Modelling the interphase of 3D printed photo-cured polymers

3D printing, in particular the Polyjet technology, has been widely employed for the production of complex heterogeneous composites such as co-continuous architectures (the so-called Interpenetrating Phase Composites, IPCs). It is a manufacturing method in which discrete photopolymer droplets of different materials can be deposited on a build tray and cured by UV light lamp. Previous research already demonstrated how the characteristics of the interface between different photopolymers can vary if formed before or after UV curing process, with the formation of a narrow or broad interphase. In the present work, the dynamic-mechanical properties of multilayer bimaterial composites (made combining a glassy and a rubbery polymer) were investigated under tensile loading, which is relatively insensitive to the spatial arrangement of layers, as opposed to bending. The use of a simple parallel configuration allowed the development of an analytical model which incorporates the properties of the two photopolymers and their interphase, considering their effect on both elastic and dissipative behaviour of the composites; in particular, the interphase behaviour is quite close to that of the glassy polymer and even small quantities are sufficient to dramatically change the overall dissipative behaviour of the composite. It is demonstrated that reliable modelling of real co-continuous architectures as obtained by Polyjet printing (and similar techniques) should include the effect of the interphase.