Oxygen Inhibition Of Radical Polymerizations Investigated With The Rheometric Quartz Crystal Microbalance

The use of the rheometric quartz crystal microbalance (QCM) as a quantitative cure monitor for photocuring systems was demonstrated. The technique requires the use of films with thicknesses in the micrometer range and is based on the direct measurement of the rheological properties of the film at a frequency of 15 MHz. To access curing processes much faster than the similar to 1 s resolution of the QCM measurement, a rotating shutter system was designed to capture processes with a temporal resolution of similar to 1 ms. The photocuring process was studied in both air and nitrogen environments and was applied to a traditional, radically polymerized acrylic system and a thiol-ene based system. Thiol-ene systems cured in oxygen reach comparable stiffnesses to those cured in nitrogen but take longer to cure and contain 1-2.5 wt % oxygen in the final materials. These results indicate that the QCM can be utilized as an effective photorheometric technique able to provide useful mechanistic insight into photoreactive processes.