Heat of polymerization is one important parameter in understanding how chemical additives impact polymerization, especially in additive manufacturing. The well-known equation for heat of polymerization is DH = m*C_p*DT, where m is the mass of the sample, C_p is the pressure constant heat capacity most commonly determined by differential scanning calorimetry (DSC), whilst the change in temperature during polymerization is DT. Normally, thermocouples are utilized to determine the change in temperature during polymerization, however, thermocouples may be affected by thermal interference from their surroundings: the monomers being held within an insulating scintillation vial or similar container.¹ Infrared cameras are commonly used, however, the mixture is held between glass plates, once again, generating interference from the surroundings.¹ Here we describe a novel technique that eliminates the need for an insulating sample container by using a standing acoustic wave to trap the monomers undergoing polymerization, during which, the change in temperature is recorded by IR thermography.

In acoustic levitation, the acoustic standing wave, , is generated utilizing 36 transducers arranged into the two parallel arrays of 3D printed apparatus which emit sound at 40 kHz². These transducers emit sound waves that interact through constructive and destructive interference to produce nodes able to hold a small volume of liquid in place. This configuration was modified to support two 405 nm lasers on the top and bottom to induce photopolymerization and a FLIR camera was placed to one side (Figure 1). In the experiment, about 5 µL of methyl methacrylate monomer resin containing a photoinitator was placed in a node of the standing wave and the two 405 nm lasers were energized to induce polymerization. A FLIR thermographic camera was used to determine the temperature of the bead throughout the polymerization process. The resulting technique allows for very little thermal interference from the surroundings.

The temperature of polymerization measured via our technique ranges from 94.7 to 95.6 °C, very similar to the literature polymerization temperatures of poly(methyl methacrylate) which range from 90 to 94.4 °C.^1,3,4 The heat capacities determined from DSC from the beads polymerized via acoustic levitation in this work are from 1.041 to 1.383 J/mol*K, again very near the literature values for neat poly(methyl methacrylate) of 1.117 to 1.38 J/mol*K.^5–8 In the end, this technique produces values of DT, and therefore, DH within the literature range without the need for the extended experiments to determine the heat transfer effects of the surrounding vessel.

(1) Suzuki, Y.; Cousins, D.; Wassgren, J.; Kappes, B. B.; Dorgan, J.; Stebner, A. P.. Compos. Part Appl. Sci. Manuf. 2018, 104, 60–67. https://doi.org/10.1016/j.compositesa.2017.10.022.

(2) UpnaLab. Acoustic Levitator https://www.instructables.com/Acoustic-Levitator/ (accessed 2022 -04 -25).

(3) Poly(methyl methacrylate) | Designerdata https://designerdata.nl/materials/plastics/thermo-plastics/poly(methyl-methacrylate) (accessed 2022 -04 -24).

(4) Jonathan, P.; Lai, C.-S.; CHEN, C.; 2015. https://doi.org/10.13140/RG.2.2.12658.20165.

(5) Melia, T. P. Polymer 1962, 3, 317–319.

(6) Pavlinov, L. I.; Rabinovich, I. B.; Okladnov, N. A.; Arzhakov, S. A. Polym. Sci. USSR 1967, 9 (3), 539–544. https://doi.org/10.1016/0032-3950(67)90072-X.

(7) Rabinovich, I. B.; Lebedev, B. V.; Melent’eva, T. I. Polym. Sci. USSR 1967, 9 (8), 1913–1921. https://doi.org/10.1016/0032-3950(67)90440-6.

(8) Dole, M. Calorimetric Studies of States and Transitions in Solid High Polymers. In Fortschritte Der Hochpolymeren-Forschung; Springer, 1960; pp 221–274.