Stress-induced Orientation of Bicontinuous Nanostructures within Randomly End-linked Copolymer Networks

Bicontinuous morphologies with preferred orientation are of interest in a variety of contexts, ranging from membranes to energy dissipating materials. Previously, we have studied the microphase separation of randomly end-linked copolymer networks (RECNs), as a robust method to generate bicontinuous structures. Here, we study the ability to introduce controlled orientation in bicontinuous polystyrene/poly(D, L-lactide) RECNs through uniaxial stretching above the glass transition temperatures, followed by quenching to room temperature. Small angle X-ray scattering (SAXS) shows a progressive increase in orientation along the stretching direction with applied strain, while the primary domain spacing remains unchanged. The evolution of SAXS patterns as a function of strain reveals that the domains initially undergo non-affine stretching, followed by domain rotation accompanied by the rearrangement of interfaces during deformation. A comparison of real-space morphologies obtained from transmission electron microscopy (TEM) tomography indicates an increase in interfacial area and Euler characteristic upon stretching, revealing a change in topology. Orientation of domains by stretching at high temperature is found to yield dramatically improved mechanical toughness.