| Abstract | The objective of this work has been to study some aspects of the ionic emulsion polymerisation of organosiloxanes in basic media, using model systems comprising of cetyltrimethylammonium bromide (CTAB) as surfactant with potassium hydroxide as initiator, and the ion — exchanged product of the surfactant as a combined initiator / stabiliser, both with octamethyltetracyclosiloxane (D,) as monomer. Other cationic surfactants were also used with potassium hydroxide, and other hydroxides used with CTAB and the ion — exchanged product, and the rates of reaction and polymer viscosity were compared to those of the model systems over a range of experimental conditions.
A review of the available patent and scientific literature on the emulsion polymerisation of organosiloxanes is given, and a review of the organosilicon chemistry relevant to the study of the reactions of the siloxane group is also included. Theoretical aspects of the emulsion polymerisation of vinyl monomers are also discussed and compared with the behaviour of the siloxane systems.
Experimental techniques were developed to monitor polymerisation rate, polymer molecular weight (via viscosity measurement), and cyclic concentration throughout the polymerisation.
The influence of reaction conditions upon the rate of polymerisation were investigated by (i)varying the monomer content of the emulsion, (ii) varying the pH of the emulsion, (iii)varying the surfactant concentration in the emulsion, (iv) using the surfactant hydroxide formed by the ion-exchange of surfactant solutions, (v) adding excess base and surfactant to polymerisations using surfactant hydroxide, and (vi) varying the temperature of polymerisation. The effects of these variables on the molecular weight and the composition of the cyclic portion of the polymerisation mixture were also studied and used to suggest a reaction mechanism which may operate in this system.
The results show that the polymerisation process is qualitatively similar to that proposed by Harkins and an aqueous phase polymerisation mechanism is proposed whereby some polymer particles are nucleated in surfactant micelles and others are formed from oligomers present in the water phase but which originate from the reaction of initiator and monomer at the surface of monomer droplets.
The reaction rate is shown to be independent of monomer concentration and monomer droplet size providing that certain conditions which maximise the rate of monomer diffusion are met. The rate of polymerisation is shown to be first order in surfactant hydroxide concentration, but the rate is proportional to (overall pH) raised to the power 0.36 when both surfactant hydroxide and potassium hydroxide are present in the system. The rate change produced by the addition of known amounts of potassium hydroxide is found to depend upon [KOH]0.89. The polymerisation rate is virtually independent of the concentration of excess surfactant salt, for a 1.25% solution of ion — exchanged surfactant, but is thought to be more influenced by the total surfactant concentration at lower surfactant hydroxide concentrations.
The results here reported are the first comprehensive data to be published on the subject of the basic emulsion polymerisation system for organosiloxanes which proceeds via an ionic mechanism, and follows a kinetic path considerably different to the established theories for vinyl emulsion polymerisation,and the bulk polymerisation of organosiloxanes using base catalysts. The overall activation energy for the polymerisation is found to be similar to that observed in bulk polymerisations. |
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