Polystyrene latex dispersions

Rheological Studies of Aqueous Concentrated Polystyrene Latex Dispersions with Adsorbed Poly(vinyl alcohol) Layers... 

Any fundamental study of the rheology of concentrated suspensions necessitates the use of simple systems of well-defined geometry and where the surface characteristics of the particles are well established. For that purpose well-characterized polymer particles of narrow size distribution are used in aqueous or non-aqueous systems. For interpretation of the rheological results, the inter-particle pair-potential must be well-defined and theories must be available for its calculation. The simplest system to consider is that where the pair potential may be represented by a hard sphere model. This, for example, is the case for polystyrene latex dispersions in organic solvents such as benzyl alcohol or cresol, whereby electrostatic interactions are well screened (1). Concentrated dispersions in non-polar media in which the particles are stabilized by a "built-in" stabilizer layer, may also be used, since the pair-potential can be represented by a hard-sphere interaction, where the hard sphere radius is given by the particles radius plus the adsorbed layer thickness. Systems of this type have been recently studied by Croucher and coworkers. (10,11) and Strivens (12). 

In this paper we report some rheological studies of aqueous concentrated polystyrene latex dispersions, in the presence of physically adsorbed poly(vinyl alcohol). This system has been chosen in view of its relevance to many practical systems and since many of the parameters needed for interpretation of the rheological results are available (15-18). The viscoelastic properties of a 20% w/w latex dispersion were investigated as a function of polymer coverage, using creep measurements. 

Figure 6.17 Plot of the pseudo-melting curve calculated from a strain sweep. This data is for a polyvinylidene fluoride latex. Similar curves can be obtained for silica and polystyrene latex dispersions...

Particle electrophoresis studies have proved to be useful in the investigation of model systems (e.g. silver halide sols and polystyrene latex dispersions) and practical situations (e.g. clay suspensions, water purification, paper-making and detergency) where colloid stability is involved. In estimating the double-layer repulsive forces between particles, it is usually assumed that /rd is the operative potential and that tf/d and (calculated from electrophoretic mobilities) are identical. 

Investigations of the electrophoretic behaviour of monodispersed carboxylated polystyrene latex dispersions as a function of particle size and electrolyte concentration by Shaw and Ottewill191 have confirmed, at least qualitatively, the existence of tea and relaxation effects. 

Experimental data are generally not in accord with the theoretical prediction in equation (8.21) regarding particle size96,196,204. For example, Ottewill and Shaw204 found no systematic variation in d log W/d log c for a number of monodispersed carboxylated polystyrene latex dispersions with the particle radius ranging from 30 nm to 200 nm. This problem still remains unresolved. 

In the methodology developed by us [24], the incompatibility of the two polymers was exploited in a positive way. The composites were obtained using a two-step method. In the first step, hydrophilic (hydrophobic) polymer latex particles were prepared using the concentrated emulsion method. The monomer-precursor of the continuous phase of the composite or water, when that monomer was hydrophilic, was selected as the continuous phase of the emulsion. In the second step, the emulsion whose dispersed phase was polymerized was dispersed in the continuous-phase monomer of the composite or its solution in water when the monomer was hydrophilic, after a suitable initiator was introduced in the continuous phase. The submicrometer size hydrophilic (hydrophobic) latexes were thus dispersed in the hydrophobic (hydrophilic) continuous phase without the addition of a dispersant. The experimental observations indicated that the above colloidal dispersions remained stable. The stability is due to both the dispersant introduced in the first step and the presence of the films of the continuous phase of the concentrated emulsion around the latex particles. These films consist of either the monomer-precursor of the continuous phase of the composite or water when the monomer-precursor is hydrophilic. This ensured the compatibility of the particles with the continuous phase. The preparation of poly(styrenesulfonic acid) salt latexes dispersed in cross-linked polystyrene matrices as well as of polystyrene latexes dispersed in crosslinked polyacrylamide matrices is described below. The two-step method is compared to the single-step ones based on concentrated emulsions or microemulsions. 

Recent studies in our laboratory (3) using polystyrene latex dispersions stabilised by the above "comb" dispersing agent have shown that polymers such as poly(ethylene oxide) induce wealc flocculation above a certain critical concentration of free polymer which was dependent on the molecular weight of the chain. 

Interpretation of rheological results The trends in the variation of Xg with are similar to those obtained recently (3) using a model polystyrene latex dispersion. The 1 values obtained in the present system are also close to tSose obtained with the model dispersion(0.017, 0.008 and 0.005 for PEO 20,000, 35,000 and 90,000 respectively). As mentioned before the sharp increase in x above + indicates that at the onset of flocculation the dispersions show marked viscoelasticity. The flocculation obtained at corresponds to the onset of the "semidilute" region, p, i.e., where the polymer coils in solution begin to arremge themselves in some... 

Fig. 1.21 Photograph of a polystyrene latex dispersion (16 vol%) in 10 mM NaCl at pH 7 with (as indicated in wt%) added hydroxyethyl cellulose (HEC) studied by Faers and Luckham [145]. In the lower photograph the tubes are tilted demonstrating the difference between rigid colloidal solid-liquid and fluid colloidal gas-liquid interfaces for the three-phase coexistence at 0.3 wt% HEC. Reprinted from M. A. Faers and P.F. Euckham, Langmuir, 13 2922, Copyright 1997, with permission from the American Chemical Society and the authors...

The effect of droplet size and its distribution on the adsorbed layer thickness may be inferred from a comparison of the results obtained with the o/w emulsions with those recently obtained using polystyrene latex dispersions containing grafted PEO chains of (molecular weight 2000) (49). As discussed earlier, the viscoelastic behavior of the system (which reflects the steric interaction) is determined by the ratio of the adsorbed layer thickness to the particle radius (8/R). The larger this ratio, the lower the volume fraction at which the system changes from predominantly viscous to predominantly elastic response. With relatively polydisperse systems, ( )cr shifts to higher values when compared to monodisperse systems with the same mean size. 

Snook and van Megen compared their computed phase diagram with the available experimental data on polystyrene latex dispersions. In agreement with experiment, they found that as the electrolyte concentration decreases so does the difference between the volume fractions of the coexisting phases (and the volume fraction at which the ordered phase first appears). The combined approximate methods of cell theory and perturbation theory agree satisfactorily with a complete Monte Carlo determination of coexisting volume fractions at c =0.1 mol m using the procedure of Hoover and Ree. ... 

The effect of droplet size and its distribution and the adsorbed layer thickness may be inferred from a comparison of the results obtained with the O/W emulsions with those obtained using polystyrene latex dispersions containing grafted... 

Fig. 14.15. Energy-distance curves for polystyrene latex dispersions with adsorbed PVA layers with various molecular weights.

Dekruif C.G., Vanlersel E.M.F., Vrij A., Russel W.B. Hard-sphere colloidal dispersions—Viscosity as a function of shear rate and volume fraction. J. Chem. Phys. 1985 83(9) 4717 725 Derooij R., Potanin A. A., Vandenende D., Mellema J. Steady shear viscosity of weakly aggregating polystyrene latex dispersions. J. Chem. Phys. 1993 99(11) 9213-9223 Derooij R., Vandenende D., Duits M.H.G., Mellema J. Elasticity of weakly aggregating polystyrene latex dispersions. Phys. Rev. E. 1994 49(4) 3038-3049 Derjaguin B.V., Landau L. Acta Physiocochim URSS. 1941 14 633-662... 

Rheological studies of sterically stabilised concentrated polystyrene latex dispersions under conditions of incipient flocculation... 

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