Emulsion polymerization clay composites

In another interesting development, Yei et al. [124] prepared POSS-polystyrene/clay nanocomposites using an emulsion polymerization technique. The emulsion polymerization for both the virgin polystyrene and the nano composite started with stirring a suspension of clay in deionized water for 4h at room temperature. A solution of surfactant ammonium salt of cetylpyridinium chloride or POSS was added and the mixture was stirred for another 4 h. Potassium hydroxide and sodium dodecyl sulphate were added into the solution and the temperature was then raised to 50 °C. Styrene monomer and potassium persulfate were later on added slowly to the flask. Polymerization was performed at 50 °C for 8 h. After cooling, 2.5% aqueous aluminium sulphate was added to the polymerized emulsion, followed by dilute hydrochloric acid, with stirring. Finally, acetone was added to break down the emulsion completely. The polymer was washed several times with methanol and distilled water and then dried overnight in a vacuum oven at 80 °C. The obtained nanocomposite was reported to be exfoliated at up to a 3 wt % content of pristine clay relative to the amount of polystyrene. 

Keywords Clays Emulsion polymerization Iron oxide Metals Organic/ inorganic composite colloids Particle morphology Pigments Quantum dots Silica Surface modification... 

Besides properties (which are usually enhanced by adding clay), another crucial factor of PCN materials elaborated through emulsion polymerization is their solid content. Although this point is less frequently addressed, most of the polymer/clay composite latexes reported in literature have solid contents below 20%. However, solid contents between 40% and 60% and sometimes higher are required for industrial applications. Using a seeded semi-batch emulsion polymerization process and a procedure otherwise very similar to that described above for Bentonite,... 

Fig. 33 Left Synthesis of ciay-armored PMMA latexes through soap-free emulsion polymerization using PDMAEMA-tethered clay as stabiUzen Right TEM image of the so-obtained PMM A/clay composite coUoid. Reproduced from [290] with permission of Wiley Periodicals...

In the second part of this article, polymer latexes surrounded by anisotropic Laponite platelets have been successfully obtained by the two routes. It was demonstrated that the clay particles play the role of a pickering stabilizer and are capable to stabilize the composite particles whose diameter depends on the amount of Laponite initially introduced into the reactor. The higher the clay concentration, the larger the composite particle number and, therefore, the higher the polymerization rate as predicted from the emulsion polymerization theory. 

Figure 4.18 Suspected morphology of polymer/MMT composite materials produced through conventional emulsion polymerization without any pretreatment of the clay particles.

The emulsion polymerization reaction was accompHshed in conventional manner using potassium persulfate as initiator and sodium dodecyl sulfate as surfactant Stable composite latexes with diameters in the range of 50 to 150 nm were successfully produced, provided that the original clay suspension was stable enough. 

D.C. Lee, L.W. Jang, Preparation and characterization of PMMA-clay hybrid composite by emulsion polymerization,... 

L, and Bourgeat-Lami, E. (2006) Polymer/laponite composite colloids through emulsion polymerization influence of the clay modification level on particle morphology. Macromolecules, 39, 9177-9184. 

There have been many other reports on direct emulsion polymerizations involving nascent clays, either using clays as stabilizing agents or with the aim of preparing polymer-clay nanocomposites. In addition, organically modified clays have also been used in direct emulsion polymerization. Improved mechanical and thermal properties, reduced vapor permeability and improved flame retardancy for the prepared polymer-clay composites were reported. 

In situ emulsion polymerization in aqueous solution is an effective and successful method for the synthesis of polymer-clay nanocomposites. The structure of the nanocomposites formed is usually characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). XRD offers a convenient method to determine the position, shape and intensity of the nanofillers. The XRD patterns shows a basal (001) reflection when the spacing between the clay layers is small. However, X-rays cannot detect the (001) reflection when the layers are exfoliated in the composite. An increase in spacing between layers is correlated with an increase in the degree of exfoliation. In contrast, TEM observation gives a direct view of the internal structure, spatial distribution of the various phases and defect structures. By combining these methods, the structure of polymer nanocomposites can be determined. 

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