Modulating Solution Phase Behavior through Block-Random Copolymer Sequence

Monomer sequence in synthetic and biological soft matter has been shown to play a critical role in material phase behavior. Recent work has used coarse-grained molecular simulations to demonstrate that small changes in sequence of intrinsically disordered proteins at fixed composition, particularly at the end of the chain, can profoundly impact the liquid-liquid phase separation of these molecules. Here, anionic polymerization was used to synthesize styrene-isoprene copolymers with similar molecular weight, polydispersity, and overall composition (50/50 wt% of styrene/isoprene) but small changes (5-10%) in monomer sequence. The sequence was altered by placing blocks of polystyrene (PS) or polyisoprene (PI) at the end or in the middle of a random copolymer chain. Dynamic light scattering was used to study the phase behavior in n-hexane. A PI block at the end of the chain significantly lowers the critical temperature (T_c) and promotes the formation of very large (100 nm) stable micelles prior to and after macroscopic liquid-liquid phase separation. Conversely, a PI block in the center of the chain increases the T_c. When the short blocks are PS, the effect is opposite and more pronounced; a block at the end of the chain increases T_c and a block in the center decreases T_c. Finally, for blocks of both PS and PI, the shift in T_c is larger when the block is placed at the end of the chain, in qualitative agreement with simulations.