Hybrid dispersion particles Morphology

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

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. 

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

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 ... 

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. 

Based on this brief literature survey, it seems quite clear that not all features of the hybrid acrylic-urethane dispersions have been studied so far. The purpose of the present study is to clarify the effect of various factors on the properties of acrylic/styrene-urethane dispersions. Some results of preliminary investigations of the particle morphology will also be revealed. 

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.

The most interesting result seems to be, however, the specific morphology of the particles of hybrid dispersions prepared according to method 2 shown in Figures 6.32 and 6.33. It is shown schematically in Figure 6.34. 

Figure 6.35 Schematically presented structure of film made from a hybrid dispersion of the particle morphology shown in Figure 6.34...

The morphology of dispersed particles revealed by TEM appeared to be very interesting. It was found that the method of hybrid synthesis had a substantial influence on the particle morphology and that usually core-shell or englued morphologies described earlier in the literature were observed. However, in one case the unusual embedded sphere morphology was seen. In these particles, the core made of polyurethane-urea is surrounded by a 25-40 nm thick sphere made of acrylic/styrene polymer and covered by a 15-20 nm thick outer layer of polyurethane-urea. The structure of film made from such dispersions is very interesting since it is a two-phase structure where nanospheres of acrylic/styrene polymer are suspended in a polyurethane-urea matrix. 

Figure 8.26. Schematic illustration of the interfacial tensions balance at the interface between three liquids. The Neumann s triangle, and the morphologies to appear in the melt-blends of three polymers are also shown. Four morphologies are predicted for dispersions of liquids 1 and 2 in liquid 3 (a) encapsulated hybrid particles (1 in 2), (a ) encapsulated hybrid particles (2 in 1), (b) stuck hybrid particles, (c) isolated particles [Nakamura and Inoue, 1990].

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