Free-radical photopolymerization plays an important role in a range of industries, including the automotive, electronic, medical, optical, and coating industries. In spite of its wide-spread application, photopolymerization models are surprisingly poor at predicting the evolution of mechanical properties. A key limitation is the lack of experimental data with sufficiently high spatial resolution to validate and parameterize such models.

In this study, photopolymerization of model acrylate resins was investigated by monitoring the changes in mechanical properties of illuminated samples via particle tracking microrheology. The technique enabled us to dissect the kinetics of the process via carefully designed experiments with spatially and temporally modulated illumination profiles.

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.

These advanced microrheology experiments provided unique information that was unattainable by other characterization methods typically used to study photopolymerization and this knowledge can be used to develop a more quantitative understanding of the process.

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