(338t) Synthesis of Polystyrene Particles Aiming at Particle Size Control By Tandem Emulsification Method

Polymer particles are used in various applications such as paints, bonding agents, and coating agents. Particle size design is required for various applications. Therefore, there is a need to investigate the effects of polymerization methods and operating conditions on particle size. Miniemulsion polymerization is one of the easiest methods to control particle size in the synthesis of polymer fine particles. Emulsions are prepared by chemical and physical methods. Especially the ultrasonic emulsification method is useful among the physical methods because it reduces the amount of surfactant used and produces emulsions with small droplet diameters. Furthermore, it has been reported that the tandem emulsification method, in which ultrasonic waves of low to high frequency are sequentially irradiated, is able to prepare very stable nanoemulsions with droplet diameters in the order of several tens of nm. However, the effect of the relationship between the crude emulsion droplet diameter before ultrasonic irradiation and ultrasonic frequency on the miniaturization has not been well investigated.

In this study, we prepared O/W emulsions of styrene and water using a two-step emulsification method and investigated the effects of crude emulsion droplet size and ultrasonic frequency on the droplet size distribution of the prepared emulsions. The prepared emulsions were heated to proceed the polymerization reaction, and the effect of emulsion droplet size on the particle size after polymerization was investigated. In addition, kinetic analysis was performed to examine the rate constants.

First, crude emulsions were prepared using a high-speed stirrer and an ultrasonic homogenizer. Styrene was used for the dispersed phase and water for the continuous phase. Sodium dodecyl sulfate (SDS) was dissolved in water as a surfactant and benzoyl peroxide (BPO) was dissolved in styrene as a polymerization initiator. Pre-prepared crude emulsions were irradiated with ultrasound at various frequencies. The effect of frequency on the change in droplet diameter caused by ultrasonic irradiation was investigated. As a result, no change in emulsion droplet diameter was observed with or without ultrasonic irradiation and at different frequencies. Moreover, there was no effect of ultrasonic irradiation on the rate constant, and the polymerization rate reached a constant value of approximately 80% after a reaction time of 4 hours. This may be one of the reasons why there was no difference in the emulsion droplet diameter before polymerization in different conditions. Next, crude emulsion diameter was changed between 2 and 10 μm to investigate the effect of crude emulsion droplet diameter. The droplet diameter after ultrasonic irradiation was measured. The results showed that there was no effect of droplet miniaturization in most of the conditions, and on the contrary, the droplet diameter increased in some of the conditions. On the other hand, a volume-averaged diameter decreased under some conditions. This suggested that large droplets outside the peak of the droplet diameter distribution were miniaturized. At the crude emulsion with a droplet diameter of 10 μm, both the number-averaged and volume-averaged diameters increased, and the rate constants also changed. It is thought that this is because the crude emulsion diameter was large and unstable, and the droplets coalesced and separated, and polymerization reaction did not proceed. Next, surfactant concentration was changed between 0 and 1.0 wt% to investigate the effect of crude emulsion droplet diameter. The droplet diameter after ultrasonic irradiation was measured. In the case of the surfactant concentration of 0 wt%, ultrasonic irradiation decreased the number-averaged diameter, but also increased the volume-averaged diameter. It is considered that the reduction of the surfactant caused the emulsion to be more sensitive to the effect of ultrasound, but the stability of the emulsion itself decreased, and the droplets coalesced and their distribution became broadened.

From these results, when the droplet diameter of the crude emulsion was large, ultrasonic irradiation increased the droplet diameter and affected the rate constant, and the droplet diameter distribution was broadened under conditions without surfactant. It is suggested that the droplet miniaturizing by ultrasonic irradiation is affected by the stability of the emulsion. Other studies have reported that high-frequency ultrasonic irradiation is effective in droplet miniaturization of crude emulsions with droplet diameters of a few hundred nm. In the future, it is expected that by decreasing the droplet size of the crude emulsion, the polystyrene particle size can be designed by appropriate selection of the ultrasonic frequency in the second step.