Effects of oxygen on interfacial strength of incremental forming of materials by photopolymerization

Photopolymerization is one of the most widely used methods for additive manufacturing and microfabrication of polymer structures. However, the mechanical properties of these materials, formed incrementally or layer-by-layer by photopolymerization, remain unclear. One critical issue is the strength of the interfaces between adjacent layers. During free radical photopolymerization, these interfaces are exposed to atmospheric oxygen, which is detrimental to the polymerization reaction due to radical inhibition. The influence of oxygen on the interfacial properties, however, is still not well understood. This paper investigates the effect of oxygen on the mechanical behavior of interfaces. In order to facilitate mechanical tests, the interfacial strength is investigated using a part-by-part method that mimics the conventional layer-by-layer photopolymerization process. The experiments found that oxygen enhances the interfacial strength by improving interfacial bridging macromolecular links. A theoretical model is developed to capture the interfacial evolution. Numerical studies further illustrate the role of several processing parameters such as curing condition and resin component.