Understanding the Chemistry and Mechanical Properties of Metal-Organic Framework-Polymer Composites

Metal-organic frameworks (MOFs) are an emerging class of materials exhibiting desirable properties and functionalities for a variety of applications, including catalysis, molecular separation, gas storage, and mechanical reinforcement. However, the majority of MOFs exist as particulate powders, limiting their transportability and applicability in practical fields. Polymers, on the other hand, are one of the most widely used materials in the world owing to their facile processability and low production cost. Combining MOFs and polymers to form MOF-polymer composites can potentially maintain the merits of both materials while overcoming drawbacks of each individual component. Specifically, MOFs are promising candidates as mechanical reinforcers for polymers because of their low density, high specific modulus, and controllable dimensions. Herein, we aim to provide a comprehensive investigation into the chemistry and mechanical properties of MOF-polymer composites. Various governing parameters, including particle aspect ratio (AR), MOF-particle interface, and intrinsic mechanical properties of MOFs, were thoroughly studied to construct an optimal pathway for fabricating mechanically reinforced MOF-polymer composites. Chapter 1 presents an introduction to MOFs, polymer composites, and mechanical properties and characterizations of polymeric materials. It serves as a foundation of this dissertation and outlines essential concepts for the scientific background. The primary factors that impact the mechanical properties of polymer composite are highlighted, leading to the following three research chapters. Comprehensive background on various characterization techniques that aim at mechanical properties is covered in detail. Chapter 2 focuses on the role of MOF AR on the mechanical properties of MOF-polymer composites. PCN-222, a Zr-MOF with porphyrin linkers, was synthesized with AR ranging from 3.4 to 54. The crystallinity and chemical structure of the MOFs remained consistent for different ARs, ensuring that the AR was the only variable in determining the mechanical reinforcement. Fabricated through the doctor-blade technique, the MOF-PMMA composite films showed homogeneous MOF distribution and alignment. Tensile tests revealed that Young's modulus of the composites increased with MOF AR, exhibiting a good agreement with a modified Halpin-Tsai model. Both storage and loss moduli were also enhanced following increased MOF AR. In addition, the thermal stability was also improved with the addition of MOF particles. In Chapter 3, the authors extend the understanding of mechanical properties of MOF-polymer composites to the interfacial properties between the two materials. Pristine MOFs often lack strong interactions with a polymer matrix due to the difference in chemical/physical properties. The authors developed a three-step synthetic route to grow PMMA on the surface of PCN-222. Owing to an efficient surface-initiated polymerization technique, the PMMA was successfully grafted with high molecular weight and grafting density. The molecular weight of PMMA could be controlled by simply varying polymerization time. The PMMA-grafted PCN-222 was manufactured along with PMMA matrix to form composite films. Mechanical analysis proved that the mechanical reinforcement was improved with increasing grafted molecular weight. Chapter 4 presents an experimental approach to unveil the structure-mechanical property of MOF single crystals, which provides insights on designing MOFs with desired mechanical strength. Zeolitic imidazolate frameworks (ZIFs), a subdivision of MOFs, were chosen as the template owing to their facile synthesis, structural diversity, and high crystallinity. Two types of micron-sized ZIFs, ZIF-8 with Zn2+ node and ZIF-67 with Co2+ node, were synthesized to compare the effect of metal-linker bond. Moreover, the linker composition was varied to examine the difference in crystal structure and defect level. The mechanical properties of these ZIF samples were revealed by nanoindentation on single particles. Overall, the stronger metal-linker bond and high crystallinity were able to yield the highest elastic modulus and hardness. Finally, Chapter 5 offers a comprehensive review on polymer-grafted MOF particles regarding the synthesis and applications associated with surface-anchored polymers. Various polymerization techniques were summarized, and their adjustment and limitations with respect to MOFs were highlighted. The novel and unique applications arisen from polymer-grafted MOFs and Mixed Matrix Membranes were thoroughly discussed.