Growth of the Polymer Particle

Burkhard E. Wagner and coworkers at Union Carbide characterized the fragmentation of silica-supported catalysts [49]. They proposed that catalysts supported on silica undergo a similar fragmentation pattern in which the silica pore volume fills with polyethylene and the shear forces of the particle growth fragments the silica particle into microspheroidal aggregates of 0.05-0.1 microns in diameter. 

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The fluidization conditions of the UNI IT)I process are more complicated due to the change in particle size during the growth of the polymer particles. 

In addition to the molecular properties, the macroscopic growth of the polymer particles is important they should be sufficiently large and dense,... 

In addition to the molecular properties, the macroscopic growth of the polymer particles is important they should be sufficiently large and dense, otherwise there is no means of collecting and processing the product. What is more, the capacity of the plant also increases with the bulk density of the product. The theoretical bulk density for ideal uniform spheres is 540 kg/m. Values obtained in modern plants are of the order of 500 kg/m. The morphology is also determined by the solid catalyst preparation. The morphology of the macroscopic catalyst particles is reproduced by the growing polymer particles. 

Monosilicic acid polymerizes in neutral or acidic solution, with a characteristic rate depending the pH [ref. 118-120]. Tarutani [ref. 118] suggested that exclusion chromatography is useful for studying the polymerization of silicic acid. The mechanism of the growth of the polymer particles was discussed on the basis of changes in the elution curves for polysilicic acids with time [ref. 118-121]. The polymerization of silicic acid is slowest at pH 2 in aqueous media [ref. 118, 119]. The eluent adjusted to pH 2 was used throughout the experiments. 

The course of a batch reaction is normally described in terms of three intervals. A particle nucleation interval which is usually short a particle growth interval where the growth of the polymer particles takes place because monomer diffuses from the droplets to the polymerization sites and a third interval where the polymerization is completed in the absence of monomer droplets. In such reactions the distribution of ages of the particles in the final latex can be quite narrow since all particles are formed during a small time period near the beginning of the reaction. 

Growth of the polymer particles leads to an increase in surface area. This increase leads to the adsorption of soap from the aqueous phase, which again leads to dissolution of micelles. 

The solubilization of the monomer in the micelles and the mechanism of growth of the polymer particles are depicted in Fig. 2-3. 

Monodispersed sols containing spherical polymer particles (e.g. polystyrene latexes22"24, 135) can be prepared by emulsion polymerisation, and are particularly useful as model systems for studying various aspects of colloidal behaviour. The seed sol is prepared with the emulsifier concentration well above the critical micelle concentration then, with the emulsifier concentration below the critical micelle concentration, subsequent growth of the seed particles is achieved without the formation of further new particles. 

Williams s core-shell theory of particle growth, however, has many unresolved conflicts. Napper [18] pointed out that the diffusion rates of species present within the polymer particle did not support the hypothesis for such large differences in the polymer concentrations between the core and shell. Moreover, Garden [19] showed that the diffusive mean free path of monomer molecules, which was much larger than the radius of the polymer particle, would not favour the core-shell equilibrium theory. Garden, as well as Friis and Hamielec [20], also indicated that Williams experimental results, i.e. a nearly constant polymerization rate, could be attributed to the concurrent decrease in [M]p due to... 

Modelling of the polymer particle growth process [82] has resulted in the conclusion that diffusion limitations are the single reason for the wide polydispersity of synthesised polymers. The model has demonstrated that the main transport limitations localise on the level of macroparticles. Modelling results are confirmed by data obtained in gas and liquid polymerisation experiments on titanium-magnesium catalysts. Authors also consider that the wide polydispersity of polymers can be explained by the existence of more than one type of active centre. Each specific type is responsible for a certain portion of polymer with a different MWD. However, the authors did not succeed in characterising the active centre [82] because it required the optimisation of many kinetic parameters. 

Nevertheless, in addition to MWD and CCD, the rate of growth and final size of the polymer particle will obviously depend very strongly on the rate of polymerization inside the particle. Since the process in question is a heterogeneous one, and the characteristic length scales of the particles vary fi-om 10s to 100s of microns, it is necessary to understand the relationship between the observed polymerization rate, intrinsic kinetics, mass and heat transfer mechanisms, and particle size. This can be done on the condition that a workable representation of the morphology of the particle can be constructed, which will be discussed in the following section. 

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