Surface heterophase polymerization

Fig. 1. The four heterophase polymerization domains are shown as a function of the emulsifier concentration and the surface tension driving force...

This chapter treats the subject of interfacial effects while a monomer is converted into its polymer. Hence, the question arises concerning the interfacial properties of polymers. For some polymers typically prepared by heterophase polymerizations, values of the surface tension (/p) and interfacial tension to water are listed in Table 8.3. Compared to the data shown in Table 8.2, the values for the polymers are higher and especially the a data can be understood in the same way as those of the low-molecular-weight compounds. 

The peculiarities of heterophase polymerizations are directly connected with the dispersed state. Figure 3 shows schematically the main difference between 1 g of polystyrene as bulk material and as a dispersion. The bulk material is a single particle with a diameter (Z)) of 1.22 cm and a total surface area (Ap) of4.676 cm, whereas the same amount of material in the dispersed state with an average particle size of 50 nm is subdivided or compartmentalized into 1.455 x 10 particles (N), with a total surface of 1.143 x 10 cm. This increase in both iV andAp causes both the advantages and problems of heterophase polymerizations and polymer dispersions compared to homogeneous counterparts. The prices mentioned in Figure 3 for that 1 g of polystyrene in different states show interestingly that the increase in the total surface area almost corresponds to the increase in the... 

Therefore, one is dealing with a heterophasic reaction which could be controlled by typical kinetic factors such as a) formation and decay of active centers with time, b) presence of a multiplicity of active centers energetically, structurally and chemically different form one another and therefore having different kinetic constants. Moreover a role could also be played by true physical phenomena such as a) variety of growing chain lifetime depending on the different degree of active centers encapsulation in the polymeric matrix, and b) limitations to heat transfer and, above all, to mass transfer from the gas phase to the liquid phase, from liquid to polymer surface and from the polymer to the surface or to the interior of the catalyst. 

The second approach or microheterogeneous model [1, 19-22] is based upon the principle, that the kinetics of the reaction in its initial stage are not that of a homophase polymerisation in a liquid monomer-polymeric solution, but a heterophase one. The reaction proceeding at the boundary liquid monomer - solid polymer microgranules surface under gel conditions. 

---