Polystyrene 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. 

Figure 16.24 shows the schematic representation of dispersed clay particles in a polymer matrix. Conventionally dispersed clay has aggregated layers in face-to-face form. Intercalated clay composites have one or more layers of polymer inserted into the clay host gallery. Exfoliated polymer/clay nanocomposites have low clay content (lower than intercalated clay composites which have clay content -50%). It was found that 1 wt% exfoliated clay such as hectorite, montmorillonite, or fluorohectorite increases the tensile modulus of epoxy resin by 50-65%. Montmorillonite was used in a two stage process of nanocomposite formation. In the first step, montmorillonite was intercalated with vinyl monomer and then used in the second step to insert polystyrene by in situ polymerization. 

Wang ° used two different organic modifications of the montmorillonite, one contains a styryl monomer on the ammonium ion while the other contains no double bond. A double bond that may be involved in the polymerization reaction is present in the cation of the clay. Polystyrene-clay nanocomposite has been prepared by bulk, solution, suspension, and emulsion polymerization as well as by melt blending. The organic modification as well as the mode of preparation may determine whether the composite is either exfoliated or intercalated. Exfoliation is more likely to occur if the ammonium ion contains a double bond... 

Layered clay nano composites have been prepared by melt intercalation for a variety of polymers, including polystyrene [221], nylon-6 [222], ethylene-vinyl acetate copolymers [223], polypropylene [224], polyimide [225], poly(styrene-fo-butadiene) [226], and PEO [227],... 

Hwang et al. [113] synthesized via in situ polymerization high-impact polystyrene (HlPS)/organically modifled montmorillonite (organoclay) nanocomposites. X-ray diffraction and TEM experiments revealed that intercalation of polymer chains into silicate layers was achieved, and the addition of nanoclay led to an increase in the size of the robber domain in the composites. In comparison with neat HIPS, they found that the HIPS/organoclay nanocomposites exhibited improved thermal stabiHly as well as an increase in both the complex viscosity and storage modulus, and they may have been influenced by a competition between the incorporation of clay and the decrease in the molecular weight of the polymer matrix. 

The preliminary study on the flammabihty of HUP-MMT compositions was performed by the determination of limiting oxygen indexes (LOI). The flame retardance was affected by styrene content in the resin and the type of clay used (Table 9.3). An additional amount of styrene has been added to some compositions in aim to improve the casting properties on the other hand the excess styrene makes the cured compositions more flammable due to the increased content of polystyrene (LOI = 18) and decreased of HUP (LOI = 24). The cured compositions containing MMT-Na as well as organoclays with DMDTA and ADA, showed... 

In immobilization, various carriers/supports have been used with enzymes. Porous glass and other derivatives (Cao et al., 1992 Marlot et al., 1985 Rucka and Turkiewicz 1990), activated carbon (Kandasamy et al, 2010), cellulose and other derivatives (Hwang et al., 2004), clay (de Oliveira et al., 2000 Lee and Akoh, 1998 Scherer et al., 2011), alumina (Bagi et al., 1997 Padmini et al., 1993), nylon (Pahujani et al., 2008), polyethylene and its derivatives (Watanabe et al., 1994), and polystyrene (Miletic et al., 2010) are the most common supports that have been used in lipase immobilization. The choice of a suitable support depends on its properties, such as its chemical composition, particle size, and surface area. These are essential for various potential applieations (Villeneuve et al., 2000). Generally, porous supports are better than nonporous ones due to their larger surface area. However, they must have a structure that allows the lipase to bind and access the substrate with a minimum internal diffusion limitation. Further details on the effect of diffusion restrictions on reaction efficiency are discussed in Chapter 4. 

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