Ultrasound initiated miniemulsion polymerization of methacrylate monomers

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Abstract

The ultrasound initiated emulsion polymerization of methyl methacrylate (MMA), n-butyl methacrylate (BMA) and 2-ethylhexyl methacrylate (2EHMA) in the presence of sodium dodecylsulfate as a stabiliser produced latex particles in the size range of 70 nm to 110 nm with molecular weights of the order of 2–6 × 106 g mol−1. The experimental data obtained show significant differences in the rates of polymerization of the methacrylate monomers in the order 2EHMA > BMA > MMA. The rate trend is discussed with respect to the physicochemical properties of the monomers. It is suggested from the results obtained that the mechanism involved in sonochemical formation of the latex particles is very similar to that of a conventional miniemulsion polymerization process.

Introduction

Miniemulsions, consisting of small meta-stable monomer droplets of size range between 50 and 500 nm, are usually prepared by shearing a mixture of monomer, surfactant, costabiliser and chemical initiator dispersed in an aqueous phase [1], [2], [3], [4]. The costabiliser, usually a linear alkane, prevents Ostwald ripening, while the growth of droplets by collision and coalescence is prevented by the addition of a surfactant. Polymerization of such systems results in latex particles with approximately the same size range as the initial droplets. This occurs because the growth of the oil droplets by collision is much slower than the polymerization time and hence there is a 1:1 copying of the monomer droplets into polymer particles.

In recent years, ultrasound has been widely used as a novel technique for polymer synthesis [5], [6], [7]. The ultrasound initiated emulsion polymerization of various vinyl monomers has been widely investigated [1], [3], [7], [8], [9], [10], [11], [12]. Compared with the conventional miniemulsion polymerization process, ultrasound induced miniemulsion polymerization has several advantages: no chemical initiators or costabilisers are required, lower reaction temperatures can be used, the polymerization rate is faster, higher monomer conversion is achieved and polymers with higher molecular weights are synthesised [1], [6], [11]. The physical effects associated with acoustic cavitation provide very efficient mixing and strong dispersion of the components making up the liquid mixture. The high shear forces around the cavitation bubbles also act to ameliorate Ostwald ripening by continually fragmenting larger oil droplets into smaller ones.

In the present study, the factors that influence the kinetics of ultrasound initiated miniemulsion polymerization of various methylacrylate monomers are considered.

Section snippets

Chemicals

Methyl methacrylate (99% purity), n-butyl methacrylate (99% purity) and 2-ethylhexyl methacrylate (98% purity) were supplied by Aldrich, Australia. These monomers were filtered twice through basic aluminium oxide to remove the inhibitor, hydroquinone. The purified monomers were sealed and stored below 4 °C until further use. High purity (99% grade) sodium dodecylsulfate (SDS) was purchased from BDH, Australia. Milli-Q filtered water (18  cm−1) was used to prepare all aqueous solutions. High

Results and discussion

Bradley and Grieser [12] have reported that ultrasound initiated miniemulsion polymerization of MMA and BA in the presence of dodecyltrimethylammonium chloride (DTAC) produces cationic latex particles in the size range of 30–100 nm and molecular weights greater than 106 g mol−1. The present study extends the previous work of Bradley and Grieser [12] by examining the kinetics of ultrasound initiated polymerization of three different methacrylate monomers in the presence an anionic surfactant, SDS.

Conclusions

In the present study, the factors contributing to the rates of polymerization in ultrasonic initiated miniemulsion polymerization of different methylacrylate monomers were investigated. The results indicate that these reactions proceed via pseudo-first-order kinetics which supports the use of a zero-one model for polymerization, such that when a radical entering a particle containing a growing radical will lead to pseudo-instantaneous termination. The experimental results are consistent with a

Acknowledgements

The authors thank James Wiltshire for his help in collecting the molecular weight data. B.T. acknowledges the receipt of a Melbourne International Research Scholarship. The financial support from ARC Particulate Fluids Processing Centre is also acknowledged.

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    Present address: School of Chemistry, University of Bristol, Bristol BS8 1TS, UK.

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