Harnessing Metastability: Process-Directed Formation of Nonequilibrium Structures in Diblock Copolymers

While the equilibrium of diblock copolymers are rather well understood, these macromolecular systems often become trapped in metastable structures rather than reaching equilibrium. This feature may result in protracted annealing times but also offers opportunities to fabricate structures with unique characteristics. Process-directed structure formation refers to reproducibly trapping the kinetics of structure formation into a desired (meta)stable target structure after a quench of the thermodynamic state, such as solvent evaporation or light-stimulated chemical conversions. This strategy leverages several unique advantages of copolymer systems, including the comprehensive knowledge of equilibrium properties and a clear timescale separation between the quench of thermodynamic variables, the system's spontaneous relaxation toward the nearest metastable structure, and its thermally activated escape toward equilibrium. Both particle-based and continuum models will be discussed, highlighting challenges for quantitatively predicting copolymer material processing at the molecular scale.

One prominent example is SNIPS, a bottom-up method for fabricating integral-asymmetric, isoporous block copolymer membranes. Initially, evaporation-induced self-assembly (EISA) creates a functional, well-ordered top layer of perpendicular cylindrical domains for selective ultrafiltration. Following this, nonsolvent-induced phase separation (NIPS) produces a macroporous support from the same material. Different applications may demand specific properties (e.g., varying thickness of the isoporous layer), yet rational design remains challenging. Highly coarse-grained particle-based and continuum models offer insights into the final nonequilibrium structure, which is shaped by multiple physical phenomena (e.g., solvent evaporation, self-assembly, solvent-nonsolvent exchange, macrophase separation, and glassy arrest) and influenced by numerous structural, thermodynamic, kinetic, and processing characteristics.