Gelation, Vitrification and Mobility Arrest in Vat Photopolymerisation: From Network Percolation to Diffusion-Controlled Cure
Segurola, Juan
2026-08-27 · Report · Version 1.1
Vat photopolymerisation is commonly characterised using incident exposure, cured depth and final functional-group conversion. These variables do not identify when a resin first becomes a system-spanning network, when the developing material loses its capacity for viscous relaxation, or when further reaction becomes limited by segmental and radical mobility. Photocuring proceeds through a sequence of physically distinct states. During the pre-gel period, growing chains, branched species and microgel-like structures remain embedded in a material that can still flow over sufficiently long times. Gelation creates a continuous network and establishes a mechanically connected solid, but substantial reactive functionality and soluble material may remain. Post-gel conversion increases network connectivity and modulus while chemical shrinkage increasingly becomes mechanically constrained. Vitrification occurs when the developing network enters a glassy or strongly mobility-restricted regime at the curing temperature. Propagation, termination and structural relaxation then become progressively controlled by diffusion, free volume and reaction-generated topology. Simultaneous photorheology and spectroscopy have demonstrated that network development and double-bond conversion need not evolve proportionally. Classical kinetic studies show strong conversion- dependent changes in propagation and termination, while recent stress-resolved photocuring experiments distinguish gelation, vitrification and reaction saturation as separate stages. Samples engineered to reach similar gelation rates can still develop different final mechanics because their diffusion and reaction histories differ. This review develops a state-resolved framework for vat photopolymerisation. It proposes reporting gel time and gel conversion, vitrification time and vitrification conversion, and the post-gel mobility window separating network percolation from severe mobility arrest. The framework distinguishes cure depth from network state, conversion from connectivity, and chemical shrinkage from stress accumulation. It is intended to support formulation development, exposure optimisation and post-cure design without assigning universal gel or vitrification thresholds to chemically different resins.