<i>In Situ</i> Study of Evaporation-Induced Surface Structure Evolution in Asymmetric Triblock Terpolymer Membranes

Evaporation-induced asymmetric triblock terpolymer membrane formation from polyisoprene-<i>block</i>-polystyrene-<i>block</i>-poly­(4-vinyl­pyridine) (ISV) that relies on self-assembly of doctor bladed solutions was studied using <i>in situ</i> grazing incidence small-angle X-ray scattering (GISAXS). Transient ordered structures were observed for two ISV terpolymers at intermediate evaporation times in the top surface layers of the films as a function of molar mass and solution concentration. Analysis of the GISAXS patterns revealed the evolution from disordered to ordered structures including a transition from body-centered cubic (BCC) to simple cubic (SC) lattices and finally to an amorphous mesoscale structure. The BCC to SC transition solves an apparent structural puzzle resulting from comparisons of, on one side, earlier quiescent solution SAXS studies suggesting BCC terpolymer micelle structures at higher concentrations and, on the other side, electron microscopy studies consistent with SC lattices originating from polymer micelles in the top separation layer of asymmetric ISV membranes. Gaining insights into the structural evolution of asymmetric triblock terpolymer film formation may enable further optimization of self-assembly plus non-solvent-induced phase separation (SNIPS) based high performance isoporous asymmetric block copolymer ultrafiltration membranes