Self-Assembly-Driven Nanomechanics in Porous Covalent Organic Framework Thin Films

Nanomechanics signifies a key tool to interpret the macroscopic mechanical properties of a porous solid in the context of molecular-level structure. However, establishing such a correlation has proved to be significantly challenging in porous covalent organic frameworks (COFs). Structural defects or packing faults within the porous matrix, poor understanding of the crystalline assembly, and surface roughness are critical factors that contribute to this difficulty. In this regard, we have fabricated two distinct types of COF thin films by controlling the internal order and self-assembly of the same building blocks. Interestingly, the defect density and the nature of supramolecular interactions played a significant role in determining the corresponding thin films' stress-strain behavior. Thin films assembled from nanofibers (similar to 1-2 mu m) underwent large deformation on the application of small external stress (Tp-Azo(fiber )film: E approximate to 1.46 GPa; H approximate to 23 MPa) due to weak internal forces. On the other hand, thin films threaded with nanospheres (similar to 600 nm) exhibit a much stiffer and harder mechanical response (Tp-Azo(sphere) film: E approximate to 15.3 GPa; H approximate to 66 MPa) due to strong covalent interactions and higher crystallinity. These porous COF films further exhibited a significant elastic recovery (similar to 80%), ideal for applications dealing with shock-resistant materials. This work provides in-depth insight into the fabrication of industrially relevant crystalline porous thin films and membranes by addressing the previously unanswered questions about the mechanical constraints in COFs.