Structure of Latex Particles

We were able to reveal the structure of latex particles and films of polar polymers (acrylate latexes) using a special technique for preparing samples... 

Makino K, Yamamoto S, Fujimoto K, Kawaguchi H, Ohshima H. Surface structure of latex particles covered with temperature-sensitive hydrogel layers. Colloid Interface Sci 1994 166 251-258. 

The foregoing discussion has clearly revealed that the structure of latex particles deriving from emulsion polymerization depends strongly on the mode in which the second monomer is introduced. A question intimately connected to seeded emulsion polymerization is related to possible inhomogeneities in swollen latex particles due to the wall-repulsion effect [99,100]. Emulsion polymerization can be used to obtain high molecular weight polymers whose radii of gyration may be of the order of the radius of the latex particles. If such a particle is swollen by a solvent, the polymer chains tend to avoid the surface of the particle because of their restricted conformations near a wall. Therefore the polymer concentration near an impenetrable surface is diminished. 

Table 9.1 Free energy changes for various morphological structures of latex particles dispersed in water" ...

Table 9J. Various mraphological structures of latex particles disposed in water under hypothetical conditions (from ref. 36)...

This chapter serves as an introduction to the origin of nonuniform latex particles. First, a brief discussion of the seeded emulsion polymerization technique that has been widely used to prepare composite polymer particles with a variety of morphological structures is given. This is followed by the illustration of the effects of important factors such as initiators, monomer addition methods, polymer molecular weight, volume ratio of the second-stage monomer to the seed polymer, and polymerization temperature that affect the morphological structures of latex particles. The development of morphological structures of nonuniform latex particles will then be covered at the end of this chapter. 

If the seed latex particles can barely be swollen by the second-stage monomer and a water-soluble initiator is used, then the subsequent seeded emulsion polymerization will be localized near the particle surface layer. Thus, the postformed polymer tends to form a surface layer around the seed latex particle. An example of this kind of morphological structure of latex particles is the seeded emulsion polymerization of methyl methacrylate in the presence of a polyvinylidene chloride seed latex. On the other hand, free radical polymerization can take place inside the seed latex particles. In this manner, various morphological structures of latex particles such as the perfect core/shell, inverted core/shell, dumbbell-shaped, and occluded structures can be achieved, depending on various physical parameters and polymerization conditions. 

Cho and Lee [6] used three different initiators, potassium persulfate, 2,2 -azobisisobutyronitrile, and 4,4 -azobis(4-cyanovaleric acid) (water-soluble, but less hydrophilic than potassium persulfate) to investigate their effects on the emulsion polymerization of styrene in the presence of polymethyl methacrylate seed latex particles. Inverted core/shell latex particles were observed when 2,2 -azobisisobutyronitrile or 4,4 -azobis(4-cyanovaleric acid) was used to initiate free radical polymerization. The use of potassium persulfate resulted in various morphological structures of latex particles, which were largely determined by the initiator concentration and polymerization temperature. 

Continuous reactors with seed latex particles in the feed stream could be an interesting polymerization system for morphological studies. The broad residence time distribution of the polymerizing latex particles associated with such a reactor configuration results in a broad particle size distribution of the effluent product. By changing the particle size distribution (monodisperse or polydisperse) of seed latex particles and operation conditions (mean residence time, monomer addition method, etc.) simultaneously, one can essentially obtain a variety of morphological structures of latex particles. 

From the electron micrographs, assuming that PVAc particles in the latex are the same size, the formation model of the porous film from the latex film can be illustrated as in Fig. 3 [19]. When the latex forms a dried film over minimum film-forming temperature, it is concluded that PVA coexisted in the latex and is not excluded to the outside of the film during filming, but is kept in spaces produced by the close-packed structure of PVAc particles. 

This paper presents the physical mechanism and the structure of a comprehensive dynamic Emulsion Polymerization Model (EPM). EPM combines the theory of coagulative nucleation of homogeneously nucleated precursors with detailed species material and energy balances to calculate the time evolution of the concentration, size, and colloidal characteristics of latex particles, the monomer conversions, the copolymer composition, and molecular weight in an emulsion system. The capabilities of EPM are demonstrated by comparisons of its predictions with experimental data from the literature covering styrene and styrene/methyl methacrylate polymerizations. EPM can successfully simulate continuous and batch reactors over a wide range of initiator and added surfactant concentrations. 

Fulda and Tieke [77] studied the effect of a bidisperse-size distribution of latex particles on the structure of the resulting LB monolayer. For this purpose, a mixed colloidal solution of particles la and lb was spread at the air-water interface. Particles la had a diameter of 434 nm, particles lb of 214 nm. The monolayer was compressed, transferred onto a solid substrate, and viewed in a scanning electron microscope (SEM). In Figure 10, SEM pictures of LB layers obtained from various bidisperse mixtures are shown. 

The expulsion of liquid from a gel as in the coagulation of latex, where the structure of rubber particles contracts and expels the serum. Synergistic... 

O. Velev, K. Furusawa, and K. Nagayama Assembly of Latex Particles by Using Emulsion Droplets as Templates. . Micro structured Hollow Spheres. Langmuir 12, 2374 (1996). 

Structural Characteristics of Latex Particles of Acrylic Polymers of Various Polarity... 

Elaissari A, Pefferkorn E. Polyelectrolyte induced aggregation of latex particles influence of the structural relaxation of adsorbed macromolecules on the colloid aggregation mode. J Colloid Interface Sci 1991 141 522-533. 

When the sample is a ternary system, analysis of the scattering data is in general much more difficult than has been discussed in this and previous chapters, but the need to investigate a ternary system is encountered often. Examples of such ternary systems are a diblock copolymer in a common solvent, a suspension of latex particles having a core-shell structure, and an incompatible binary polymer blend in which one of the polymers is semicrystalline. By employing the technique of contrast matching... 

In the present article, we review recent SAXS-studies conducted on polymeric latexes. We will first give an exposition on the theory of SAKS including contrast variation based on references [56-60]. The main purpose of the theoretical exposition is a clear assessment of the structural information embodied in the SAXS-intensities. This discussion will also be helpful to delineate possible limitations of this method. It will reveal which parameters can be gained from a SAXS-analysis of latex particles and their relation to the structure of the particles. 

Since a meaningful analysis of latex particles requires measurements down to smallest q-values, a detailed description of an optimized small-angle camera together with the discussion of experimental problems of SAXS will be given. Also, we shall discuss the subsequent treatment of data and the steps necessary to extract the structural information from the SAXS-intensities. Section 4 is devoted to a discussion of the experimental results obtained in recent investigations. 

---