Quantifying Dual‐Color Photopolymerization in Xolography via Raman Spectroscopy

ABSTRACT Xolography is a volumetric 3D printing technology utilizing two different wavelengths of light to produce objects with micrometer resolution and excellent surface quality within minutes. Dual‐color photoinitiators (DCPIs) are crucial for spatial control of polymerization. However, their performance has mainly been assessed indirectly by evaluating printed objects. In this study, we present a methodology using in situ Raman spectroscopy to characterize the dual‐color curing response of a representative spiropyran‐based DCPI in a methacrylate‐based resin composition. A dual‐wavelength irradiation setup has been developed to monitor the degree of curing (DoC) as a function of the applied light doses, providing quantitative, real‐time insights into bond conversion and polymerization kinetics, as well as enabling the Xolography process window to be derived. The DCPI exhibited a contrast of 89%, showing improved performance compared to first generation systems, while its efficiency was benchmarked against a benzophenone reference. Spatially resolved Raman mapping of volumetric test prints revealed DoC gradients ranging from 40% to 80% directly after printing. We demonstrate that subsequent posttreatments can eliminate these nonuniformities, increasing the DoC to > 90%. This approach provides a framework for evaluating novel dual‐color photoinitiator systems, facilitating the rational design of resins, illumination protocols and optimized configurations for volumetric 3D printing.