Stable concentrated emulsions as precursors for hydrophilic-hydrophobic polymer composites

Abstract

A new method to prepare polymer composites based on concentrated emulsions was developed in this laboratory. The composite is synthesized starting from a concentrated emulsion of a hydrophobic phase, containing a hydrophobic monomer, dispersed in a continuous hydrophilic phase, containing a hydrophilic monomer. Of course, inverted dispersions in which the hydrophilic phase is dispersed in a hydrophobic phase are also possible. The concentrated emulsions have the same appearance and behaviour as a gel; the volume fraction of their dispersed phase is very large (it can be as large as 0.99). Not all pairs of monomers can lead to concentrated emulsions. In addition, concentrated emulsions prepared and stable at room temperature may not remain stable at the polymerization temperature of 50°C. The present paper contains a large number of experiments whose goal is to identify the conditions under which a coupling between two monomers can lead to hydrophilic-hydrophobic or hydrophobic-hydrophilic composites. The nature of the hydrophilic and hydrophobic monomers (particularly their polarity), the pH and the ionic strength are found to be important factors in the formation at room temperature and stability at 50°C of the concentrated emulsions. It is found that the instability at 50°C of the concentrated emulsions (expressed as weight per cent of bulk phases separated from the emulsion after heating for a certain time) can be correlated with the interfacial tension at 25°C between the hydrophilic and hydrophobic phases free of surfactant. The larger this interfacial tension, the higher the stability of the concentrated emulsion. Hence, the lower the polarity of the hydrophobic monomer and the higher the polarity of the hydrophilic monomer, the more stable the emulsion. The film-forming capability of the surfactant is also important. Improved mechanical strength of the interfacial film is achieved using blends of non-ionic surfactants or mixtures of ionic surfactants and long-chain alcohols as dispersants. Increased stability is exhibited by adding methyl cellulose in the aqueous continuous phase when hydrophobic monomers with higher polarity are employed in the dispersed phase. High temperatures and higher volume fractions of the dispersed phase decrease the stability of the emulsion. Several hydrophilic-hydrophobic polymer composites have been prepared on the basis of the above observations.