Search

In this work, the microemulsion polymerization modeling problem is addressed with an integrodifferential approach. The procedure was applied to experimental data, previously presented, on the microemulsion polymerization of hexyl methacrylate (C6MA) and styrene (STY). It was found that: (i) the nucleation rate is not linear with time, as assumed before, (ii) a vitreous effect is observed even in reactions where the polymer's glass transition temperature is lower than the reaction temperature, (iii) radical entry to polymer particles and coagulation among particles are negligible, (iv) the rate decrease interval is also caused by a reduction of active sites, (v) a mechanism in which micelles provide monomer to living particles was detected, and (vi) a simple three‐parameter mechanistic model was obtained, capable of describing the studied systems.

The synthesis of nanostructured poly(N-isopropylacrylamide) (polyNIPA) hydrogels by a two-stage polymerization process is reported here. The process involves the synthesis of slightly crosslinked polyNIPA nanoparticles by inverse (w/o) microemulsion polymerization; then, these particles are dried, cleaned and dispersed in an aqueous solution of NIPA and a crosslinking agent (N,N-methylene-bis-acrylamide or NMBA) and polymerized to produce the nanostructured hydrogels. Their swelling and de-swelling kinetics, volume phase transition temperatures (TVPT) and mechanical properties at the equilibrium swollen state are investigated as a function of the weight ratio of polyNIPA particles to monomer (NIPA). The nanostructured gels exhibit larger equilibrium water uptake, faster swelling and de-swelling rates and similar TVPTthan those of the conventional ones; moreover, the elastic and Young moduli are larger than those of the conventional hydrogels at similar swelling ratios. The fast swelling and de-swelling kinetics are explained in terms of the controlled inhomogeneities introduced by the method of synthesis.The synthesis of nanostructured poly(N-isopropylacrylamide) (polyNIPA) hydrogels by a two-stage polymerization process is reported here. The process involves the synthesis of slightly crosslinked polyNIPA nanoparticles by inverse (w/o) microemulsion polymerization; then, these particles are dried, cleaned and dispersed in an aqueous solution of NIPA and a crosslinking agent (N,N-methylene-bis-acrylamide or NMBA) and polymerized to produce the nanostructured hydrogels. Their swelling and de-swelling kinetics, volume phase transition temperatures (TVPT) and mechanical properties at the equilibrium swollen state are investigated as a function of the weight ratio of polyNIPA particles to monomer (NIPA). The nanostructured gels exhibit larger equilibrium water uptake, faster swelling and de-swelling rates and similar TVPTthan those of the conventional ones; moreover, the elastic and Young moduli are larger than those of the conventional hydrogels at similar swelling ratios. The fast swelling and de-swelling kinetics are explained in terms of the controlled inhomogeneities introduced by the method of synthesis.

A method to produce poly(vinyl acetate) lattices with high-solid content (ca. 30wt.%) without losing the characteristics of the microemulsion-made particles is presented. The method is based on the multi-stage addition of monomer to a latex produced by the polymerization of 3wt.% vinyl acetate in three-component microemulsions stabilized with low concentrations (<1wt.%) of the anionic surfactant, Aerosol OT, to produce lattices with up to 30wt% solids. The results demonstrated that the poly(vinyl acetate) in the high solid-content latex has much smaller molar masses than the poly(vinyl acetate) in emulsion-made lattices with similar solid content. Also the microemulsion-made lattices contain particles two- to three-fold smaller than those obtained by emulsion polymerization.

The synthesis by a two-stage polymerization process of microstructured polyacrylamide hydrogels with large swelling capacity and improved mechanical properties is reported. First, crosslinked polyacrylamide particles of nanosize scale are made by inverse microemulsion polymerization. These particles are then dried and redispersed in an aqueous solution of acrylamide and polymerized in the presence of a crosslinking agent. The microstructured hydrogels, in contrast to transparent conventional polyacrylamide hydrogels, are translucid due to the presence of the dispersed particles. The swelling capacity of these hydrogels increases as the particle content increases and their Young and elastic moduli (at equilibrium swelling) diminish only slightly. Mechanical tests disclose that the microstructured hydrogels have larger Young moduli than conventional hydrogels with an identical degree of swelling.

A recently developed model for o/w microemulsion polymerization, tested only with styrene, is validated here by comparing its predictions with experimental data from the polymerization of three monomers with different chemical structures and water solubilities. The model reproduces well the kinetic data as well as the characteristics of the latex and polymer (particle size, average molar mass, number of polymer chains per particle, etc.) for the three monomers. The model predicts only two reaction rate intervals and that the conversion at which the maximum reaction rate occurs, depends only on the type of monomer and reaction conditions, in agreement with experimentally reported data. In the model, both micellar and homogeneous nucleation mechanisms are permitted, even for low water-solubility monomers such as styrene, because of the large amounts of surfactant typically employed in microemulsion polymerization. The role of these two mechanisms in the polymerization process is examined here.

Summary: The equilibrium swelling and the plateau elastic modulus of a family of hydrogels made by the polymerization of acrylamide with itaconic acid or some of its esters were investigated as a function of composition and crosslinking degree to find materials with satisfactory swelling and elastic properties. We show that an appropriate selection of the comonomers and the concentration of the crosslinking agent is very important to produce hydrogels with large swelling capacity and good mechanical attributes. Tailoring of mechanical properties and swelling can also be achieved by this method.

The influence of pH on the incorporation of a functional monomer (itaconic acid or methacrylic acid) on core-shell polymers of styrene-butyl acrylate and on their mechanical properties is investigated. The structured polymers were prepared by a two-stage emulsion polymerization process. It was found that the composition of the shell is strongly affected by the pH of the reacting medium. The amount of functional monomer that is incorporated in the shell increases as the pH decreases and becomes a maximum when the pH falls below the pKaof the functional monomer. The mechanical properties (ultimate elongation, ultimate strength, and toughness) improve as the amount of incorporated functional monomer increases. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 3299–3304, 1999

Summary: Polymerization of vinyl acetate (VA) in three component oil/water (o/w) microemulsions stabilized with a mixture of two cationic surfactants (DTAB/DDAB in a weight ratio of 3), was carried out at 40, 50 and 60°C using a water soluble initiator, V-50. In all cases studied, stable latexes containing particles with diameters between 70 and 110 nm were obtained. Particle size increased with conversion yielding uncommonly large particles for microemulsion polymerization, probably because particle coagulation. Multimodal molar mass distributions with average molar masses between 1.2 to 2.0×106 g/mol were obtained. Chain transfer to polymer and bimolecular termination reactions play important roles in the microemulsion polymerization of this monomer.

A spectrophotometric method for porphobilinogen deaminase assay in erythrocytes is described. This test is determinant for the definite diagnosis of acute intermittent porphyria. In the method described, d-aminolevulinic acid is used as substrate. Mercaptoethanol and zinc ions are introduced to maintain d-aminolevulinic acid dehydratase in optimal conditions and to guarantee thein vitroproduction of porphobilinogen. An incubation temperature of 45°C leads to the production of uroporphyrins, which are measured spectrophotometrically at 405 nm, giving reproducible results. The assay can be performed easily in any clinical laboratory and is valuable for detecting both patients and carriers of acute intermittent porphyria.

The synthesis of styrene-acrylonitrile (SAN) copolymers by emulsion or microemulsion polymerization using either a water-soluble (potassium persulfate or KPS) or a water-insoluble (AIBN) initiator is presented here. The surfactant used was dodecyltrimethylammonium bromide (DTAB) or sodium dodecylbenzenesulfonate (SDBS). Polymerization in DTAB microemulsions initiated with AIBN are faster and have higher conversions than those initiated with KPS, but the opposite effect is observed for emulsion polymerization. In both emulsion and microemulsion polymerizations, high molecular weights (2-4 × 106 Dalton) SAN copolymers are produced with composition richer in styrene (S/AN = 81/19 w/w) than the initial feed composition (75/25 w/w). Latex produced by microemulsion polymerization contain particles two- to three-fold smaller than those prepared by emulsion polymerization.

The grafting polymerization of methyl methacrylate (MMA) and cellulose from henequen (Agave fourcroydes) is investigated as a function of the initiator concentration (cerium-and-ammonium nitrate) and the monomer/cellulose ratio. The formation of cellulose-g-PMMA is confirmed by IR spectroscopy, DSC, and TGA. Both the initiator concentration and the MMA/cellulose ratio have a strong influence over the grafting parameters and over the molecular weight of the grafted PMMA. A higher initiator concentration and a lower monomer/cellulose ratio result in a lower molecular weight of the grafted polymer. Increasing the amount and the molecular weight of the grafted PMMA increases the compatibility of the fibers with SAN and PVC, as demonstrated by a mechanical test and scanning electron microscopy. SAN and PVC composites made with grafted cellulose exhibit higher flexural and tensile moduli, respectively, than those produced with the ungrafted fibers. Both moduli increase as the amount of reinforcement increases. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 339–346, 1997

A spectrophotometric method for porphobilinogen deaminase assay in erythrocytes is described. This test is determinant for the definite diagnosis of acute intermittent porphyria. In the method described, d-aminolevulinic acid is used as substrate. Mercaptoethanol and zinc ions are introduced to maintain d-aminolevulinic acid dehydratase in optimal conditions and to guarantee thein vitro production of porphobilinogen. An incubation temperature of 45°C leads to the production of uroporphyrins, which are measured spectrophotometrically at 405 nm, giving reproducible results. The assay can be performed easily in any clinical laboratory and is valuable for detecting both patients and carriers of acute intermittent porphyria.

The polymerization of vinyl acetate (VA) in three component o/w microemulsions stabilized with the cationic surfactant, CT AB, is presented. Initiation is achieved thermally with a water soluble initiator (V-50). Stable latex containing small particles (ca. 35 nm) with molecular weights (Mw) of around 4 × 105are obtained. Analysis of the molecular weight distribution suggests that chain transfer to monomer (and notto polymer, which is the typical termination mechanism in emulsion polymerization, specially at high conversions) is the dominant mechanism of termination.

On the basis of the data processing technique associated with the integrodifferential ID modeling assessment methodology, the styreneacrylonitrile microemulsion copolymerization system is studied, with a threestate conversion, active particles and micelles and fourparameter model entrance and exit of radicals to and from particles, monomer transport from micelles to growing particles and initial micelle number, in conjunction with experimental conversion measurements. Comparing with the standard regression method, employed in previous studies, the ID approach incorporates additional information contained in the smooth conversion measurement derivative and does not require relying on a constant number of particles, as the slopeandintercept method does. The resulting model, tested at three initiator concentrations, temperatures and comonomer feed, describes the Sshaped conversion and the bellshaped active particles evolutions. This last finding is contrary to previous assumptions on linear growth with time of active particles. As expected, the entry and exit rate coefficients increase with initiator and styrene concentration and temperature. The surfactant transport from micelles decreases with initiator concentration and the initial micelles concentration value only affects the entry rate.