Latex dispersions structure

The study of materials by the use of any type of microscope. The structure of latex, dispersion of compounding ingredients in elastomers and identification of blooms are typical uses in rubber technology. 

Shear thinning refers to the observation of a decrease in dispersion viscosity as the applied shear rate is increased. It is because of the breakdown of structure in dispersions as the shear rate is increased. For hard sphere latex dispersions, where the interaction potential is zero except at contact, there is found to be a shear thinning behavior for volume fractions above 50%.P ... 

Various vinyl monomers such as vinyl chloride were added to a natural rubber latex dispersion, and polymerized to make a coreshell structure. 

The creation of 2D crystals of both micron sized and nanometre sized particles remains a somewhat empirical process due to the ill-defined role of the substrate or surface on which nucleation takes place. Perrin first observed diffusion and ordering of micron sized gamboge 2D crystals in 1909 under an optical microscope [32]. Several techniques have been proposed for the formation of 2D arrays at either solid-liquid surfaces or at the air-water interface. Pieranski [33], Murray and van Winkle [34] and later Micheletto et al. [14] have simply evaporated latex dispersions. Dimitrov and coworkers used a dip-coating procedure, which can produce continuous 2D arrays [35,36]. The method involves the adsorption of particles from the bulk solution at the tricontact phase line. Evaporation of the thin water film leads to an attractive surface capillary force which aids condensation into an ordered structure. By withdrawing the film at the same rate as deposition is occurring, a continuous film of monolayered particles is created. Since the rate of deposition is measured with a CCD camera, it is not possible to use nanometer sized particles with this method, unless a nonoptical monitor for the deposition process can be found. 

AFM images of smaller PBMA latex (100 nm diameter) containing 2 wt% SDS exhibited a more disordered structure [7,15], Curiously, the last drop region of the film was well ordered. As these dispersions dried, the surface exhibited a hexagonal array of cells, consistent with van Tent s observation of ordering in the centre region of his latex dispersions. 

One obvious example of this is opal, the gemstone formed by adhesion of silica particles over geological time. The colors seen by diffraction of white light from the particles suggest that the structuring of the particles extends for millimetres, a milhon times further than would be expected from the range of atomic forces. The same sorts of colors can be seen in polymer latex dispersions. It is important to inquire about the origins of such structures. 

The multiple-scattering problem has so far restricted the study of structure to two types of dispersion (i) dilute latex dispersions at very low salt concentration (ku 1), where structure results from the repulsion between long-range double-layers,and (ii) polymer latex particles dispersed in an organic solvent of closely matched refractive index. 

Fig. 3-6. Comparison of the experimental interference function S(q) obtained by neutron scattering for latex dispersions by Cebida et al. [48] with theoretical lattice factors Zfq), The filled circles, triangles and open circles represent S(q) observed for a latex particle of R = 157 A at volume fractions 0.04,0.08, and 0.13, respectivdy. Curves I, 2. and 3 represent Z(q) for fee structures with [a = 830 A, g = 0.24], [a = 679 A, g = 0.21], [a = 539 A, g = 0.18], respet ely. Taken ftom [45] with the permission of the American Physical Society...

As already mentioned, latex particles in dispersions at very low ionic strength form an ordered structure. This phenomenon has been confirmed by ultra-microscopy techniques [28,59]. Figure 4-4 shows USAXS patterns of latex dispersions at various concentrations. In the profile, clear and distinct Bragg peaks were observed, reflecting the formation erf the ordered structure. Hawe, we contend that the ordered structure was formed in the interior portion of the dispersion in addition to the IcKation close to the cell wall. From the peak position, the interparticle distance 2D xp could be estimated to be 7200 A for a 6 vol.% dispersion, which is very close to the calculated value, 2Dq. The... 

For latex dispersions, the two-state structure has been confirmed by microscopy. In polyelectrolyte solutions it has been inferred from the fact that 2D xp < 2Do as described in Sect. 2. However, the size of the ordered duster was not estimated. For polyelectrolyte solutions, the intermoleculm- distmice calculated from the Bragg peak position was of the order of 100 A. Hence, it may be a reasonable estimate that the size of the polyel arolyte cluster would be of the order of several thousand A to several micrometers. Because the USAXS intensity was not sufficiently high enough for solutions of ionic polymers such as NaPSS, we determined the size of the duster by using neutron scattering [65]. In the following the SANS measurements for NaPSS-D20 solutions are described. 

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