Polymer-colloid-solvent mixtures polymeric stabilization

Dispersion polymerization differs from emulsion polymerization in that the reaction mixture, consisting of monomer, initiator, and solvent (aqueous or nonaque-ous), is usually homogeneous. As polymerization proceeds, polymer separates out and the reaction continues in a heterogeneous manner. A polymeric surfactant of the block or graft type (referred to as protective colloid ) is added to stabilize the particles once formed. 

Stability in mixtures of colloidal particles and polymer molecules, dispersed in a solvent, has been the subject of experimental and theoretical investigations for a long time and it has applications in diverse fields such as paint technology, wastewater treatment, emulsion polymerization, biology etc. It has now been well recognized that polymer molecules can be used to induce either stabilization or flocculation (phase separation) in colloidal dispersions. It is important to distinguish between polymers which are adsorbed on the particle surface and those that are free in solution because the two situations usually lead to qualitatively different effects. Stability imparted by adsorbed polymers is known as steric stabilization and the flocculation or phase separation caused by the free polymer is due... 

In the above descriptions we concentrated on situations where a polar background solvent was implicitly assumed. In apolar solvents double layer repulsion is diflhcult to achieve because dissociation, leading to charged surface groups, is less likely to occur and it becomes essential to stabilize colloids with polymers as to prevent instabilities. In the first decades after the establishment of the DLVO theory most papers on forces between colloidal particles focused on Van der Waals and double layer interactions. Forces of other origin such as polymeric steric stabilization [17], depletion [40] or effects of a critical solvent mixture [41] gained interest at a later stage. 

In contrast to suspension polymerization, the fabrication of microspheres using dispersion polymerization produces colloid particles with a low polydispersity (Arshady 1992). In this process, the monomers, steric stabilizers, and initiators are soluble in a solvent as a continuous phase. Polymerization occurs initially within the homogeneous reaction mixture. Ultimately, it is the insolubility of formed polymer particles in the continuous phase that allows for the collection of these microspheres. The colloid particles formed using this technique are held together with the help of a colloidal stabilizing agent (Arshady 1992). 

Latex is a colloidal dispersion of polymer in an aqueous solvent. This method is more suitable for those polymers that can be prepared via emulsion polymerization or those that have the ability to form emulsion. It consists of an aqueous dispersion/ stabilization of filler using a surfactant followed by the addition of the dispersed filler into the polymer latex. Nanocomposites can be obtained after freeze-drying the above mixture followed by melt processing. The latex method has several advantages including no requirement for organic solvent, reliability, ease of processing, and improved dispersion of the filler in the viscous polymer matrix [70]. 

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