Tensile Testing. Tensile testing was carried out on an Instron 3365 with a 5 kN load cell, using an AVE2 video extensometer to measure strain. Dumbbell samples were prepared to ASTM D638 Type IV specifications, and specimen measurements read off before testing using digital callipers. The procedure for evaluating the tensile modulus is described in the SI. A rule of mixtures (RoM) was fit to the modulus (𝐸), and vol.% (𝑉) data: