In-situ monitoring of the mechanical properties during the photopolymerization of acrylate resins using particle tracking microrheology
Slopek, Ryan Patrick
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The fundamentals of the photopolymerization process are not well understood. As a result, issues affecting the cure speed and overall quality of the final product (shape, size, and surface finish) of photopolymerization impose significant limitations on applications that require fast processing and high spatial resolution. To address this issue, microrheology was employed to perform in-situ monitoring of the liquid-to-gel transition during free-radical photopolymerization. Photosensitive acrylate and hydrogel resins were exposed to ultraviolet light, while the Brownian motion of micrometer sized, inert fluorescent tracer particles was tracked via optical videomicroscopy. Statistical analysis of particle motion yielded the rheological properties of the embedding medium as a function of time and location, thereby relating UV exposure to the progress of polymerization and gelation. The microrheological setup enabled a detailed study of three-dimensional gelation profiles; other experimental parameters that were initially varied include photoinitiator concentration, monomer composition, and light intensity. Significant changes in gelation time were observed with varying UV intensity and UV penetration depth into the sample. In addition, oxygen inhibition was found to significantly impact the cure speed of monomeric resins. The preliminary results were used to test the accuracy of the energy threshold model, which is often used to empirically predict the outcome of photopolymerization reactions. By using lithographic masks to generate well-defined UV illumination patterns with characteristic dimensions of tens of micrometers, it could be shown unambiguously that the diffusion of oxygen, an inhibitor, plays a critical role in the polymerization reaction. The experiments are in excellent agreement with a simple two-step model of oxygen consumption followed by polymerization. The use of high-speed electronic shutters in the UV light path enabled us to control the illumination time of the samples with high precision. Microrheological analysis could be used to reconstruct three-dimensional profiles of partially polymerized samples. Traditional photorheometry is not capable of resolving the evolution of sample rheology with such spatial resolution. In addition, experiments with pulsed illumination were used to quantify the role of dark reactions due to residual free radicals after termination of UV illumination.