Ultrasonic Detection of the Interface Formation between the Polymer-Rich and Monomer-Rich Phases during Bulk Polymerization of Methyl Methacrylate

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Polymeric materials are essential for a wide range of applications; however, understanding and controlling their reactive processing, especially in the later stages, remains challenging. One notable phenomenon in the polymerization of methyl methacrylate (MMA) via bulk radical polymerization is the Trommsdorff effect; this effect causes a rapid acceleration in the reaction rate, and its mechanism remains debatable. In this study, we investigated MMA polymerization by integrating ultrasonic monitoring, temperature tracking, and video recording. This approach enabled the observation of the formation of a polymer-rich phase and the propagation of the polymerization front. Contrary to the assumption that MMA undergoes homogeneous polymerization, our results revealed macroscopic heterogeneity during its transition from the liquid to the glassy state. The polymerization process can be categorized into six distinct stages based on its response to ultrasonic propagation. These results fundamentally enhance our understanding of bulk polymerization.