Seeded emulsion polymerization of styrene

The kinetics of the seeded emulsion polymerization of styrene, with the swollen radii of the seed ranging from 44 to 79 nm, have been measured dilatometrically. The conversion against time curves displayed an increase in the instantaneous rate of polymerization, followed by an apparent steady state domain. These measurements permitted for the first time the direct evaluation of the kinetic parameters that govern both the entry of free radicals into the latex particles and the first-order loss of free radicals from the particles. The latter rate coefficient was found to vary with the inverse square of the swollen particle radius, as expected for the diffusion controlled exit of free radicals from the particles. The magnitudes of the observed entry rate coefficients, when coupled with their dependence upon the initiator concentration, suggest that free radical capture by the seed particles was relatively inefficient. The capture efficiency increased significantly, however, with decreasing initiator concentration if the number of seed latex particles was held constant. The results obtained show clearly that the average number of free radicals per particle can be < 1/2 for styrene emulsion polymerizations under suitable conditions (e.g., low initiator concentrations and/or small particle sizes). Thus styrene can follow Smith-Ewart case 1 kinetics. The validity of the interpretation of the data is strongly supported by the excellent agreement with literature values obtained for various kinetic parameters. A background initiation process that causes the emulsion polymerization of styrene to proceed in the absence of added initiator was detected. This appears to be the emulsion polymerization analogue of the thermally induced bulk polymerization of styrene

Retardation by oxygen in emulsion polymerisation

The extended retardation of the free-radical seeded emulsion polymerisation of vinyl acetate by oxygen has been modelled and fitted to experimental data. It is argued that the unusually long retardation observed in this system is due to the high entry efficiency of the aqueous-phase oligomeric radicals which allows latex particles to compete with dissolved oxygen for these initiating radicals. This, in the case of VAc, is due to the high value of the product of the propagation rate coefficient and the water solubility. As oxygen is consumed the competition increasingly favours entry of initiating radicals into particles and the polymerisation rate gradually increases. The model also qualitatively predicts the much shorter retardation behaviour of styrene and MMA which is due to the lower values of the product of the propagation rate coefficient and the water solubility for these monomers

General solution to the Smith-Ewart equation for emulsion polymerization kinetics

Using matrix methods, complete time-dependent solutions are derived for the Smith-Ewart equations describing polymerization in emulsion systems with a constant number of polymerizing loci, where the mechanisms of free radical entry, exit and termination all operate simultaneously. The results, which are valid for any system where the average number of free radicals per particle is < ∼ 0.6, reduce to previous, more restricted, solutions under the appropriate limits

Characterization of an ionic liquid ferrofluid electrospray emission pattern

© 2014 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. 2014 by Brandon Jackson, Lyon B. King

Water-binding and oxygen permeability in poly(vinyl alcohol) films

The measurement and interpretation of isotherms for sorption of water onto, and oxygen permeability of, polymer substrates are important for designing polymers in applications such as agricultural seed coatings. In a preliminary study of water sorption in the commonly used copolymer poly( vinyl alcohol-co-vinyl acetate), moisture sorption isotherms were measured for a series of substrates of different comonomer composition at 25degreesC. The data were fitted to the D'Arcy-Watt model, which gave physically reasonable values for parameters related to the strength of monolayer adsorption to high affinity sites on the substrate and the number of sites for secondary water adsorption per unit mass. The oxygen permeability of poly( vinyl alcohol) films was found to be a function only of the water content of the films, with no other obvious contributing factors. Marked differences were seen in the populations of 'bound' and 'unbound' water characterized by H-1 nuclear magnetic resonance (NMR) spectroscopy and moisture sorption isotherm fitting. There was no correlation of oxygen permeability with 'bound' or 'unbound' water measured by any means, a fact suggesting that there is a continuum of water environments within the materials

Synthesis of anisotropic nanoparticles by seeded emulsion polymerization

Anisotropic polystyrene nanoparticles of diameters below 0.5 mu m were prepared by coating the surface of crosslinked polystyrene latex particles with a thin hydrophilic polymer layer prior to swelling the particles with styrene and then initiating second-stage free-radical polymerization. Conditions were found so that all particles had uniform asymmetry. The effect of surface chemistry on the development of particle anisotropy during seeded emulsion polymerization of sub-0.5 mu m diameter particles was studied. The extent and uniformity of the anisotropy of the final particles depended strongly on the presence of the hydrophilic surface coating. Systematic variation of the degree of hydrophilicity of the surface coating provided qualitative insight into the mechanism responsible for anisotropy. Conditions were chosen so that the surface free energy favored the extrusion of a hydrophobic bulge of monomer on the hydrophilic surface of the particle during the swelling phase: the presence of a hydrophilic layer on the particle surface causes this asymmetry to be favored above uniform wetting of the particle surface by the monomer. Kinetic effects, arising from the finite time required for the seed to swell with the monomer, also play a role

Self-Assembling Array of magnetoelectrostatic jets from the surface of a superparamagnetic ionic liquid

© 2014 American Chemical Society. Electrospray is a versatile technology used, for example, to ionize biomolecules for mass spectrometry, create nanofibers and nanowires, and propel spacecraft in orbit. Traditionally, electrospray is achieved via microfabricated capillary needle electrodes that are used to create the fluid jets. Here we report on multiple parallel jetting instabilities realized through the application of simultaneous electric and magnetic fields to the surface of a superparamagnetic electrically conducting ionic liquid with no needle electrodes. The ionic liquid ferrofluid is synthesized by suspending magnetic nanoparticles in a room-temperature molten salt carrier liquid. Two ILFFs are reported: one based on ethylammonium nitrate (EAN) and the other based on EMIM-NTf2. The ILFFs display an electrical conductivity of 0.63 S/m and a relative magnetic permeability as high as 10. When coincident electric and magnetic fields are applied to these liquids, the result is a self-assembling array of emitters that are composed entirely of the colloidal fluid. An analysis of the magnetic surface stress induced on the ILFF shows that the electric field required for transition to spray can be reduced by as much as 4.5 × 107 V/m compared to purely electrostatic spray. Ferrofluid mode studies in nonuniform magnetic fields show that it is feasible to realize arrays with up to 16 emitters/mm2