Tailoring the morphology in partially and fully miscible mixtures of PEEK and PEI

Over the last three decades, the vast body of work related to the control of the morphology of multiphase polymers has concentrated on systems that are inherently immiscible. On the other hand, detailed morphological studies of phase separation in miscible blends have been limited by the highly unstable nature of the structures generated as a function of time and temperature. In this work, we present one basic polymer system, with a slight variation in chemical structure, that allows for a high level of morphological tailoring and control in the miscible to partially miscible region. Two close isomeric forms of polyetherimides (PEI) yield fundamentally different types of morphologies in polyetheretherketone (PEEK)/PEI melt-processed multiphase systems: a partially miscible, phase-separated microstructure for the barely studied PEEK/para-PEI (p-PEI) system, and the typically reported fully miscible PEEK/meta-PEI system (m-PEI). The PEEK/p-PEI system displays sub-μm, matrix/dispersed phase or co-continuous types of morphologies, with the latter quickly coarsening over tens of μm in length scale under quiescent annealing conditions due to the PEEK/p-PEI interfacial tension, which was measured by the breaking thread method at 0.14 mN/m, one of the lowest values ever reported for a polymer pair. On the other hand, for the PEEK/m-PEI system, controlling the thermal annealing temperature promotes PEEK recrystallization and the formation of a nanostructured PEI-rich phase. Considering both types of blends, it is then possible to control the morphological length scale of PEEK and PEI domains over nearly 4 orders of magnitude, from ≈5 nm to over 15 μm. This is the smallest domain size ever reported for co-continuous systems. The selective extraction of the PEI phase then results in porous PEEK monoliths with full pore interconnectivity, with an average pore size spanning the same considerable range – without any interfacial modifier or block copolymer.