Abstract
We investigate the frontal photopolymerization of a thiol–ene system with a combination of experiments and modeling, focusing on the interfacial conversion profile and its planar wave propagation. We spatially resolve the solid-to-liquid front by FT-IR and AFM mechanical measurements, supplemented by differential scanning calorimetry. A simple coarse-grained model is found to describe remarkably well the frontal kinetics and the sigmoidal interface, capturing the effects of UV light exposure time (or dose) and temperature, as well as the front position and resulting patterned dimensions after development. Analytical solutions for the conversion profile enable the description of all conditions with a single master curve in the moving frame of the front position. Building on this understanding, we demonstrate the design and fabrication of gradient polymer materials, with tunable properties along the direction of illumination, which can be coupled with lateral patterning by modulated illumination or grayscale lithography.
Highlights
Photoinitiated polymerization of multifunctional monomers provides a facile and rapid method for the synthesis of three-dimensional crosslinked polymer networks.[1, 2] Advantages of photopolymerization include that it can be carried out at room temperature, without solvents, and within very short conversion times
We explicitly investigate the interfacial profiles generated by frontal photopolymerization (FPP), the factors governing the profile shape and propagation, and the conditions relevant for pattern development and manufacturing
We find that the FPP model can describe the evolution of the front profile of a thiol-ene system with remarkable accuracy, despite its simplicity
Summary
Photoinitiated polymerization of multifunctional monomers provides a facile and rapid method for the synthesis of three-dimensional crosslinked polymer networks.[1, 2] Advantages of photopolymerization include that it can be carried out at room temperature, without solvents, and within very short conversion times. The solidification front starts from the surface closest to the illuminating source, initially forming a ‘skin’, and invades the uncrosslinked medium. This frontal aspect of the polymerization process is apparent in the photopolymerization (and crosslinking) of thick (mm-cm) material sections, and permits rapid 3D patterning by modulated or multistep illumination[9] (without resorting to stereo-, two-photon or multistep lithography and alignment). FPP is directional and generally isothermal, being controllably initiated and stopped by collimated light exposure, which is advantageous from a manufacturing perspective
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