown to be due to a combination of nonlinear monomer partitioning, nonnegligible bimolecular termination, and, in some cases, diffusion limitations to propagation.
Journal of Applied Polymer Science, 2009
Emulsifier-free emulsion polymerization of styrene was performed in the presence of small amount of methacrylic and itaconic acids as carboxylic acid monomers and potassium persulfate as an initiator at 70 C to prepare monodisperse polymer particles. Diameter of monomer swollen polymer particles (d pswol ) was measured by dynamic light scattering for samples taken from the reaction mixture during the Intervals II and III of the emulsion polymerization. Graphically treatment of d pswol versus conversion data allowed us for the first time to directly determine the critical monomer conversion (x c ), from which constant monomer concentration in the polymer particles (C MP ) dur-ing the Interval II was then calculated. x c and C MP were obtained to be 0.379 and 5.68, respectively. C MP value is in good agreement with that obtained by centrifugation method and those reported in the literature for the similar system. Attempts were also made to evaluate the average number of growing chain per particle (n) during the Interval II of emulsion polymerization of styrene.
Colloid and Polymer Science, 2015
This paper presents the UV synthesis in miniemulsion of nanoparticles from hexyl acrylate monomers using radical photoinitiator type I benzoin methyl ether. The optimization of such a miniemulsion composition is studied to produce monodisperse nanoparticles with a mean size d p inferior to 200 nm after photopolymerization of the droplets. The correlation between the size distributions of the droplets and the corresponding nanoparticles was verified. Therefore, a composite design was built as regards to the amount of the surfactant (sodium dodecyl sulfate), the co-stabilizer (pentadecane), and the monomer (hexyl acrylate) in order to model the mean size and the polydispersity of the miniemulsion droplets. A range of monodisperse droplets size between 150 and 175 nm was achieved. After UV irradiation, high monomer conversion yield (>90 %) was reached leading to nanoparticles with mean size ranging from 100 to 165 nm. These hexyl acrylate nanoparticles have been obtained after 10 min of irradiation, offering new opportunities for nanoparticles synthesis in enclosed and/or plastic vessels.