Effect of Versatile Structural Design on Properties and Applications of Two-Dimensional Conjugated Microporous Polymer Thin Films
Miller, Kristen A.
Ajayan, Pulickel M.; Thomas, Edwin L.
Doctor of Philosophy
Microporous two-dimensional (2D) polymers have great potential in many applications given their covalent bonding in two dimensions, extended conjugated structures, high surface areas and functional design. However, their widespread implementation in practical application spaces has so far been limited by the lack of facile and scalable processing methods from their generally insoluble forms. In this work, the inspiration for structural design is drawn from the molecular construct of high-strength, high-modulus 1D polymers that have produced some of the strongest materials to date, particularly poly(p-phenylene-2,6-benzobisoxazole) (PBO) or Zylon. Herein, a series of novel solvothermal bottom-up methods were developed to directly synthesize 2D covalently linked polymer films from starting material solutions. Benzoxazole-linked films were produced via a two-step process that allows the deposition of a uniform intermediate film network via reversible, non-covalent interactions, followed by a subsequent solid-state annealing step that facilitates the irreversible conversion to the desired polymer product. The versatility of this synthesis method is demonstrated by producing films with four different aromatic core units. Slight modification of the synthetic methods enables the direct synthesis of reversibly linked imine and hydrazone-based films using a metal triflate catalyst with a single processing step. The resulting 2D polymer films are amorphous yet demonstrate microporosity and an anisotropic layered morphology that can be exfoliated into few-layer nanosheets. The hydrazone-linked films exhibit more compact planar stacking and bright photoluminescence due to the constrained planar conformation induced by intramolecular hydrogen-bonding. These 2D polymer films are comprised of highly aromatic, conjugated building blocks providing an opportunity to translate the mechanical performance of classic rigid-rod 1D polymers across a plane by extending covalent bonding into two dimensions, while simultaneously reducing density. The demonstrated mechanical properties from tensile testing and nanoindentation show axial tensile and transverse compressive elastic moduli on the scale of several GPa, rivalling the performance of solution-cast films of 1D PBO, as well as several other 1D high-strength polymer films. The structural effect of linkage chemistry is compared via nanoindentation, showing that the benzoxazole-linked films exhibit higher modulus and hardness relative to the more compliant reversibly linked counterparts.