Victoria L. Dimonie

Encapsulation of inorganic particles via miniemulsion polymerization. I. Dispersion of titanium dioxide particles in organic media using OLOA 370 as stabilizer

The dispersion of hydrophilic and hydrophobic titanium dioxide (TiO2) particles in organic media (styrene and cyclohexane) was studied to evaluate the effect of dispersion quality (i.e., size and stability) on the encapsulation efficiencies of subsequent miniemulsion polymerizations. Through screening studies of various block copolymers, OLOA 370 (polybutene–succinimide pentamine) was chosen as the stabilizer for detailed dispersion studies on both types of TiO2 particles. As a result of strong interactions between the amine end group of the OLOA 370 stabilizer and the hydroxyl groups on the surface of the hydrophilic TiO2 particles, a good dispersion stability and small particle size (Dv = 39–45 nm) was obtained using 1.0 wt % stabilizer and 20 min of sonification. The dispersions of the hydrophobic TiO2 particles resulted in a larger average particle size (Dv = 60 nm) and poorer stability.

Encapsulation of inorganic particles via miniemulsion polymerization. II. Preparation and characterization of styrene miniemulsion droplets containing TiO2 particles

The encapsulation of titanium dioxide (TiO2) particles via styrene miniemulsion polymerization requires two successive dispersion steps. First, the TiO2 particles must be successfully dispersed in the monomer phase. Second, this phase must be dispersed in an aqueous surfactant solution to form stable submicron droplets. The oil/water interface between the droplets and the aqueous phase can be affected not only by the surfactant used but also the components present in the oil phase. Interfacial tensions between the phases were measured. The presence of insufficient or excess stabilizer both reduced the interfacial tensions. This was attributed to migration of the partially covered hydrophilic TiO2 particles or the excess OLOA 370 stabilizer (polybutene–succinimide pentamine) to the oil/water interface. The free surfactant in the aqueous phase and the average droplet size of the miniemulsions were characterized as a function of the process variables. The presence of TiO2 particles within the droplets limited the ability of the process to reduce the droplet size although miniemulsion droplets in the size range of 150–200 nm were still obtained for subsequent polymerizations.

Encapsulation of inorganic particles via miniemulsion polymerization. III. Characterization of encapsulation

The application of the miniemulsion polymerization technique to the encapsulation of titanium dioxide (TiO2) inside polystyrene latex particles was investigated. Complete encapsulation, meaning all of the polystyrene encapsulating all of the TiO2 in the colloidal particles, was not achieved. The most successful encapsulations were only achieved when the TiO2 particles, either hydrophilic or hydrophobic, were well dispersed in the styrene monomer prior to formation and polymerization of the miniemulsions. The TiO2 dispersions in styrene were prepared by sonification of the TiO2 in the presence of an adsorbing steric stabilizer OLOA 370 (polybutene–succinimide pentamine). Miniemulsions were prepared by dispersing the monomer phase (also containing hexadecane as a costabilizer and polystyrene to enhance the nucleation of the droplets) in water using sodium lauryl sulfate as surfactant. The latexes resulting from the subsequent polymerizations were characterized in terms of the encapsulation efficiencies (via density gradient column separations) and particle size. The maximum encapsulation efficiencies (83% TiO2 and 73% polystyrene) were achieved using hydrophilic TiO2 particles stabilized with 1.0 wt % OLOA 370. As the density of the particles collected from the density gradient column increased from zone to zone, both the average particle size and number of the TiO2 particles contained in each latex particle increased with the largest particle size (209 nm) containing an estimated 22 TiO2 particles.

Multicomponent latex IPN materials: 2. Damping and mechanical behavior

The integrals of the linear loss shear modulus vs. temperature (loss area, LA) and linear tan δ vs. temperature (tan δ area, TA) were characterized for various core/shell latex particles with synthetic rubber, poly(butadiene-stat-styrene) [P (Bd/S), 90/10], and interpenetrating polymer networks (IPN) as the cores. The IPN cores were composed of P(Bd/S) (Tg ≃ − 70°C) and an acrylate based copolymer (Tg around 10°C) for potential impact and damping improvement in thermoplastics. Poly(styrene-stat-acrylonitrile) (SAN, 72/28) was the shell polymer for all these polymers. Under the same loading, for both toughening and damping controls, among the IPN core/shell, blend of separate core/shell, and multilayered core/shell polymers, the IPN core/shell polymers were the best dampers. However, the other core/shell polymers also showed higher LA values than P(Bd/S)/SAN core/shell polymer. A comparison of LA values via a group contribution analysis method was made, the effect of particle morphology and phase continuity on damping being studied. Inverted core/shell latex particles (glassy polymer SAN was synthesized first) showed much higher LA and TA values than normal core/shell ones (rubbery polymer was synthesized first). Models for maximum LA and TA behavior are proposed. The damping property was essentially controlled by the phase miscibility and morphology of the core/shell latex particles. The LA values for each peak in these multiphase materials provided some indication of the several fractional phase volumes.

Encapsulation of inorganic particles via miniemulsion polymerization

The encapsulation of TiO2 particles via miniemulsion polymerization is strongly dependent on the size and stability of the inorganic particles in the monomer medium in which they are initially dispersed. It was found from XPS and FT-IR studies that both the hydrophilic and hydrophobic TiO2 particles, which were studied, have hydroxyl groups present on their surfaces, which can strongly interact with the amine end-groups of the polymeric stabilizer, OLOA370 (polybutene-succinimide diethyl triamine). It was found from the dispersion and adsorption studies that the amount of OLOA370 retained on the TiO2 particles is strongly dependent on the area exposed by the sonification that is applied to break up the aggregates in the dispersion process. The TiO2 dispersions in styrene monomer were themselves dispersed as miniemulsion droplets and subsequently polymerized. It was concluded from the density gradient column (DGC) analysis of the latexes obtained from the encapsulation polymerizations, that the stability of the inorganic particles in the monomer, as well as their particle size, significantly influence the encapsulation efficiencies. The use of the hydrophilic titanium dioxide particles in combination with the stabilizer, OLOA370, resulted in a good dispersibility, dispersion stability, and small TiO2 particle size. This lead to better encapsulation efficiencies compared to the hydrophobic particles. The poorer results obtained with the hydrophobic TiO2 particles were attributed to their larger particle size, which resulted from the reduced adsorption of the OLOA370. Fewer hydroxyls and the presence of the trimethoxy octyl silane (TMOS) groups, which themselves are unable to provide sufficient steric stability, are proposed to explain these findings.

The role of dispersed phase morphology on toughening of epoxies

The use of structural core/shell latex particles as toughening agents provides a model system which allows independent control of several key factors that influence the fracture toughness of modified plastics. This paper focuses on varying the shell composition of poly(butadiene-co-styrene) [P(B-S)] core/poly(methyl methacrylate) (PPMA) shell particles by incorporating acrylonitrile (AN) comonomer into the PMMA shell at various AN/MMA ratios and by crosslinking of the shell at various AN/MMA ratios. It was found that the degree of particle dispersability in the epoxy matrix can be precisely controlled by the AN content in the PMMA shell and by crosslinking the PMMA in the shell. It was also found that the degree of particle dispersability plays a crucial role on the fracture toughness of the modified epoxies. A microclustered morphology provides a much higher toughness than a uniform particle distribution.

Nitroxide-mediated living free radical miniemulsion polymerization of styrene

The miniemulsion technique was applied to the controlled/living free radical polymerization of styrene utilizing TEMPO-terminated oligomers of polystyrene (TTOPS) as a macroinitiator species to initiate the polymerization at 125 °C. Miniemulsion polymerizations of styrene using different amounts of TTOPS were studied. It was found that the polymerization rate of a miniemulsion containing no TTOPS was significantly faster than those containing TTOPS. In the latter reactions, the molecular weight of the polymer increased almost linearly with conversion up to 75% conversion. The slope of the molecular weight vs conversion curve increased with decreasing TTOPS concentration as expected. Particle size distributions were measured by both TEM and CHDF and were found to be broad. Using the kinetics and particle size data, the average number of active free radicals per particle (n in classical emulsion polymerization systems) was estimated and found to be low (≤0.005).

Anionic dispersion polymerization of styrene. I. Investigation of parameters for preparation of uniform micron-size polystyrene particles with narrow molecular weight distribution

Anionic dispersion polymerization in a hexane medium has been applied to the synthesis of monodisperse polystyrene particles in the size range of 1.41–6.16 μm, and having narrow molecular weight distributions Mw/Mn of 1.02–1.28. sec-Butyllithium was used as the initiator. Polystyrene-block-polybutadiene diblock copolymer containing 23% polystyrene block, (i.e., Stereon 730A) with a molecular weight of 147,000 g/mol and a polydispersity of 1.05, was found to be a suitable steric stabilizer for the preparation of micron-size polystyrene particles with narrow size distribution. Tetrahydrofuran (THF) was used as a promoter for obtaining narrow molecular weight distributions. However, this study revealed that the addition of small amounts of THF as promoter broadened the particle size distribution. High solids content polystyrene dispersions were also prepared without using any promoter by both batch and/or multi-addition monomer processes.

Morphology, design and characterization of IPN-containing structured latex particles for damping applications

Abstract We have prepared a series of novel structured latex particles with interpenetrating polymer network (IPN) cores and glassy SAN shells. The IPN cores were composed of two polymers: polybutadiene-based and acrylic-based. The morphologies of these latex particles were determined by TEM. The glass transition temperature and mechanical behavior of the polymers were characterized by DMS. The effect of different components on the final core/shell particle morphologies and mechanical properties was studied. The mechanical behavior of core/shell particles with IPN cores was also compared with that of separate core/shell and multilayered core/shell particles. In addition, normal core/shell synthesis (rubbery part first, then the glassy part) and inverted core/shell synthesis (glassy part first, then the rubbery part) were performed to provide another access for morphology design and control. It was found that the core/shell latex particles with poly(butyl acrylate)-based copolymers are more miscible than poly(ethylhexyl methacrylate)-based copolymers. The high grafting efficiency of poly(butyl acrylate) plays an important role in governing phase miscibility. The latex particles synthesized by the inverted core/shell mode showed higher miscibility than the normal synthesized ones. The damping properties of different core/shell particles were evaluated based on the loss area (LA) from dynamic mechanical spectroscopy measurements. The IPN core/shell polymers were found to be the best dampers due to their more miscible chemistry. The highest level of damping was achieved by inverting core/shell particles with dual-phase continuity compared to normal core/shell particles.

Synthesis and characterization of a macromonomer crosslinker

A macromonomer crosslinker was synthesized by reacting an hydroxy-terminated ethylene–butylene diol copolymer with an excess of acrylic acid in toluene. 1H-, 13C-, and attached proton test (APT) NMR, Fourier transform IR, and gel permeation chromatography were used to characterize the macromonomer crosslinker. Its dilute solution behavior was also compared with the base diol. The addition of the ester moiety to the copolymer backbone did not involve any other side reactions, as evidenced by the similarities in the structures of the macromonomer crosslinker and the diol.