(694g) Analysis of Two-Dimensional Polymer Films Fabricated Via Solution-Casting

Due to the high efficiency of solution-casting two-dimensional polymers (2DPs) to produce continuous films, there is interest in exploring potential applications of these materials. Correspondingly, there is a need to develop methodologies that will establish the mechanism of formation, physical properties, and techniques for modulating the fabrication of 2DP films. It is suspected that these materials could have physical properties favorable for a variety of applications due to their customizable and highly ordered molecular structure: continuous sheets of molecules that stack due to hydrogen bonding interactions.¹

Due to its simplicity, the 2D-polymer composed of 1,3,5-tris(4-aminophenyl)benzene (TAPB) and terephthalaldehyde (PDA) (the TAPB-PDA 2DP) is used as the basis for developing methodologies that will be used to study other 2DPs that our group is synthesizing. Fourier transform-infrared spectroscopy (FT-IR) is used to observe the reaction that forms the TAPB-PDA 2DP suspension, and quartz crystal microscopy with dissipation (QCM-D) is used to observe the formation of a film after solution-casting TAPB-PDA 2DP suspensions and allowing the solvent to evaporate. Density functional theory vibrational frequency calculations and Noda’s rules for interpreting 2D correlation spectra are used for further analysis. A phase-field model of the propagation of 2DPs that form in solution is also in development.

FTIR is used to observe the reaction that forms the exfoliated TAPB-PDA 2DP suspension in a trifluoroacetic acid (TFA)/water (95/5 v/v%) solvent. This technique is used to observe formation of the TAPB-PDA 2DP suspension that is formed using stoichiometric amounts of reactants (3:2 PDA:TAPB) and using excess PDA (20:1 PDA:TAPB). Comparing these results led to identification of an intermediate species and multiple products: the TAPB-PDA 2DP forms along with a “capped TAPB” structure where each amine of a TAPB molecule reacts with a PDA molecule but does not contribute to the formation of a larger macrostructure. kv correlation analysis and fitting FT-IR absorbance-wavenumber data will be used to yield rate laws that model the kinetics of these reactions.²

The QCM-D technique measures the mass of a substance that is bound to the surface of a sensor as a function of the sensor’s vibrational frequency and associated resonance modes. It also measures energy losses (dissipation) of these resonance modes, which indicates the extent of viscoelasticity or stiffness of a sample. The decrease of penetration depth into a sample with respect to the order of the resonance mode allows for identifying the depth-dependence of these properties in a sample. Fitting QCM-D data to a properly selected model allows for elucidating the local physical state of a system. Therefore, QCM-D can be used to establish the nucleation mechanism by which a solution-casted 2DP suspension forms into film upon solvent evaporation.

QCM-D is used to study the film formation process that occurs during the evaporation of TAPB-PDA 2DP suspensions solution-casted on gold-coated sensors. The evaporation rate of the solvent is controlled by using a flow cell that is attached to a QCM-D module, within which nitrogen gas flows at a controlled mass flow rate. The film formation is analyzed as a function of the concentration and volume of 2DP suspension deposited on the QCM-D sensor and the solvent evaporation rate. Higher dissipation values are detected by the higher resonance modes during solvent evaporation. This indicates that a more rigid and solid-like structure forms further from the sensor, which suggests that the 2DP film forms close to the air-liquid interface. Additional QCM-D data will be interpreted using existing models to identify the nucleation mechanism and kinetics by which 2DP film formation occurs.

[1] Burke, D. W. et. al, Angew. Chem. Int. Ed. 2020, 59, 5165–5171.

[2] Shanmukh, S.; Dluhy, R. A., J. Phys. Chem. A 2004, 108, 5625–5634.