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Flow induced polymer-filler interactions: Bound polymer properties and bound polymer-free polymer phase separation and subsequent phase inversion during mixing

Akay, G.
The irreversible absorption of macromolecules on to solid filler particles during mixing in the melt is investigated. The molecular weight and concentration dependence of the absorbed layer thickness are evaluated and the chemical and morphological nature of the irreversibly absorbed polymer (bound polymer) are determined. It is found that the thickness of the bound polymer is not only dependent on the filler concentration but also dependent on polymer molecular weight. Bound polymer in high density polyethylene/colloidal silica systems is more amorphous and contains higher concentration of oxidation products (which are induced by a mechanochemical reaction during mixing) when compared with free polymer. As a result of bound polymer formation, flow induced crystallization and polymer entrapment during mixing, a phase separation between bound polymer and free polymer takes place which eventually leads to a liquid to solid phase inversion when the phase volume of the solids exceed a critical value under isothermal conditions. If the mixing is continued after phase inversion, the particle size of the newly formed powder is reduced and particle size distribution, is narrowed. This process of phase inversion and subsequent size reductions are termed as crumbling which can take place at filler concentrations well below the expected maximum packing fraction of solids, if the filler particle size is small and/or the molecular weight of the polymer is high. Crumbling phenomena are utilized in the agglomeration of fine powders and microencapsulation of solids and/or liquids.