Monodisperse latex micrograph

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. 

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. 1.8 Electron micrograph of cross-linked monodisperse polystyrene latex particles. The latex is a commercial product (d = 0.500 jun) sold as a calibration standard. (Photograph courtesy of R. S. Daniel and L. X. Oakford, California State Polytechnic University, Pomona, CA.)... 

Monodisperse spheres are not only uniquely easy to characterize, but also very rarely encountered. Polymerization under carefully controlled conditions allows the preparation of the polystyrene latex shown in Figure 1.8. Latexes of this sort are used as standards for the size calibration of optical and electron micrographs (also see Section 1.5a.3). However, in the majority of colloidal systems, the particles are neither spherical nor monodisperse, but it is often useful to define convenient effective linear dimensions that are representative of the sizes and shapes of the particles. There are many ways of doing this, and whether they are appropriate or not depends on the use of such dimensions in practice. There are excellent books devoted to this topic (see, for example, Allen 1990) and, therefore, we consider only a few examples here for the purpose of illustration. 

Figure 1.3 Electron micrographs of colloidal materials in which three, two, and one dimensions lie in the colloid range (bars indicate 1/am) (a) spherical particles of monodisperse polystyrene latex, (b) packed spherical particles of polystyrene latex, (c) fibres of chrysotile asbestos, (d) thin plates of kaolinite.

Fig. 1. Monodisperse polyvinyl toluene latex. Surface replica electron micrograph of a dried film (E. B. Bradford, Dow Chemical Co., Midland, Mich.), 27, 830 x magnification.

TEM micrograph of monodisperse polystyrene latex particles produced by emulsion polymerisation... 

Dried films of certain monodisperse polystyrene latexes display brilliant iridescent colors. These colors have been attributed to the diffraction of visible light by latex particle crystallites (46-48). When dried at room temperature, polystyrene latexes form opaque, white, friable, discontinuous films. Electron micrographs of surface replicas of these films show the monodisperse spheres packed in uniform hexagonal arrays without appreciable coalescence (49). If the distance between the particle centers is within a certain range, the crystallite diffracts visible light and the interference colors are observed. These color combinations are specific for the latex particle diameter (Table VII). 

Figure 9.2 Scanning electron micrographs (SEM) of various particles, (a) Polystyrene latex (( ) 50 OOOx magnification) and AgCI ((2) 15 OOOx magnification) particles are seen. Both size distribution and aggregation (maybe due to crosslinking) of polystyrene (PS) particles can be distinguished. Notice that the PS dispersion is monodisperse. Shaw (1992). Reproduced with permission from Elsevier, (b) Two different types of particles that represent extreme variations from spherical particles are seen (( ) shows tobacco virus particles and (2) shows clay particles). The clay (sodium kaolin-Ite) particles have a mean diameter of 0.2 pm. Hiementz and Rajagopalan (1997). Reproduced with permission from Taylor Francis...

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