Polymer grains

For the analysis of the role of monomer diffusion during ethylene polymerization while forming a solid polymer a model of the polymer grain (see Fig. 2) has been suggested (95). This model is consistent with the results of the study of nascent morphology of the polymer and its porosity (95, 100, 103). According to this model three levels are considered in the analysis of transport phenomena. 

Fig. 2. Models of the primary particle (a) and polymer grain (b) for the analysis of the role of monomer diffusion to tbe catalyst surface, (a) 1—catalyst 2—polymer film, (b) 1—micrograin 2—macropore.

Thus, two factors may be pointed out that determine the possibility of obtaining high yields of crystalline polyethylene on a solid catalyst with no diffusional restriction (1) the splitting up of the catalyst into small particles (< 1000 A), possible when using supports with low resistance to breaking (2) the formation of polymer grains with polydispersed porosity. 

During the collision of two polymer grains areas with a size of a few square micrometers come into contact. In the contact zone an electron transfer can take place as described in [2] (Fig. 2a). In case of the collision of two chemically different polymer particles one of them preferably interacts as electron pair donator and the other species as electron pair acceptor. In this way laterally expanded charge domains can be formed (Fig. 2b). The low electrical conductivity of the polymer bulk and surface prevents a rapid charge dissipation, hence the formed domain structure seems to be permanently stable. 

Fig. 2 Model concept of the contact charging of polymer grains, a Contact between an electron pair donator domain (empty dots) of the particle 1 and an electron pair acceptor domain (grey dots) of the particle 2. Charge transfers (e-) are taking place. After separation the two particles (b), a positively charged ( ) and a negative charged ( ) domain, remain on the particles surface...

Figure 8 Simplified model for the fragmentation of MgCh-supported Ziegler-Natta catalyst grains to primary catalyst particles and shape conservation of growing polymer grains.

A key process of the slurry polymerization is the particle-forming process, which involves the transformation of a catalyst particle into a polymer grain. It has been found that each catalyst particle is transferred into one polymer grain, as shown in Fig. 5. 

Maintenance of the commercial parameters of the process, during the synthesis of the calcium stearate antiagglomerator in tubular turbulent reactors, allows an aqueous suspension with pH = 10.5-12 to be obtained. The hydrogen indicator of the solution is optimal both for the stabilisation of polymer grains and for the separation of the unreacted monomer and solvent from the reaction mixture at the degassing stage. 

The baseline resistance of the sensors also depends on the posttreatment. There are various explanations for the mechanism behind the response behavior modification due to the posttreatment. The lower baseline resistance and the faster response transients after the acetone treatment could be caused by dissolving some of the acrylic matrix during this posttreatment. Dissolving the matrix can result in a closer contact between the conducting polymer grains and enhanced diffusion due to a smaller path length through the composite layer. 

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