Efficient Synthesis of Poly(methacrylic acid)-block-Poly(styrene-alt-N-phenylmaleimide) Diblock Copolymer Lamellae Using RAFT Dispersion Polymerization

Abstract

RAFT dispersion polymerization is used to prepare diblock copolymer nano-objects using a poly(methacrylic acid) macromolecular chain transfer agent (PMAA macro-CTA) as the steric stabilizer and AIBN initiator at 70 °C. The core-forming block is a 1:1 alternating copolymer comprising styrene (St) and N-phenylmaleimide (NMI), and the continuous phase is an ethanol/1,4-dioxane mixture. The 1,4-dioxane cosolvent is essential for this formulation because it aids solubilization of the NMI comonomer within the growing diblock copolymer micelles. Even so, kinetic studies reveal a significant retardation effect once micellar nucleation has occurred. More importantly, the relatively high glass transition temperature of the P(St-alt-NMI) core-forming block (Tg = 219 °C) has an interesting influence on the evolution of the copolymer morphology with conversion. At the polymerization temperature of 70 °C, this alternating copolymer is so stiff that 2D lamellae are formed, rather than the vesicular phase that is commonly observed for other RAFT dispersion polymerization formulations. A detailed phase diagram is reported for a series of PMAA79–P(St-alt-NMI)x diblock copolymers, which enables the reproducible synthesis of pure spheres, worms, and the lamellar phase. It is also noteworthy that the worm phase region is unusually broad compared to previous polymerization-induced self-assembly (PISA) formulations. The worms are relatively short and stiff but form free-standing gels above 9% w/w. Increasing the mean degree of polymerization of the core-forming block leads to stronger, more brittle gels. On transferring the diblock copolymer nano-objects into water via dialysis, highly negative zeta potentials are observed above the pKa of the PMAA stabilizer chains, regardless of the copolymer morphology. Thermogravimetric analyses indicate that these diblock copolymer nano-objects have relatively high thermal stabilities, with little or no mass loss being observed on heating in air up to 347 °C.