  Bicontinuous microstructures are essential to the function of diverse natural
and synthetic systems. Their synthesis has been based on two approaches:
arrested phase separation or self-assembly of block copolymers. The former is
attractive for its chemical simplicity, the latter for its thermodynamic
robustness. Here, we introduce Elastic MicroPhase Separation (EMPS) as an
alternative approach to make bicontinuous microstructures. Conceptually, EMPS
balances the molecular-scale forces that drive demixing with large-scale
elasticity to encode a thermodynamic length scale. This process features a
continuous phase transition, reversible without hysteresis. Practically, we
trigger EMPS by simply super-saturating an elastomeric matrix with a liquid.
This results in uniform bicontinuous materials with a well-defined microscopic
length-scale tuned by the matrix stiffness. The versatility and robustness of
EMPS is further demonstrated by fabricating bicontinuous materials with
superior mechanical properties and controlled anisotropy and microstructural
gradients.
