Hybrid dispersion particles

The most important factor that affects the morphology of hybrid dispersion particles... 

The other factors that may influence hybrid dispersion particle morphology will be discussed in detail in Section 6.3.2 of this Chapter. 

When the process is carried out according to Method la it may be assumed that the kind of initiating system (water-soluble initiator or redox system) does not have any significant effect on the morphology of hybrid dispersion particles since the polymerisation proceeds mainly on the surface of DPUR particles and monomer droplets and in the micelles of emulsifier. In the case of water-soluble initiator (potassium persulphate) the polymerisation starts in water where the ion-radicals are formed from the monomer molecules dissolved in water. These ion-radicals can diffuse to DPUR particles, monomer droplets and emulsifier micelles. After this happens the system will consist of ... 

This experiment was aimed at clarifying what is the maximum amount of monomer that would he able to swell polyurethane-urea, i.e., would be able to take part in the formation of hybrid dispersion particles, assuming that enough time is allowed to achieve equilibrium. Films made from DPUR 25 8 were used in this experiment. The following resnlts were obtained ... 

The required properties of the hybrid particles can only be obtained when the magnetic particles are dispersed as single particles in the matrix. So, initially the magnetic particles (stabilized by surfactants or protective coUoids) should be well-dispersed in the aqueous phase and aggregation should be avoided during the polymerization. Additionally, in combination with surfactants, ultrasound can be used to make well-dispersed particles. 

If a comparison were made between hybrid dispersion systems taking into account the kind of dispersion particles they contain, it would appear that the best properties should generally be observed for systems obtained by synthesis for which the particles have a uniform structure (they may be called true hybrid particles) and the worst - for simple blends of the two dispersions [14] (see Figure 6.1). 

Obviously, both types of dispersion particle morphology presented in Figure 6.1 ( coreshell and true hybrid ) should be considered as idealised cases. In practice, a variety of different particle morphologies may be observed. Three of them are shown in Figure 6.2. 

Figure 6.1 Comparison of hybrid dispersion systems obtained by (a) blending, (b) and (c) by synthesis, taking into account particle type.

Figure 6.2 Examples of different particle morphologies of hybrid dispersions obtained by synthesis (such morphologies may be considered intermediate between core-shell and true hybrid morphology shown in Figure 6.1).

A recently published patent [33] from Zeneca described in detail the methods of preparation of hybrid polyurethane/vinyl polymer dispersions but neither discussed the effect of various factors on the properties of the hybrid dispersions nor provided any information on their particle morphology. 

Particle size distribution and zeta potential of a typical hybrid dispersion obtained using a water-soluble initiator are presented in Figures 6.17 and 6.18, respectively. 

Figure 6.17 Particle size distribution for a typical polyurethane-urea-acrylic/styrene hybrid dispersion synthesised in this study (MDPUR-ASD 97 from Table 6.7 prepared according to method 2 using a water-soluble initiator).

TEM photos of the particles of two hybrid dispersions prepared using less hydrophobic and more hydrophilic monomer prepared according to method la are presented in Figures 6.23 and 6.24. A TEM micrograph of the particles of a hybrid dispersion prepared according to method 3 using more hydrophobic monomer is presented in Figure 6.25. 

The morphology of dispersion particles was investigated using the method described in Section 6.4.1. Examples of different morphologies of particles of hybrid dispersions synthesised in this study according to methods la, lb and 3 are presented in Figure 6.31 in comparison with particles of the starting dispersion of BA/MM/S copolymer. The contrast was selected so that in pictures d and c white colour represents the polyurethane-urea part of the hybrid and in picture b the same colour represents the acrylic/styrene part of the hybrid. 

The morphology of particles of hybrid dispersion synthesised according to method 2 using water-soluble and redox initiators is presented in Figures 6.32 and 6.33, respectively. Both pictures show both the single particles and the coalesced particles to demonstrate what happens to the particle morphology in the process of film formation. White colour represents the polyurethane-urea part of the hybrid. 

Figure 6.32 Morphology of particles of hybrid polyurethane-urea-acrylic/styrene hybrid dispersion prepared according to method 2 (See Section 6.3.2) using water-soluble initiator (MDPUR-ASD 97). Micrograph was taken using TEM. Both single particle and coalesced particles are shown. Reproduced with permission from Professor A. E. Czalych, Institure of Chemical Physics of the Russian Academy of Sciences, Moscow.

Table 6.24 shows that the chemical structure of the acrylic/styrene part of the hybrid has no effect on the macroscopic properties of hybrid dispersions, although the appearance of the dispersion particles may be quite different (compare Figures 6.23 and 6.24). Difference in the appearance of particles may result from the degree of hydrophobicity of the monomer (this effect is explained in Section 6.3.2.1). 

Investigations of swelling of DPUR particles with monomers indicate that although all monomer being fed into the system may theoretically enter DPUR particles, since the maximum degree of swelling is more than twice that of the monomer/ DPUR solids w/w ratio used commercially in synthesis of hybrid dispersions, in practice there will always be a natural tendency of the system to reach equilibrium between monomer droplets, non-swollen DPUR particles and swollen DPUR particles. Therefore, when hybrid dispersions are prepared according to method la, lb or 2 (see Section 6.3.2) the result will always be a mixture of particles of hybrid structure with DPUR and ASD particles, as has been explained in Section 6.3.2.1). 

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