Ordering and Grain Growth in Charged Block Copolymer Bulk Films

Recent technological advances have already demonstrated that the morphologies of block copolymer bulk films can be controlled through cautious chemical design and thermal annealing techniques. This has in turn led to sudden interest primarily focusing on how to regulate the orientation of block copolymers nanostructures and minimize the prevalent defects. Presently, thermal processes have been developed in bid to circumvent this shortcoming, however, such processes have been deemed unsuitable for orienting microphase-ordered block copolymers composed of at least one block with charged moieties that can form thermally stable ionic clusters. As a probable solution, solvent-vapor (SV) annealing has been applied to block ionomer bulk films composed of midblock-sulfonated pentablock copolymers. Using this technique, highly ordered morphologies that display evidence of improved in-plane orientation have been achieved. Nonetheless, it is imperative that other similar and yet alternative solvent annealing strategies be assessed.

To this note, Justin J. Ryan (PhD candidate) and Professor Richard J. Spontak at North Carolina State University in collaboration with Dr. Byeongdu Lee at Argonne National Laboratory and Professor Kenneth P. Mineart at Bucknell University compared the effectiveness of three different solvent-related processes and reported the technique that yields the most promising route to morphological ordering and in-plane grain growth. Their work is currently published in the research journal Advanced Materials Interfaces.

The research method employed entailed the application of small-angle X-ray scattering to compare the effectiveness of three solvent-related processes–SV annealing, SV permeation, and SV sorption–on block ionomer ordering and grain growth, and offer explanations for observed differences on the basis of thermodynamic- and transport-related considerations. The methodology employed also involved an in-depth assessment on the influence of the three solvent-related processes in three midblock-sulfonated block ionomers.

The authors observed that, while the SV-permeation approach succeeded, the resultant morphology was not as highly ordered as those generated by either SV-annealing or SV-sorption. In fact, they noted that only SV-annealing yielded highly ordered nanostructural elements that likewise exhibited a moderate degree of orientation with an orientation parameter measuring ≈0.47. According to the authors, the in-plane lamellar grain growth observed could be attributed to the mold-induced lateral constraint with accompanying in-plane epitaxy.

In summary, controlling the orientation of microphase-ordered block copolymers and ionomers is of substantial interest in a variety of nanotechnologies. The study presented a thorough investigation of the influence of SV-annealing, SV-permeation, and SV-sorption on morphological ordering and in-plane grain growth, using three midblock-sulfonated block ionomers. In general, differences in the experimental design of the solvent-related processes were found to affect nanostructural development, as evidenced by the extent of in-plane grain growth. Altogether, insights gleaned from their study provide guidance for the production of block ionomer bulk films requiring anisotropic morphologies that are both highly ordered and oriented, as well as a better understanding of the experimental design of solvent-related processes intended to alter the spatial characteristics of polymer nanostructures.