Conversion-Dependent Evolution of Viscoelastic Properties during Copolymer Photocuring

Light-cured copolymers are essential in diverse and rapidly advancing fields, including dental restoratives, coatings, 3D printing, and adhesives, where their mechanical performance critically dictates material applicability and long-term durability. While the development of modulus during copolymerization has been examined as a function of degree of conversion (DC), the underlying mechanism governing this evolution remains thus-far insufficiently understood. Here, we systematically investigated the DC-dependent evolution of viscoelastic properties in typical photocured methacrylate copolymers using a combination of in situ near-infrared spectroscopy, equilibrium swelling, and dynamic nanoindentation. The results show that both storage and loss moduli ( E′ and E″ ) exhibit reproducible, stagewise characteristics with increasing DC. A unified four-stage modulus evolution pathway─rubbery state, vitrification transition, early glass state, and deep-glass state─is established based on the characteristic points of viscoelastic moduli, which can serve as the experimental markers for stage identification. Furthermore, we demonstrate that monomer composition of the copolymer system regulates the entire modulus-conversion trajectory and its stage boundaries, rather than merely affecting end-state moduli. This work provides a conversion-governed viscoelastic evolution map that offers a mechanistic foundation for formulation design, curing optimization, and performance prediction of light-cured cross-linked copolymer systems.