Polydispersed latexes

If polydisperse latexes are desired in batch emulsion polymerizations above CMC, high values and low initial initiator concentrations (/q) prolong interval I enhancing the formation of negatively skewed [169] or even bimodal PSDs [127]. 

Since hydrolysis of SSC during polymerization might be responsible for colloidal instability and polydisperse latexes, its stability in water was tested as follows at 25 °C and at 60 °C (the temperature used for emulsion polymerization). D2O was added to a solution of acetone/SSC to give a two-phase mixture, and the... 

C and 40 % conversion in 90 min at room temperature. At the end of the reaction the p-styrenesulfonic acid was completely soluble. Although these conditions were not identical to those of the emulsion polymerization, they do indicate that SSC monomer would at least partly hydrolyze during polymerization at 60 C. The water soluble /r-styrenesulfonic acid could form water soluble polymer and/or cause secondary particle nucleation resulting in polydisperse latexes with varied compositions. 

None of the three styrenesulfonate monomers produced monodisperse colloidally stable latexes. SSC hydrolyzed competitively with emulsion polymerization, and the resulting p-styrenesulfonic acid caused secondary particle formation. MSS and meta-tich MVBS gave unstable and/or polydisperse latexes due to either low solubility in VBC/styrene latexes or too high reactivity during copolymerization, para-rich MVBS was incompletely miscible even with VBC and styrene monomers. Methacrylic acid is now under investigation as the source of negative charge in cross-linked polyampholyte latexes. 

Even this equivalent size distribution may be biased due to sampling efficiency as a function of particle size. Let us assume a calibration curve obtained using five monodisperse latex samples. In the measurement of a real sample of polydisperse latex particles, even though each pulse will correspond to the right size, the number of pulses in each size segment will be distorted due to non-constant sampling efficiency therefore the particle size distribution will... 

Emulsion polymeriza tion of ABS (241) gives a mbber-phase particle morphology which is mostly deterrnined by the mbbet-seed latex. Since the mbber particle size, polydispersity, and cross-linking ate estabhshed before the preparation, the main variables relate to grafting, molecular weight... 

Natural latex is polydisperse (size of individual particles may vary from 0.01 to 5 p.m). Flowever, synthetic latex has a relatively narrow particle size, and therefore the viscosity at a given rubber content is higher in synthetic rubber (polyisoprene) solutions. The average molecular weight is typically about I million g/mol, although it depends on the gel content. 

The micrographs however, revealed that the latex particle standards were not monodispersed as claimed by the suppliers. This can clearly be noted from the micrographs in Fig. U,a-e. They indicate a distinct polydispersity the micrographs of the 2T5 and 312 nm samples in fact reveal two distinct particle populations. 

M=2879, is recommended as a primary calibration standard in preference to pure liquids (too sensitive to impurities) or uniform-particle-size latexes (too sensitive to residual polydispersity). 

Since the latex is slightly polydisperse the specific surface area of the latex cannot be calculated with sufficient accuracy. We will therefore present the adsorption results per unit mass of the latex. Note, that we are in this work only concerned with the surfactant composition on the surface and not the absolute value of the amount of adsorbed surfactant per unit area. 

Figure 1.8 shows an electron micrograph of latex particles made from polystyrene cross-linked with divinylbenzene. Note that these latex particles are not the same as simple polystyrene molecules in a true solution. The particles shown in the figure display a remarkable degree of homogeneity with respect to particle size. Such a sample is said to be monodisperse (in size), in contrast to polydisperse systems, which contain a variety of particle sizes. We have a good... 

FIG. 2.1 Sedimentation field flow fractionation (SdFFF) (a) an illustration of the concentration profile and elutant velocity profile in an FFF chamber and (b) a schematic representation of an SdFFF apparatus and of the separation of particles in the flow channel. A typical fractionation obtained through SdFFF using a polydispersed suspension of polystyrene latex spheres is also shown. (Adapted from Giddings 1991.)... 

Most technological suspensions consist of very polydisperse particles. In order to simplify his experimental system Stotz employed monodispersed latex suspensions (particle diameter = 6 or 30y). In an interesting comparative experiment, he also measured the particle mobilities using the simple... 

It can be established by the following reasoning. If n = %, each particle contains at most one free radical. Growing chains in the latex particles can thus either grow or be terminated instantaneously by entrant free radicals. These mutually exclusive kinetic events immediately prescribe the Flory most probable distribution function for the growing chains (12) this is an exponential distribution function with a polydispersity index of 2.00 (13). 

The latexes investigated were the 357 nm Dow monodisperse polystyrene (LS-1010) and two polydisperse polystyrene latexes prepared in our laboratory (2) where the concentration of functional monomer, Cops II (Alcolac-ammonium salt of a short chain vinyl sulfonate), added to the recipe was 10 3 and 10 -M for and C, respectively. 

The constant value of core diameter and low polydispersity oQ in core diameter is consistent with the known uniformity of the latexes. Either the S or the CS analysis gives a core diameter of 0.37 ym and refractive index of 1.18-1.20. The core diameter is slightly smaller than the value of 0.41 ym which came from an independent analysis using a turbidimetric technique. The turbidimetric technique was an approximate method that required the assumption of refractive index and density. 

Latex particle sizes and polydispersity were measured by use of an ICI Joyce Loebl disc centrifuge (JLDC) photosedimentometer (11,12). 

Reimers and Schork [94, 95] report the use of PMMA to stabihze MM A miniemulsions enough to effect predominant droplet nucleation. Emulsions stabilized against diffusional degradation by incorporating a polymeric costabilizer were produced and polymerized. The presence of large numbers of small droplets shifted the nucleation mechanism from micellar or homogeneous nucleation, to droplet nucleation. Droplet diameters were in the miniemulsion range and reasonably narrowly distributed. On-hne conductance measurements were used to confirm predominant droplet nucleation. The observed reaction rates were dependent on the amount of polymeric costabilizer present. The latexes prepared with polymeric costabilizer had lower polydispersities (1.006) than either latexes prepared from macroemulsions (1.049) or from alkane-stabilized miniemulsions (1.037). 

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