Electrical anisotropy and its mitigation in conductive polymers printed by vat photopolymerization

In most additive manufacturing (AM) technologies, objects are realized layer by layer. This layer-by-layer construction leads to inherent anisotropic physical properties. Controlling, understanding and sometimes mitigating such anisotropy is a critical issue in the development of AM. Electrical anisotropy in conductive nanocomposites processed by Vat photopolymerization is demonstrated and quantified in the present work. In the used method, which enjoys high resolution and high speed, layers of an acrylate based resin, are successively cross-linked by UV irradiation of 2D patterns. Carbon nanotubes are used as conductive fillers for their low percolation threshold that allows realizing conductive and still sufficiently transparent materials for UV polymerization. Conductivity parallel to the layers of 3D objects is found to be much greater than conductivity perpendicular to the layers. This electrical anisotropy is explained by the high contact resistance between printed layers. High contact resistance results from the slow diffusion of carbon nanotubes from the uncured material towards the interface of the cured object. It is found that implementing a delay time before curing successive layers, or decreasing the matrix viscosity with temperature, to promote diffusion of the conductive particles allow substantial reduction of the contact resistance between layers. As a result, conductivity anisotropy can be reduced by almost two orders of magnitude. This control and mitigation of conductivity anisotropy allows reconciliation of the high resolution of the Vat photopolymerization technology with the possibility to realize uniform 3D materials.