No abstract

Polystyrene/silica (PS/SiO(2)) and poly(styrene-co-methyl methacrylate)/SiO(2) composite latex particles were prepared by surfactant-free emulsion polymerization in the presence of a poly(ethylene glycol) monomethylether methacrylate (PEGMA) macromonomer. The resulting composite particles were stabilized by the negatively charged silica particles that adhered to the surface of the latex particles. Different process parameters were investigated in order to optimize the latex stability and maximize the reaction rate. Mixing in such a surfactant-free process is of major importance and is mainly determined by the type of impeller used during the emulsification. The concentrations of PEGMA and silica particles were also optimized in order to improve the interaction between the organic and inorganic phases and ensure a good latex stability. The presence of silica particles on the polymer particle surface was found to affect radical absorption and decrease therefore the reaction rate.

Core−shell nanoparticles with a polystyrene core and a hybrid copolymer shell were synthesized via emulsion polymerization of styrene and subsequent addition of γ-methacryloxypropyltrimethoxysilane (MPS) to produce the shell by copolymerization reaction of MPS with the residual amount of styrene. The influences of pH, the amount of MPS, and the addition mode of MPS on the particles size, morphology, and copolymer architecture in the shell were studied using dynamic light scattering (DLS), transmission electron microscopy (TEM), gas chromatography (GC), differential scanning calorimetry (DSC), solid-state NMR, and infrared (IR) spectroscopy.

Clay-armored polymer particles were prepared by emulsion polymerization in the presence of Laponite platelets that adsorb at the surface of latex particles and act as stabilizers during the course of the polymerization. While Laponite RDS clay platelets are most often used, the choice of the type of clay still remains an open issue that is addressed in the present article. Four different grades of Laponite were investigated as stabilizers in the emulsion polymerization of styrene. First, the adsorption isotherms of the clays, on preformed polystyrene particles, were determined by ICP-AES analysis of the residual clay in the aqueous phase. Adsorption of clay depended on the type of clay at low concentrations corresponding to adsorption as a monolayer. Adsorption of clay particles as multilayers was observed for all the grades above a certain concentration under the considered ionic strength (mainly due to the initiator ionic species). The stabilization efficiency of these clays was investigated during the polymerization reaction (free of any other stabilizer). The clays did not have the same effect on stabilization, which was related to differences in their compositions and in their adsorption isotherms. The different grades led to different polymer particles sizes and therefore to different polymerization reaction rates. Laponite RDS and S482 gave similar results, ensuring the best stabilization efficiency and the fastest reaction rate; the number of particles increased as the clay concentration increased. Stabilization with Laponite XLS gave the same particles size and number as the latter two clays at low clay concentrations, but it reached an upper limit in the number of nucleated polymer particles at higher concentrations indicating a decrease of stabilization efficiency at high concentrations. Laponite JS did not ensure a sufficient stability of the polymer particles, as the polymerization results were comparable to a stabilizer-free polymerization system.

Partitioning of laponite disklike clay platelets between polymer particles and bulk aqueous phase was investigated in Pickering surfactant-free emulsion polymerization of styrene. Adsorption of laponite clay platelets plays an important role in the stabilization of this system, influencing the particle size and the number of particles, and, hence, the reaction rate. Adsorption isotherms show that, while the laponite clay platelets are almost fully exfoliated in water, they form multilayers on the surface of the polymer particles by the end of polymerization, as confirmed by transmission electron microscopy (TEM). This observation is supported by quartz crystal microbalance, conductivity, and TEM measurements, which reveal interactions between the clay and polystyrene, as a function of the ionic strength. The strong adsorption of clay platelets leaves a low residual concentration in the aqueous phase that cannot cause further nucleation of polymer particles, as demonstrated during seeded emulsion polymerization experiments in the presence of a high excess of clay. A Brunauer-Emmett-Teller (BET)-type model for laponite adsorption on polystyrene particles matches the adsorption isotherms.

No abstract

International audienceA novel in situ video probe with automated image analysis was used to develop a population balance model for a breakage-dominated liquid liquid emulsification system. Experiments were performed in a 2 L tank, agitated by an axial flow propeller. The dispersed phase (ethylene glycol distearate) concentration was varied from 0.2 to 1.0% (w/w), and agitation rates were varied from 0.2 to 0.5 W/kg, in the presence of excess surfactant. Three numerical discretization methods were compared: fixed pivot, cell average, and finite volumes. The latter was then chosen for the subsequent simulations due to its rapidity and higher precision. An investigation of the different theories for bubble/droplet breakage was done and the frequencies (or breakage rate kernels) were compared. Four models were found applicable: the models developed by Coulaloglou and Tavlarides (Coulaloglou, C. A.; Tavlarides, L. L. Chem. Eng. Sci. 1977, 32, 1289); Sathyagal and Ramkrishna (Sathyagal, A. N.; Ramkrishna, D. Chem. Eng. Sci. 1996, 51, 1377); Alopaeus, Koskinen, and Keskinen (Alopaeus, V.; Koskinen, J.; Keskinen, K. I. Chem. Eng. Sci. 1999, 54, 5887); and Baldyga and Podgorska (Baldyga, J.; Podgorska, W. Can. J. Chem. Eng. 1998, 76, 456). The one by Sathygal and Ramkrishna included the daughter size distribution. A log-normal daughter size distribution was chosen for the models by Coulaloglou and Tavlarides and Alopeus et al. Also, a normal distribution was used in the model by Baldyga and Podgorska. These models were compared with the experimental data to allow parameter identification. The model by Baldyga and Podgorska was found to give the best prediction of the shape of the distribution, its mean diameter, and standard deviation

Experimental data about both the liquid and the solid phases during seeded batch crystallizations of citric acid (CA) in water were obtained using in situ Raman spectroscopy and image acquisition. These experimental results were reported in a previous paper in this issue (Caillet, A., et al. Cryst. Growth Des. 2007, 7, 2080-2087. The present paper is now focused on the mathematical modeling of the desupersaturation process during the crystallization of monohydrate citric acid (MCA), which is the stable form at 15 °C. Both the crystallization of MCA (monohydrate) and the dissolution of the anhydrous (ACA) form were investigated. The model is based upon population balance equations (PBEs) describing the evolution of the crystal size distribution (CSD) during batch seeded operations. The estimation of the kinetic parameters of MCA nucleation and growth, and of the dissolution of ACA, was performed using nonlinear optimization techniques. For various operating conditions (modifications of the initial supersaturation and of the seed amount), the two PBE models represent satisfactorily the experimental behavior of the process. In particular, activated secondary nucleation is shown to explain particular features of the solute/solvent system that were observed previously.