Emulsion polymerisation in the presence of hydrophilic comonomers

Abstract

Emulsion polymerisations are preferably stabilised electrosterically through the addition of hydrophilic comonomers having carboxyl functional groups. Furthermore, the incorporation of these comonomers can enhance the properties of the final polymer product. Despite their industrial importance, there is a lack of kinetic studies on these systems. The hydrophilic comonomer preferentially polymerises in the aqueous phase, leading to grafted or embedded hydrophilic chains anchored on the particle surface forming a hairy layer. Developing a mathematical model, it was found that the inhibition effects of the hairy layer of poly-AA chains were more profound compared to poly-MAA chains. The copolymerisation of styrene with MAA was monitored using reaction calorimetry technique for the first time. Using this technique, the enthalpy of copolymerisation of styrene with MAA was estimated and it was found that though the enthalpy of copolymerisation of styrene and MAA is lower than that of styrene alone, the heat of reaction was much higher. This was attributed to a higher polymerisation rate in the presence of MAA comonomer. Since various rapid events occur during emulsion polymerisation that may not be detected using temperature monitoring a novel on-line monitoring technique via high resolution electrical impedance spectroscopy (EIS) was developed for this purpose. It was shown that conductance changes with time at 1 kHz could provide information about properties of various stages of emulsion polymerisation systems. At lower frequencies (i.e., 1 Hz), however, EIS data could successfully detect the particle size evolution. Also, the addition of MAA to the reactor facilitated particle formation, and was reflected by a shorter pre-nucleation period compared with similar experiments without MAA. Furthermore, it was observed that polymerisation of MAA preceded that of styrene.