Co-continuous Nanostructure Formation by Randomly-Branched Copolymers of Polystyrene and Poly(D,L-lactide)

Co-continuous polymer nanostructures (CPNs) have drawn much attention due to their synergistic combination of the properties of constituting polymers in three dimensions. While CPNs have been found to form robustly across a wide composition window of ~ 30 wt. % within randomly linked copolymer networks, these systems do not easily allow the formation of thin membranes due to their covalent network architectures. As a solution-processible alternative, we selected randomly-branched block copolymers, with the expectation that thin-films of CPNs could be fabricated while maintaining wide co-continuous composition windows. Mono- and di-azido-terminated polystyrene (PS) and poly(D,L-lactide) (PLA) were chosen as starting blocks due to their incompatibility, and they were coupled with multi-alkynes by copper-catalyzed azide-alkyne cycloaddition. To prevent gelation, the average polymer functionality (f_p) was tuned by the ratio of mono- and di-azido terminated polymers, and crosslinker functionality (f_c) was controlled by selecting one of three multi-functional alkynes. For each system, the composition of PS and PLA was varied and the window of co-continuity was determined by gravimetry and small-angle X-ray scattering. We found that the breadth of the co-continuous window widened upon increasing f_p from unity toward the gel point for each value of f_c, and that systems with f_c = 4 could achieve a co-continuous window as wide as the previous network architectures, while maintaining the ability to form thin films by solution processing.