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INTRODUCTION
(a) Polvmensation
Emulsion polymerisation is one of the fundamental methods whereby homo and copolymers
can be prepared. [1]. Its particular value is that the polymers are produced as small particles
having a narrow molecular weight range. By way of contrast suspension polymerisatión
produces particles usually of large size with the molecular weight distribution being far greater
than that obtained by emulsion polymerisation.
For emulsion polymerisation the reactants have to be dispersed in water with the aid of a
surfactant and vigorous agitation. The nature of the surfactant, or emulsifier, influences the
polymerisation rate and nature of the latex produced including particle size, particle size
distribution and stability of the latex The surfactants oremulsifiers used are classical soaps and
consist of a hydrophobic alkane tail together with a hydrophilic head group which can either be
charged (anionic, or cationic'or zwitterionic) or neutral (eg. a polyethylene oxide). The function
of the surfactant or emulsifier is to form micelles in solution which dissolve some of the
matenals to be polymerised. When radicals produced from the initiator arc fonned in the
aqueous phase, they have to diffuse' to the micelles in order to initiate polymerisation. At an
early stage of the polymerisation a large number of small polymer particles will be produced
which are isolated from each other by the aqueous phase and by the emulsifier layer adsorbed
on the surface; Monomer fr&n the oil droplets will diffuse intO the growing particles and during
this period the rate of reaction will remain approximately constant. , Cleafly the number of
particles will be influenced by the emulsifier concentration and the behaviour of the radicals in
the latex particles. A positive advantage of the aqueous environment is that it takes away the
heat liberated during the polymerisation and hence the products are very different to those
obtained by bulk polymensation
Usually emulsion polymerisations are carried out with an auxilliary stabiliser being present the
function of which is to control the size distribution and stability of the latex particles. These
stabilisers are usually high molecular weight water soluble polymers which are adsorbed onto
the surface of the polymer particles and protect the particles by steric hinderance. Materials in,
common use include poly(vinylalcohol), 'sodium polyacrylate, carbohydrates (gum arabic),
polyethyleneglycols and water soluble cellulose derivatives eg. methyl and
carboxymethylcellulose.
