Insights into the photopolymerisation dynamics of crosslinker-free resin, at 3D printer voxel resolution

We study monomer-photopolymerisation-printing without a cross-linker, at voxel scale. The model system is the free radical polymerisation of iso-bornylacrylate monomer (IBOA), photoinitiated by (2,4,6-trimethylbenzoyl) diphenylphosphine oxide (TPO-L), examined in situ by confocal fluorescence microscopy. Specifically, we examine an ≈ 10 μ m thick sample, diameter ≈ 100 μ m between two glass coverslips. These conditions are representative of a single voxel in an incremental layer in commercial digital light printing devices. Fluorescence lifetime imaging of probe species, BODIPY-C12 and perylene diimide, monitors and maps viscosity in and around the irradiated area, highlighting diffusive flows competing with photo-initiation. The expected outward flux of polymer chains escaping from the irradiated area is shown directly. The balance between this flux and chain entanglement determines formation of a stable polymer voxel. The balance dependends notably on the rate of initiation (UV irradiance and photoinitiator concentration) and the size of the UV irradiated area. We thus exhibit and understand the conditions for printing a stable thermoplastic voxel.