Abstract
Many reactions are known to be affected by ultrasound. In the particular case of polymer systems, whilst there is a wealth of information on the degradative effects of ultrasound, there is very little on the effect of ultrasound on a polymerising system. This project has attempted a systematic investigation of the emulsion polymerisation of styrene and the effect of ultrasound on the components of the reaction and the emulsion polymerisation as a whole.The decomposition of the initiator, potassium persulphate, was found to be enhanced by 20 kHz ultrasound at reaction temperatures lower than 70⁰C. The usual sonochemical correlation with solvent vapour pressure did not apply and therefore an alternative explanation was developed. The data was analysed by assuming that, since potassium persulphate is involatile, the sonically assisted rate of decomposition must be due to the reaction taking place at an elevated temperature in the interface between the cavitation bubble and the bulk solvent (and not in the cavitation bubble itself). The analysis indicated that the enhanced decomposition was proportional to both the acoustic pressure and the square root of the acoustic intensity.
The investigation of the emulsion polymerisation of styrene with and without sonication identified two major effects of ultrasound. Sonication enhanced the emulsification of styrene in water and significantly reduced the induction period of the polymerisation. The two effects are related since enhanced emulsification causes an increase in the surface area of the styrene which in turn increases the efficiency of the initiator and decreases the induction period of the polymerisation. The polymerisation rate constant , kp, , was found to be increased significantly when ultrasound was applied and the ultrasonic intensity increased. This can be rationalised in terms of a decrease in the activation energy for the emulsion polymerisation of styrene.
The latices produced in the presence and absence of ultrasound were analysed for particle size, molecular weight and polydispersity to monitor any change in the characteristics of the polymer. The particle size data was also used to calculate the relative particle number of each latex. Sonicated latices were found to have lower particle numbers and larger particle sizes than comparable latices produced thermally. These results were rationalised in terms of the enhanced emulsification effect of ultrasound. The molecular weights and the polydispersity of the latices did not show any real trend as ultrasound was applied and the ultrasonic intensity increased. This suggests that ultrasound produces latices of similar molecular weight and polydispersity to conventional emulsion polymerisations.
| Date of Award | 1993 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Timothy Mason (Supervisor) & j Philip Lorimer (Supervisor) |