Closed-loop control of polymerization fronts during Frontal Polymerization of DCPD

Frontal polymerization (FP) is a widely studied, autocatalytic curing process that enables fast and efficient manufacturing of polymers and composites. Of particular interest is the extent to which FP makes it possible to rapidly and simply introduce spatial variation in material properties. In prior work, control of this spatial variation has been the result of open-loop methods, for example, embedding phase-changing particles in resin undergoing FP. These methods make alterations to the system only before or during initiation and therefore do not account for disturbances to the front’s behavior due to uncertainty in initial conditions, boundary conditions, and formulation. Additionally, whereas the front’s profile, i.e., the shape and speed of propagation, is altered by these methods, they cannot converge the profile to a specific target. Instead, its alteration is a byproduct of controlling other states such as front instabilities or cured surface morphology. In this work, for the first time, we demonstrate control of FP by directly controlling the front’s profile during propagation via closed-loop control, an approach that is able to automatically detect unexpected disturbances during curing and alter how it behaves accordingly. Our approach has two parts. The first part is a real-time algorithm that detects FP fronts from streams of thermal images. We show that this algorithm is robust to changes in camera, component geometry, and composition, performing equally well when applied to neat, composite, and gelled and extruded resin FP systems. The second part is a real-time algorithm that chooses where to apply thermal inputs with a moving ultraviolet (UV) source to steer detected FP fronts so that they converge to a target profile. Proof-of-concept hardware experiments show that closed-loop control of front propagation allows us to enforce two representative target profiles—each differing significantly from uncontrolled profiles. We also find that closed-loop control reduces the fronts’ variability when compared to trivial open-loop control, i.e., uncontrolled FP, despite uncertainty in initial conditions, boundary conditions, and formulation. These results suggest that our approach could be used, in the future, to tailor the speed of manufacturing, the cured shape of polymer and composite components, and polymer patterning induced by front merging in ways that were previously unattainable.