Latex freeze drying

Particle-Size Determination Particle-size of the cleaned1 latexes were determined using transmission electron microscopy after freeze-drying the samples and counting the particles with a Quantimet image analyzer. The number average particle diameters ( n) of the homopolymer, the 85/15 VA/BA and 70/30 VA/BA latexes were found to be 0.Q57/ m, 0.062/<.m and 0.073 m, respectively. 

At the steps before the elaboration of carbon nanotube nanocom-posites, wet-STEM can be used for the characterization of nanotubes dispersed in a liquid (see Figure 3.18), and for polymer latex/ nanotubes mixing (before evaporation or freeze-drying to elaborate polymer/carbon nanotube nanocomposites). 

Latex samples used for particle size analysis on the electron microscope were diluted in emulsifier solutions containing HDQ. The MMA latexes were placed on a grid, dried, and platinum shadowed prior to photographing. The PVA latexes were diluted, freeze dried, and shadowed on a Formar support film. 

A trap-to-trap (T-T) distillation apparatus was employed to separate the latex solids from the volatile components. The distillation procedure allows freeze drying of the latexes with condensation of all volatiles ftiat are quantitatively condensed. 

In parallel, the dry latexes, obtained by the freeze drying procedure, and coated latexes were analyzed by solid-state Si NMR in order to determine the extent of condensation of silanols... 

Jong et prepared NR composites reinforced with hybrid filler consisting of defatted soy flour (DSF) and CB. Aqueous dispersions of DSF and CB were first mixed, and then blended with NR latex and sulfur dispersion, respectively. The homogenous composite mixtures were quickly freeze-dried and compression moulded to offer the NR composites. They found that the NR composites reinforced with 40% of hybrid filler (the ratio of DSF to CB was 1 1) exhibited a 90-fold improvement in the rubber plateau modulus compared with unfilled NR, showing a significant reinforcement effect by the hybrid filler. 

Latex is a colloidal dispersion of polymer in an aqueous solvent. This method is more suitable for those polymers that can be prepared via emulsion polymerization or those that have the ability to form emulsion. It consists of an aqueous dispersion/ stabilization of filler using a surfactant followed by the addition of the dispersed filler into the polymer latex. Nanocomposites can be obtained after freeze-drying the above mixture followed by melt processing. The latex method has several advantages including no requirement for organic solvent, reliability, ease of processing, and improved dispersion of the filler in the viscous polymer matrix [70]. 

Walter and Bryant [414] described a method for freeze drying latex specimens in a home made vacuum system rather than a commercially available device (as was typical of the state of the art at that time). Later, a freeze drying/image analysis method using commercially available equipment was described [413]. Important details of that method included specimen preparation, placement onto a TEM (or SEM) grid, the hardware for the experiment and the metal coating. 

Results of the experiment described are shown in Fig. 4.41. A monodisperse latex of known particle size (Fig. 4.41A) was used both as a control and for calibration of the particle size distribution measurement [419]. A film forming latex is shown following both air drying (Fig. 4.41B) and freeze drying (Fig. 4.41C). Clearly, the flat, film forming, air dried particles are three dimensional following freeze dr5ong. 

Fig. 4.41 TEM micrographs of freeze dried polystyrene latex (A) used as a control for the experiment are three dimensional and show no deformation, whereas an air dried film forming latex (B) shows barely visible, flat regions that have no shadow. The same latex as in (B) after the freeze drying experiment has shadows (C), showing that the particles are three dimensional.

Fig. 5.78 A typical film forming latex is shown in the air dried preparation (A) where discrete particles are not obvious compared to a freeze dried and shadowed preparation (B).

The latex solids were determined gravimetrically, the average particle sizes by electron microscopy, and the number average molecular weights by osmometry of tetrahydrofuran solutions of freeze-dried, reprecipitated polymer some osmometry results were checked by gel permeation chromatography. 

In the case of emulsions, latexes, some adhesives and wet membranes, the specimen of interest is wet, generally with water and must be dried prior to electron microscopy. The deleterious effects of air drying result from the stress of surface tension forces. The methods used by biologists [334] to avoid this problem are (1) the replacement of water with an organic solvent of lower surface tension, (2) freeze drying or (3) critical point drying. Some latexes can be fixed in their... 

TEM has proven to be the most effective technique for characterization of the particle size distribution in emulsions. A dilute solution cast on a carbon film and metal shadowed shows an agglomerate of latex particles in a commonly encountered drying pattern where the shadowing method shows the particles are flat and obviously not separate or discrete (Fig. 5.66A). The area in Fig. 5.66B would also be difficult to use for particle size measurement whereas in Fig. 5.66C the emulsion particles are well separated and the shadows show they are three dimensional in shape due to freeze drying. 

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