Microemulsion Polymerization of Butyl Acrylate under Ultrasound Irradiation

Abstract

The microemulsion polymerization (MEP) of butyl acrylate (BA) stabilized by ionic emulsifier and initiated by oil-soluble dibenzoyl peroxide (DBP) or lauroyl peroxide (LPO) initiators have been studied under conven- tional (without ultrasound irradiation, CMEP) and ultrasonic (NMEP) conditions. The polymerization rate vs. conver- sion curve of the microemulsion polymerization of BA initiated by DBP was described by two nonstationary rate intervals. Four nonstationary rate intervals with two rate maxima appear in the microemulsion polymerization of BA initiated by LPO. The maximal rate of polymerization increases with temperature and the increase is much more pronounced under the conventional conditions. The overall activation energy is much larger under the conventional conditions (¼ 84 kJmol � 1 ) than under the ultrasound (¼ 20 kJmol � 1 ) conditions. The exit (desorption) rate constants k 0 des (cm 2 s � 1 ) and kdes (s � 1 ) (Ugelstadt/O'Tool approach, Nomura model and Gilbert model) as a function of temper- ature and the initiator type and concentration for the CMEP and NMEP were evaluated. Generally, the k 0 des (cm 2 s � 1 ) slightly increase with increasing the reaction temperature (except for the highest and lowest temperatures estimated by Tmodel). The values of kdes (s � 1 ) and k 0 des (cm 2 s � 1 ) are larger under the ultrasound conditions. The increased degrada- The principle behind the formation of transparent or semitransparent microemulsions (fine emulsion with a droplet size 10-60 nm) is the very low interfacial ten- sion caused by the penetration of coemulsifier into the droplet surface layer. Three- or four-component mix- tures containing water, monomer, emulsifier and co- emulsifier can form thermodynamically stable micellar solutions (microemulsions). The addition of coemulsi- fier increases the thermodynamic stability of micelles driving more emulsifier into micellar state, while on the other hand it can decrease the kinetic stability of micelles leading to faster formation and dissolution of micellar aggregates in solution. The thermodynam- ic stability of micelles is discussed in terms of how emulsifier distributes between the monomeric emulsi- fier and micellar state. The kinetic stability is dis- cussed in terms of the average time of micellar aggre- gate. Cosolvents (coemulsifiers) are known to pene- trate into micelles. The effect of such penetrating co- solvents can be discussed in terms of two aspects: an increase in distance between emulsifier groups and a decrease in the dielectric constant of micellar layer. This can explain the decrease of CMC as a result of dilution of micellar surface charge. 1