(611c) 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 model intrinsically disordered proteins using solvophilic and solvophobic segmental beads. The model demonstrates that at fixed composition, small changes in monomer sequence, particularly at the end of the chain, can profoundly impact liquid-liquid phase separation. Here, we show experimentally that sequence controlled synthetic polymers can be used to modulate the critical temperature (T_c). 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 homopolymer blocks of polystyrene (PS) or polyisoprene (PI) in the middle or at the end of a random copolymer chain. Turbidimetry and dynamic light scattering were used to study the phase behavior in n-hexane, a PI-selective solvent, and dimethylacetamide (DMAC), a PS-selective solvent. This selectivity means that a PI block in n-hexane is solvophilic and a PI block in DMAC is solvophobic. Conversely, a PS block in DMAC is is solvophilic and a PS block in n-hexane is solvophobic. When solvophilic homopolymer blocks are placed in the center of the chain, T_c decreases, and when solvophobic homopolymer blocks are placed in the center of the chain, T_c increases. When solvophilic homopolymer blocks are placed at the end of the chain, the T_c decreases due to the formation of very large (100 nm) stable micelles prior to and after macroscopic liquid-liquid phase separation. Finally, when solvophobic homopolymer blocks are placed at the end of the chain, we observe varying phenomena; PI end blocks in DMAC decrease in T_c due to the formation of micelles, whereas PS end blocks in n-hexane do not form micelles and drastically increase T_c. This difference in behavior is due to differing compatibility between the random block and homopolymer block. When the compatibility is sufficiently high, the polymers will not microphase separate, and are therefore, highly incompatible with the solvent. These results are in good quantitative agreement with simulation where the segments at the ends of the chain have a larger impact on phase behavior than those at the center of the chains.