Precision Library Synthesis of PS–PMMA Block Copolymers with Discrete Junctions: Cooperative Self-Assembly to Break the Phase-Separation Limit

We achieved the formation of a lamellar structure via phase separation of a polystyrene-poly(methyl methacrylate) (PS–PMMA) block copolymer (BCP) with a relatively low molecular weight [M_n ∼ 11,400, DP_n = 42 (PS), 69 (PMMA)] through the introduction of a discrete bis-urea component at the junction. This is a remarkable achievement, given that the theoretical minimum molecular weight required for phase separation of PS–PMMA BCPs is 28,000. A library synthesis of the junction structure, while keeping the molecular weights of the two polymer segments fixed, led to the discovery of this unprecedented behavior. A cyclohexylene spacer between the urea bonds is crucial for affording an ordered structure, and even a slight change of the spacer structure resulted in the formation of a disordered structure. Fourier-transform infrared spectroscopy analyses revealed that hydrogen-bonding interactions between the bis-urea junctions are indispensable for a successful phase separation. The hydrogen-bonding interactions at the junction cooperate with the phase separation of the BCP to stabilize transient lamellae formation during thermal annealing at 150 °C. Further evidence for this cooperative self-assembly process was obtained from experiments with blends of junction-matched/mismatched BCPs.