Ultra-Low-Frequency Longitudinal Acoustic Phonon Modes Provide Insight into Polymerization Kinetics of Epoxy and Dual-Cure Resins

In resin-based additive manufacturing, the extent of cure influences the printability and fidelity of the final product. This process has generally been monitored with infrared spectroscopy to compare conversion-sensitive bands to a reference band, but the approach cannot be used for in-situ measurements. On the other hand, Raman spectroscopy, which complements infrared, is ideal for in-situ measurement and has recently become readily accessible for use in applied studies. In addition to the chemical vibrational bands, the Raman spectrum of polymers has an additional peak at ~10–20 cm^-1 due to the amorphous structure. This feature, also known as the Boson peak, arises due to longitudinal acoustic phonon modes (LAMs) which are directly related to the phonon density of states. Utilizing the LAMs, we have developed a "chemically agnostic" method to monitor polymerization kinetics with much better signal-to-noise than traditionally possible. This technique is particularly useful for in-situ studies of epoxy-based resins as the opening of the epoxy ring is extremely difficult to monitor with vibrational spectroscopy. Finally, we demonstrate the detection of transient structural changes during the curing process by monitoring both the Stokes and anti-Stokes lines of the LAMs.