Latexes optical techniques

Bauer et al. describe the use of a noncontact probe coupled by fiber optics to an FT-Raman system to measure the percentage of dry extractibles and styrene monomer in a styrene/butadiene latex emulsion polymerization reaction using PLS models [201]. Elizalde et al. have examined the use of Raman spectroscopy to monitor the emulsion polymerization of n-butyl acrylate with methyl methacrylate under starved, or low monomer [202], and with high soUds-content [203] conditions. In both cases, models could be built to predict multiple properties, including solids content, residual monomer, and cumulative copolymer composition. Another study compared reaction calorimetry and Raman spectroscopy for monitoring n-butyl acrylate/methyl methacrylate and for vinyl acetate/butyl acrylate, under conditions of normal and instantaneous conversion [204], Both techniques performed well for normal conversion conditions and for overall conversion estimate, but Raman spectroscopy was better at estimating free monomer concentration and instantaneous conversion rate. However, the authors also point out that in certain situations, alternative techniques such as calorimetry can be cheaper, faster, and often easier to maintain accurate models for than Raman spectroscopy, hi a subsequent article, Elizalde et al. found that updating calibration models after... 

Small grains, powders, or large particle size latexes can be examined directly once dispersed on a microscopic slide and, should the optical system demand it, covered with a coverslip. Bulk samples can be prepared by a variety of techniques [2], as highlighted below. 

The mass versus particle size distribution of several polymer latices with diameters in the range of 30 nm to 1500 nm was determined in less than 20 minutes using an integrated hydrodynamic chromatograph. Distributions obtained were compared with those found by other particle sizing techniques such as electron microscopy to verify validity of the technique. The instrument employed was able to analyze latices re-producibly with different optical properties, even though some of the injected particles may have been trapped within the column. Latex properties were correlated with particle size distribution data to illustrate the benefit of this particle sizing technique. 

This effect can be used to position and manipulate single cells or large viruses in solution. For other biophysical studies, individual macromolecules can be tethered to one or more latex or plastic beads and manipulated using optical tweezers. Other techniques combine similar approaches using small magnetic beads. 

The method consists essentially of repetitive filtration of a series of dispersions through a filter bed at different constant pressures. The minimum coagulation pressure can be estimated from the number of latex particles retained in the bed (measured either gravimetrically or by an optical difference technique). This shows a sharp increase at the pressure corresponding to the onset of coagulation. 

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. 

Microscopy is a key approach which is frequently used for the characterizatitm of composite latex particles. There are a wide range of microscopes which can be used to analyze latexes, such as the optical microscope (OM), scanning electron microscope (SEM), transmission electron microscope (TEM), scanning transmission electron microscope (STEM) and the atomic force microscope (AFM). The choice of the microscope technique depends on the resolution and size range needed (i.e. nanometres to microns). The most important factor in microscopy is contrast. If the contrast is low, it becomes very difficult to distinguish between... 

The particle morphology can have important ramifications for the latex product performance. Because multi-lobed particles have a larger hydrodynamic volume than a spherical particle of equal polymer mass, such types of latexes have been used to raise the viscosity in coatings applications. Hollow particles are used in paper coatings to improve the optical properties and surface smoothness. Particles with core-shell morphologies or with domains have been developed for impact modification. In addition, various microencapsulation techniques have been employed to enclose a wide variety of materials (47, 97,239) for pharmaceutical, agricultural and cosmetic applications. 

Emulsion processes are not used for making solid grades of polystyrene. This is because these processes lead to polymer containing large quantities of soap residues which impair the electrical insulation properties and optical clarity. Emulsion polymerization does, however, find limited application in the production of polystyrene latex used in water-based surface coatings. The techniques employed are very similar to those used for other polymer latices, e.g. poly(vinyl acetate) latex (Chapter 5). 

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