Ordering and Grain Growth in Charged Block Copolymer Bulk Films: A Comparison of Solvent‐Related Processes

Abstract

for ubiquitous nanotechnologies. [1][2][3][4] Thermodynamic incompatibility [5][6][7] drives BCs to order into an assortment of classical (e.g., spherical, cylindrical, or lamellar) or spatially complex (e.g., gyroid, [8] helical, [9] or Fddd [10,11] ) morphologies. Judicious chemical design of BCs in terms of factors, such as chemical constitution [12][13][14] and molecular composition, weight and architecture, [15] as well as targeted blending with solvents [16,17] or other macromolecules, [18,19] yields customized phase behavior and properties, as well as morphologies with specific spatial characteristics. While some BC morphologies are intentionally defective for connectivity purposes, [20,21] most efforts have endeavored to generate near-equilibrium nanostructures that can compare directly with theory [22,23] or establish definitive structure-property relationships. [24] More recently, directed self-assembly strategies that rely on BC crystallization, [25,26] chemical coordination, [26] or surface-induced alignment [27] have resulted in nearly monodisperse nanofibers grown in living fashion, designer superstructures with hierarchical features, or nanotemplates with precise spatial modulation, respectively. Another important aspect of morphological control includes orientation, which is crucial in the development of anisotropic mechanical properties, [28] defect-free diffusive pathways, [29] or wavelength-specific photonic guides. [30,31] Methods that promote morphological refinement and orientation in nonpolar BCs rely on thermal [32,33] and solvent [34,35] treatment.Block ionomers (BIs), charged block copolymers possessing at least one block with ionic moieties, combine the ability of BCs to microphase order [36,37] with the hydrophilic/ionophilic nature of polyelectrolytes and are therefore of interest in fuel cells [38] and water purification. [39,40] In addition, they have been utilized as electroactive media, [41] amphoteric gas-separation membranes, [42][43][44] and organic photovoltaics. [45] Unlike their BC analogs, BIs suffer from a significant complication-the formation of ionic clusters, which often trap nonequilibrium morphologies. To avoid degradation issues of BI bulk films induced by thermal annealing, we have previously reported that the nanostructural elements in BIs can be not only solvent templated [46] during film casting but also solvent annealed [47] to promote equilibration of dried films. Here, we consider three poly[While prior efforts have demonstrated that the morphologies of block copolymer (BC) bulk films can be controlled through judicious chemical design and thermal annealing, recent interest has focused on regulating the orientation of BC nanostructures and minimizing defects. Thermal processes developed to achieve this purpose for nonpolar BCs are not, however, suitable for orienting microphase-ordered BCs composed of at least one block with charged moieties that can form thermally stable ionic clusters. To overcome this challenge, we have previously applied solvent-vapor (...

100th Anniversary of Macromolecular Science Viewpoint: Block Copolymers with Tethered Acid Groups: Challenges and Opportunities

“…To improve the mechanical properties of the acid-tethered block copolymers, crystalline hydrophobic blocks with high T g were tested. − These effectively mitigated the swelling behavior; however, obtaining equilibrium morphologies with thermal annealing became difficult. More complex block architectures were also envisaged, as shown by the representative examples in Figure b–d. − , Placing the acid-tethered polymers in the midblock and hydrophobic polymers at both ends of the triblock copolymers created firm bridges. − …”

“…The current research progress in acid-tethered polymers is closely related to the controlled synthesis of high-precision polymers with tailor-made monomers. − To date, various types of acid moieties have been added to several polymers with tunable glass transition temperatures ( T g ) − and polarities. − This has enabled the high-yield preparation of proton-conducting polymers with linear diblock, ,,− triblock, , pentablock, , and comb shapes . Therefore, the exploration of the structure–transport relationship became possible.…”

“…47−50 To date, various types of acid moieties have been added to several polymers with tunable glass transition temperatures (T g ) 51−53 and polarities. 52−54 This has enabled the high-yield preparation of proton-conducting polymers with linear diblock, 46,52,55−65 triblock, 52,66 pentablock, 67,68 and comb shapes. 69 Therefore, the exploration of the structure−transport relationship became possible.…”

“…More complex block architectures were also envisaged, as shown by the representative examples in Figure 1b−d. [66][67][68]87 Placing the acid-tethered polymers in the midblock and hydrophobic polymers at both ends of the triblock copolymers created firm bridges. 66−68 While the postmodification approach is the most powerful and efficient means of preparing the acid-tethered polymers, the inherent heterogeneity of the chemical compositions resulting from the random reactions is a critical obstacle.…”