Particle distribution in matrix

Idealized distribution of filler particles in a matrix can be predicted by various models as discussed in Chapter 5. Here, an attempt is made to examine empirical data on filler distribution and to determine factors in actual filler which cause that distribution differs from an ideal model used to predict packing density of the filler. 

Filler particles generated in situ can be perceived as ideally distributed within the matrix. Experimental studies show that the situation is more complex. Polyfdimethyl siloxane) network was swollen to equilibrium in tetraethyl-orthosilicate which was then hydrolyzed to produce an in situ filler. Such an experiment gives the almost ideal conditions of uniform distribution because both matrix and the filler precursor are chemically similar. There are numerous factors which affect how uniformly a filler is distributed. These include  

The most uniform distribution was obtained when the hydrolysis time was long, sample was thin, and the catalyst basic. If conditions were reversed (short hydrolysis time, bulky sample, and acidic catalyst), filler was preferentially formed on the peripheries of the sample. What is the force which drives the precursor out of its initial equilibrium The most likely scenario is that a fast process leads to the 

In another similar example nanocomposite was formed in a polyurethane matrix. Solvent soluble polyurethane had pyridine groups attached which formed complexes with metal salts. Films were then formed and subjected to a reducing agent in order to produce particulate metal filler. In this case the distribution of the filler which was formed was not uniform because the filler had tendency to aggregate (even though it was chemically attached to the matrix prior to the reduction). The following were factors controlling size and shape of these metal particles  

Polymeric segments could prevent excessive aggregation. These experiments show that there is no reasons to expect a polymer-filler system to have a homogeneous distribution. Even under such ideal conditions as described in the above two cases (the chemical affinity of substrates and the anchoring of the filler precursor) it did not occur. The general conclusion from these experiments is that a homogeneous distribution of filler in the matrix is rather the exception than the rule. 

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