Ultrasonic initiation of polystyrene latex synthesis

Polystyrene latex samples have been synthesised from an oil-in-water (o/w) emulsion of the monomer using ultrasonic irradiation as the initiation sour ce. No added chemical initiators were used in any of the reactions. The fre e radicals formed from the cavitation process under the action of the ultra sound are sufficient to initiate the polymerisation. In the current study, the effects of varying the input intensity of the ultrasound were investiga ted. A marked increase in the monomer conversion rate as a function of time was seen as the power was increased. Despite this increase in conversion r ate, the increasing intensity did not affect the resultant latex particle s izes. In all cases, the particle diameters at the conclusion of a polymeris ation were seen to be in the range of 40 to 50 nm. The effects of changing the concentration of surfactant in the polymerisation medium and of changin g the temperature were also probed. Increases in both were seen to lead to an increased rate of conversion at a fixed input intensity. Again, the part icle diameters were invariant as a function of changing conditions. The sma ll particle sizes and high conversion rates are thought to be due to a cont inuous nucleation process that is caused by the scavenging of free radicals throughout the polymerisation by freshly formed small oil droplets. (C) 20 00 Elsevier Science B.V. All rights reserved.