Particles with Strongly Bonded Polymer

Particles can be made with strongly bound soluble polymer molecules at the surface as shown schematically in Fig. 1020. 

When two such particles approach through the solvent, there is a slight van der Waals attraction at large separations. But as the gap equates to two polymer molecule diameters, an increasing repulsion is observed. This eventually breaks down into the primary minimum if sufficient force is applied to debond the polymer molecules. Thus, there is a small seomdary minimum in this situation, separated from the primary minimum by a large energy barrier. Such particles behave very much like hard sjAeres, as shown in Fig. 10.20(c), because there is 

Latex particles, such as polystyrene or poly(methyl methacrylate), surface-grafted with water soluble polymer, behave in this way. For example, Pusey and his colleagues have made latex dispersions with strongly bonded polymer chains at the surface, matching the refractive index of latex and solvent to reduce van der Waals forces to a minimum. These dispersions appeared to be fully stable and exhibited phase transitions when concentrated to particle volume fractions of 0.5, producing ordered colloidal crystals which displayed opalescent colors. The conclusion was that the particles were behaving as hard spheres with zero adhesion. 

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Figure 10.20. (a) Particle with strongly bonded soluble polymer on Its surface, (b) Interaction energy between two surfaces in solvent with bonded soluble polymer, (c) Hard sphere approximation to interaction energy. 

Such complex restructuring of the interfaces, with polymer molecules changing positions substantially as the particles approach, is quite different from the situation when polymer is strongly bonded to the particles. 

Colloidal systems can be divided into lyophilic and lyophobic systems. Lyophilic colloids have a strong affinity with the dispersion medium by which a solvation shell around the particle is formed. This process is called solvation and if the dispersion medium is water it is called hydration. A polysaccharide dissolved in water is an example of a lyophilic colloidal system. The solvation shell is formed by hydrogen bonds between the hydroxyl groups of the polymer molecules and the water molecules. Pharmaceutical examples are solutions of dextran, used as plasma expanders. Micelles are also lyophilic colloids. Example of such a system is the aqueous cholecalciferol oral mixture (Table 18.15). In these preparations, a lipophilic fluid is dissolved in an aqueous medium by incorporating it in micelles. Because this type of colloids falls apart on dilution to concentrations below the CMC, they are also known as association colloids. Lyophobic colloids have no affinity with the dispersion medium. Thus, in this type of colloids no solvation shell is formed around the particles. An example of lyophobic particles are colloidal gold particles (with a diameter of 1 nm - 1 pm) dispersed in water. There are no... 

The most important filler parameter affecting modulus is its shape. Unfortunately, when the filler is non-spherical theories become much more complicated and the reader is advised to refer to Chow s review [61]. Shape factors can be incorporated in the models mentioned previously but are only useful when applied to very high aspect ratio materials, e.g., fibres. There is also an almost insurmountable problem with particulate fillers the difficulty and effort to measure aspect ratios of micrometre sized particles. Pukansky examined the effects of 11 different fillers in polypropylene [69] and concluded that Young s modulus is affected by the amount of bonded polymer, which is in turn related to surface area, and therefore to both particle size and shape. That observation helps to explain the strong effect that nano-fillers have on the modulus of a composite. Schreiber and Germain showed that modulus depends on the strength of interaction between the polymer and the filler surface [62]. 

In a later study, Millili et al. proposed a bonding mechanism, referred to as autohesion, to explain the differences in the properties of spheres granulated with water and ethanol. Autohesion is a term used to describe the strong bonds formed by the interdiffusion of free polymer chain ends across particle-particle interfaces (59). 

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