Polymer plus colloids

Organosol. A mixture of polymer and plasticizer used for niolding. Organosol comes from the words organic and solvent. The polymer, which is the organic and is typically PVC, in fine particles is rrtixed as a colloid in plasticizer plus a little solvent. It-all forms goo that can be put in a mold. With a little heat, the solvent evaporates, and the plasticizer and polymer-particle colloid form a gel, the final product. 

In this chapter we consider the depletion interaction between two flat plates and between two spherical colloidal particles for different depletants (polymers, small colloidal spheres, rods and plates). First of all we focus on the depletion interaction due to a somewhat hypothetical model depletant, the penetrable hard sphere (phs), to mimic a (ideal) polymer molecule. This model, implicitly introduced by Asakura and Oosawa [1] and considered in detail by Vrij [2], is characterized by the fact that the spheres freely overlap each other but act as hard spheres with diameter a when interacting with a wall or a colloidal particle. The thermodynamic properties of a system of hard spheres plus added penetrable hard spheres have been considered by Widom and Rowlinson [3] and provided much of the inspiration for the theory of phase behavior developed in Chap. 3. 

Adsorption behavior and the effect on colloid stability of water soluble polymers with a lower critical solution temperature(LCST) have been studied using polystyrene latices plus hydroxy propyl cellulose(HPC). Saturated adsorption(As) of HPC depended significantly on the adsorption temperature and the As obtained at the LCST was 1.5 times as large as the value at room temperature. The high As value obtained at the LCST remained for a long time at room temperature, and the dense adsorption layer formed on the latex particles showed strong protective action against salt and temperature. Furthermore, the dense adsorption layer of HPC on silica particles was very effective in the encapsulation process with polystyrene via emulsion polymerization in which the HPC-coated silica particles were used as seed. 

In this study, adsorption behavior of water soluble polymers and their effect on colloid stability have been studied using polystyrene latices plus cellulose derivatives. As the aqueous solution of hydroxy propyl cellulose(HPC) has a lower critical solution temperature(LCST), near 50 °C(6 ), an increased adsorption and strong protection can be expected by treating the latices with HPC at the LCST. 

Emulsion Adhesives. The most widely used emulsion-based adhesive is that based upon poly (vinyl acetate)—polytyinyl alcohol) copolymers formed by free-radical polymerization in an emulsion system Poly(vinyl alcohol) is typically formed by hydrolysis of the poly (vinyl acetate). The properties of the emulsion are derived from the polymer employed in the polymerization as well as from the system used to emulsify the polymer in water. The emulsion is stabilized by a combination of a surfactant plus a colloid protection system. The protective colloids are similar to those used paint (qv) to stabilize latex. For poly (vinyl acetate), the protective colloids are isolated from natural gums and cellulosic resins (carboxymethylcellulose or hydroxyethylcellulose). The hydrolized polymer may also be used. The physical properties of the poly (vinyl acetate) polymer can be modified by changing the co-monomer used in the polymerization. Any material which is free-radically active and participates in an emulsion polymerization can be employed. Plasticizers (qv), tackifiers, viscosity modifiers, solvents (added to coalesce the emulsion particles), fillers, humectants, and other materials are often added to the adhesive to meet specifications for the intended application. Because the presence of foam in the bond line could decrease performance of the adhesion joint, agents that control the amount of air entrapped in an adhesive bond must be added. Biocides are also necessary many of the materials that are used to stabilize poly (vinyl acetate) emulsions are natural products. Poly(vinyl acetate) adhesives known as "white glue" or "carpenter s glue" are available under a number of different trade names. Applications are found mostly in the area of adhesion to paper and wood (see VlNYL POLYMERS). 

One application area between 1962 and 1979 that has an impact on today s problems is the manufacture of wallboard. Wallboard was prepared by sandwiching a gypsum mixture between heavy papers. A typical core consists of gypsum, starch, potash, a pulp slurry, an asphalt or rosin-size emulsion, and gauging water (i.e., a colloidal silica plus polymer binder system). The mixture is blended together and placed between liners made on linerboard machines. 

FIGURE 11.1. There are four primary mechanisms for the stabilization of emulsions (plus combinations, of course). Some emulsions may be weakly stabilized by the presence of adsorbed ions and nonsurface-active salts (a). The presence of colloidal sols partially wetted by both phases of the emulsion may form a mechanical barrier to drop contact and coalescence (b). Many emulsions are stabilized by adsorbed polymer molecules (c). Along with polymers, adsorbed surfactant molecules represent the most common stabilization mechanism (d). 

Einarson and Berg (1993) have attempted to explain the data on flocculation kinetics of latex particles with a block copolymer adsorbed on them. The polymer was polyethylene oxide (PEO)/polypropylene oxide (PPO). PPO is water insoluble and forms the part that adsorbs on the latex PEO forms streaming tails into water. Some charge effects remain after the polymer adsorption. The total potential is DLVO plus elastic plus osmotic effects. After fitting the model to the experimental data, they were able to calculate the value of 6, which they called the adlayer thickness. Their data on the stability ratio of latex with and without the polymer and as a fimction of NaCl concentration are shown in Figure 3.23. Note that the polymer stabilizes the colloid by almost one order of magnimde in NaQ concentration. That is, polymers may be necessary to maintain stability in aqueous media containing substantial electrolyte. 

This relates the polymer activity (which determines B2) to the colloid volume fraction at the spinodal. De Hek and Vrij [56] could give a good description of the phase line of mixtures of polystyrene chains plus small volume fractions of (hard-sphere like) octadecyl silica spheres dispersed in cyclohexane [109]. 

A semi-grand canonical treatment for the phase behaviour of colloidal spheres plus non-adsorbing polymers was proposed by Lekkerkerker [141], who developed free volume theory (also called osmotic equilibrium theory ), see Chap. 3. The main difference with TPT [115] is that free volume theory (FVT) accounts for polymer partitioning between the phases and corrects for multiple overlap of depletion layers, hence avoids the assumption of pair-wise additivity which becomes inaccurate for relatively thick depletion layers. These effects are incorporated through scaled particle theory (see for instance [136] and references therein). The resulting free volume theory (FVT) phase diagrams calculated by Lekkerkerker et al. [142] revealed that for <0.3 coexisting fluid-solid phases are predicted, whereas at low colloid volume fractions a gas-hquid coexistence is found for q > 0.3, as was predicted by TPT. 

In this overview on the history of depletion in colloidal dispersions we have focused on nuxtures of colloidal spheres and non-adsorbing polymers, which have received most attention. Since the 1990s depletion phenomena have also been studied systematically in dispersions of colloidal rods [227, 228], platelets [229], rocks [230] (colloidal particles with an irregular surface) or cubes [231] plus nonadsorbing polymers or in mixtures of different colloids with large size asymmetry [232-235]. In Chap. 5 we concentrate on mixtures of colloidal large spheres plus added small spheres or added colloidal rods. Finally, in Chap. 6 we concentrate on the phase behaviour of colloidal rod plus polymer dispersions. 

In this chapter we have presented the free volume theory for hard spheres plus depletants and focused on the simplest possible case of hard spheres + penetrable hard spheres. In the next chapters we will extend the free volume theory to more realistic situations (Chap. 4 hard spheres + polymers. Chap. 5 hard spheres -I- small colloidal particles. Chap. 6 hard rods -I- polymers) and compare the results with experiments and simulations. 

In the introductory chapter we saw that many systematic depletion studies were performed on mixtures of spherical colloids plus non-adsorbing or free polymers. The reason is obvious spherical colloids are of industrial and fundamental relevance, and can be prepared in a relatively controlled way (rather monodisperse, hard-sphere like), while polymers are ubiquitous, and are efficient depletants. 

Fig. 4.3 State diagrams of colloid-polymer mixtures for q = 0.08 (top), q = 0.57 (middle) and = 1.0 (bottom). Experimental data PMMA spheres plus polystyrene polymers in cis-decalin [16, 17]. Curves free volume theory [18] with S = Rg. For high q a triple triangle (hatched) is predicted by the theory...

To summarize, theory and experiment clearly demonstrate that the types of phase equilibria encountered in unmixed colloid-polymer mixtures are rather sensitive to the size ratio q. For sufficiently large ( 0.3) a colloidal gas-liquid phase separation is encountered. For 0.4, the simple model of hard spheres plus penetrable hard spheres fails to accurately describe the phase behaviour of well-defined hard-sphere eolloid plus polymer mixtures. For large -values it is essential to improve the simple description of polymer chains as penetrable hard spheres. 

Fig. 4.6 Osmotic equilibrium between a reservoir containing polymer chains and a system of colloids plus polymers where (in this example) unmixing resulted in a colloidal gas in equilibrium with a colloidal liquid...

We now incorporate the correct depletion thickness into free volume theory presented in Sect. 3.3. We consider the osmotic equilibrium between a polymer solution (reservoir) and the colloid-polymer mixture (system) of interest, see Fig. 4.6. The general expression for the semi-grand potential for Nc hard spheres plus interacting polymers as depletants, see (3.18), is... 

Fig. 4.13 Comparison of experimental gas-liquid coexistence binodals (data) compared to GFVT (curves). Left panel, spherical colloids mixed with polymer chains in a -solvent for q = 0.84 (open triangles, [20]), 1.4 (stars, [21]) and 2.2 (crosses, [21]). Right panel, colloidal spheres plus polymers in a good solvent for q = 0.67 (open squares, [20]), 0.86 (inverse filled triangle, [54]) and 1.4 (pluses, [20])...

The spinodal decomposition can be studied in much more detail using confocal scanning laser microscopy (CSLM). Aarts et al. [98] studied the phase separation kinetics of a PMMA colloid plus PS polymer mixture in decalin with q = 0.56. In Fig. 4.17 typical spinodal structures are observed that coarsen in time. 

Phase Transitions in Suspensions of Rod-Like Colloids Plus Polymers... 

So far we have considered the phase behaviour of colloidal spheres plus deple-tants. In Chap. 3 we considered the simplest type of depletant, the penetrable hard sphere. We then extended this treatment in Chap. 4 to ideal and excluded volume polymers and in Chap. 5 we considered small colloidal spheres (ineluding mieelles) and colloidal rods as depletants. In this chapter we consider the phase behaviour of mixtures of colloidal rods plus polymeric depletants. For an overview of several types of colloidal rods encountered in practice we refer to [1]. 

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