Transmission electron latexes

Paine et al. [99] tried different stabilizers [i.e., hydroxy propylcellulose, poly(N-vinylpyrollidone), and poly(acrylic acid)] in the dispersion polymerization of styrene initiated with AIBN in the ethanol medium. The direct observation of the stained thin sections of the particles by transmission electron microscopy showed the existence of stabilizer layer in 10-20 nm thickness on the surface of the polystyrene particles. When the polystyrene latexes were dissolved in dioxane and precipitated with methanol, new latex particles with a similar surface stabilizer morphology were obtained. These results supported the grafting mechanism of stabilization during dispersion polymerization of styrene in polar solvents. 

In 1997, a Chinese research group [78] used the colloidal solution of 70-nm-sized carboxylated latex particles as a subphase and spread mixtures of cationic and other surfactants at the air-solution interface. If the pH was sufficiently low (1.5-3.0), the electrostatic interaction between the polar headgroups of the monolayer and the surface groups of the latex particles was strong enough to attract the latex to the surface. A fairly densely packed array of particles could be obtained if a 2 1 mixture of octadecylamine and stearic acid was spread at the interface. The particle films could be transferred onto solid substrates using the LB technique. The structure was studied using transmission electron microscopy. 

The useful range of the transmission electron microscope for particle size measurement is c. 1 nm-5 p,m diameter. Owing to the complexity of calculating the degree of magnification directly, this is usually determined by calibration using characterised polystyrene latex particles or a diffraction grating. 

The diameter of latex particles was measured from their transmission electron micrographs which were obtained by use of a Hitachi electron microscope HU-12AF. The uniformity ratio of particle size (U) was calculated from eq. 2 ... 

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. 

The transmission electron microscopy results are consistent with a segregated latex particle consisting of a polystyrene rich core and a soft poly-n-butyl acrylate rich shell. 

Transmission electron microscopy (TEM) analysis (6-8) was used also to characterize the latexes in this particle growth monitoring study. The electron micrographs were taken at a magnification of 30.000X. 

Due to the high water solubility of MAA, partitioning of the MAA in the water phase was expected. After polymerization, the obtained miniemulsions (latexes) and the colloidal nanoMIPs were characterized by gravimetric analysis, dynamic light scattering (DLS), gas adsorption measurements (BET), and transmission electron microscopy (TEM) as shown in Fig. 9. 

The various latexes were characterized with respect to particle size and size distribution, surface charge and functional group density, and electrophoretic mobility behavior. As observed by transmission electron microscopy all latexes were found highly monodisperse with a uniformity ratio between 1.001 and 1.010, a property due to the short duration of the nucleation period involved in the various radical-initiated heterogeneous polymerization processes. The surface charge density was determined by a colorimetric titration method reported elsewhere [13]. 

Koehler and Provder [317] sized monodisperse PMMA latexes with a range of instruments Disc centrifugal sedimentation (DCP), sedimentation field flow fractionation (SFFF), hydrodynamic chromatography (HDC), photon correlation spectroscopy (PCS), turbidimetry and transmission electron microscopy (TEM). TEM gave the smallest sizes, DCP and SFFF were in fair agreement in the center and PCS the highest sizes. 

Fig. 10a, b. Transmission electron micrographs of cross-sections of hollow Au-Si02 spheres, prepared by calcination of 640 nm latex coated with five monolayers of Au Si02 nanoparticles (core size 15 nm, shell thickness 2 nm). Reproduced with permission from Ref. [19]. Copyright 2001, Wiley-VCH... 

There are a number of procedures for determining the particle size of the latex polymer. Transmission electron microscopy (TEM) has been used for some time. The method is necessary to confirm the sphericity of the particles. Since a microscopist selects the images for further study, and the number of slides to be taken is limited for practical reasons, the statistical randomization of the measurements is somewhat questionable. Results, by the latter technique have actually been surprisingly good. 

Figure 9.9 Transmission electron micrographs of 1/0.435 core-shell latex panicles stained with uranyl acetate and RuOa. The core is 1% crosslinked PBA. The shell is PBM synthesized (a) in the absence of chain transfer agent, and (b) in the presence of chain transfer agent. Lighter regions = PBA, darker regions = PBM. (Reprinted from ref. 67. Copyright 1995 John Wiley Sons, Inc.)...

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... 

Figure 9.6 Transmission electron micrographs of (a) PS-300/PEMA-AIBN and (b) PS-190/PEMA-AIBN composite latex particles with a 40/60 weight ratio of seed to shell. The dark regions are polystyrene stained with RuOa and the lighter regions are acrylate domains outlined with phosphotungstic acid stain. (Reprinted from ref. 36. Copyright 1991 John Wley Sons, Inc.)...

Figure 14.9 Freeze-fracture transmission electron micrograph images of a latex film prepared at 36°C from a surfrtctant-free PBMA latex (

Mondragon et al. [250] used unmodified and modified natural mbber latex (uNRL and mNRL) to prepare thermoplastic starch/natural rubber/montmorillonite type clay (TPS/NR/Na+-MMT) nanocomposites by twin-screw extrusion. Transmission electron microscopy showed that clay nanoparticles were preferentially intercalated into the mbber phase. Elastic modulus and tensile strength of TPS/NR blends were dramatically improved as a result of mbber modification. Properties of blends were almost unaffected by the dispersion of the clay except for the TPS/ mNR blend loading 2 % MMT. This was attributed to the exfoliation of the MMT. 

Zhang et al. [63] prepared styrene-butadiene nanocomposites by dispersing an aqueous dispersion of montmoril-lonite and latex and flocculating the dispersion with acid. The performance of the rubber nanocomposites were compared with clay, carbon black, and silica rubber composites prepared by standard compotmding methods. The montmoriUonite loadings for the rubber nanocomposite were up to 60 phr. The morphology of the rubber nanocomposites by transmission electron microscopy appears to indicate intercalated structures. The mechanical properties of the rubber nanocomposites were superior to all of the other additives up to about 30 phr. However, rebound resistance was inferior to all of the additives except sUica. The state of cure was not evaluated. 

Mondragon et al ° reported that unmodified and modified NR latex were used to prepare thermoplastic starch/NR/MMT nanoeomposites by twin-screw extrusion. After drying, the nanoeomposites were injection moulded to produce test specimens. SEM of fractured samples revealed that chemical modification of NR latex enhanced the interfacial adhesion between NR and thermoplastic starch (TPS), and improved their dispersion. X-ray diffraction (XRD) showed that the nanoeomposites exhibited partially intercalated/exfoKated structures. Surprisingly, transmission electron microscopy (TEM) showed that clay nanoparticles were preferentially intercalated into the rubber phase. Elastic modulus and tensile strength of TPS/NR blends were dramatically improved from 1.5 to 43 MPa and from 0.03 to 1.5 MPa, respectively, as a result of rubber modification. 

Latex with hydroxyl functionalised cores of a methyl methacrylate/butyl acrylate/2-hydroxyethyl methacrylate copolymer, and carboxyl functionalised shells of a methyl methacrylate/butyl acrylate/methacrylic acid copolymer was prepared by free radical polymerisation. The latex was crosslinked using a cycloaliphatic diepoxide added by three alternative modes with the monomers during synthesis dissolved in the solvent and added after latex preparation and emulsified separately, then added. The latex film properties, including viscoelasticity, hardness, tensile properties, and water adsorption were evaluated as functions of crosslinker addition mode. Latex morphology was studied by transmission electron and atomic force microscopy. Optimum results were achieved by introducing half the epoxide by two-step emulsion polymerisation, the balance being added to the latex either in solution or as an emulsion. 8 refs. 

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