In Situ Bimodal or Broad Particle Size Distribution Using Miniemulsion Polymerization Ashwini Sood Department of Chemical Engineering, Harcourt Butler Technological Institute, Kanpur 208002, India Received 27 March 2008; accepted 9 December 2008 DOI 10.1002/app.30169 Published online 28 May 2009 in Wiley InterScience (www.interscience.wiley.com). ABSTRACT: Calculations for maximum volume fraction (u m ) for a monomodal and a bimodal dispersion are given. These are extended to express the volume fraction of dis- persed phase (u < u m ) for a bimodal distribution. By substi- tuting the volume fraction, so obtained, various semiempirical laws relating relative viscosity to the volume fraction of the dispersed phase for monomodal dispersions can be extended to bimodal dispersions also. It was mathe- matically shown that the viscosity of a bimodal dispersion shows a minimum for a particular size ratio of small to large particles for a given relative number concentrations of small to large particles and the interspacing between the small and the large particles. Also, it was shown that an increase in the relative number concentrations of small to large par- ticles, keeping the size ratio of small to large particles and the interspacing between the small and the large particles constant, always increases viscosity. These findings also have practical significance because they can be used to obtain high solid content dispersions with minimum viscos- ity. Candidate recipe and operating variables that can be varied to obtain either bimodal or very broad distributions through miniemulsion polymerization are finally identified. V V C 2009 Wiley Periodicals, Inc. J Appl Polym Sci 114: 49–61, 2009 Key words: calculations; dispersions; lattice models; particle size distribution; viscosity INTRODUCTION Emulsions are dispersed systems with liquid mono- mer droplets (dispersed phase) in a liquid (continu- ous phase). Miniemulsion involves creating a stable emulsion of very small monomer droplets in the size range of 50–500 nm. The droplet is stabilized against Ostwald ripening (or molecular diffusion) and against coalescence by collisions. In creating a minie- mulsion, diffusional stabilization is achieved by add- ing a small quantity of a highly monomer-soluble and water-insoluble agent, which is called a hydro- phobe or costabilizer (cosurfactant in early litera- ture). Stabilization against coalescence is provided by adding an appropriate surfactant. In miniemul- sion polymerization, the polymerization process is initiated in small, stabilized nano-droplets. In 1973, Ugelstad et al. 1 were the first to publish results in which monomer droplets with sizes of less than 700 nm were nucleated, leading to polystyrene polymer particles of similar sizes and having a broad particle size distribution. Creation of miniemulsion droplets was obtained by stirring; sodium dodecyl sulfate (SDS) was used as a surfactant and cetyl alcohol (CA) was used as a costabilizer. Characteristics of miniemulsions can be summarized as follows: (1) The creation of a miniemulsion requires high me- chanical agitation or intense shear to reach a steady state given by rate equilibrium of droplet fission and fusion. (2) The osmotic stability of miniemulsion droplets results from an osmotic pressure in the droplets, which balances the Laplace pressure and thus prevents monomer diffusion. The osmotic pres- sure results from the addition of the hydrophobe or costabilizer, which has extremely low water solubil- ity. Miniemulsions undergo structural changes to es- tablish a situation of zero effective pressure. (3) The colloidal stability of miniemulsion droplets is achieved by adding a surfactant. The surface cover- age of the miniemulsion droplets by surfactant mole- cules is not complete. The amount of surfactant required to form a polymerizable miniemulsion is small, usually between 0.5% and 25% with respect to the monomer phase. The miniemulsion polymeriza- tion process has the potential of revolutionizing the production of latexes with unusual performance characteristics. The various aspects and advantages of this polymerization process have been docu- mented in the review papers of the leading research- ers in this area. 2–6 In this work, the potential of miniemulsion poly- merization as a means to produce in situ bimodal or very broad particle size distribution is explored. This is based on our previous works. 7–10 In the first of these works, 7 we had found that the experimen- tally reported instability by Miller et al. 11,12 is due to a broad initial droplet size distribution in which, Journal of Applied Polymer Science, Vol. 114, 49–61 (2009) V V C 2009 Wiley Periodicals, Inc. Correspondence to: A. Sood (sood.ashwini@rediffmail. com).