(426g) Facile Characterization of Pore Accessibility in Metal-Organic Framework (MOF)/Polymer Composites

Metal-organic framework (MOF)/polymer composites provide the possibility of combining the desired reactive and sorptive properties of highly porous MOFs with the desired mechanical properties of polymers to develop novel functional materials. Both MOF and polymer chemistries are complex leading to various degrees of material compatibility. Varying chemical compatibility can influence microscopic properties such as pore intrusion of polymer strands or MOF particle encapsulation that can negatively impact the MOF performance by limiting pore accessibility. It is desired to develop a facile measurement of the accessibility of MOF pore space within the composite matrix. Traditionally, N₂ isotherms at 77K have been used to characterize pore space in porous materials. However, we found that using N₂ isotherms to assess pore accessibility in MOF/polymer composites tends to underestimate the true value. We hypothesized that this was due to the cryogenic temperature of the measurement falling below the glass transition temperature of the polymers in use. We proposed instead to use CO₂ isotherms at 0° C as a measurement of pore accessibility. Both measurements were performed on pure MOF powders HKUST-1, UiO-66-NH₂, and MOF-808 to record BET surface area or CO₂ uptake at STP. Various MOF/polymer composites were synthesized via film casting, electrospinning, or solution blow spinning. TGA was used to calculate the true MOF wt% of the composites. After measuring N₂ and CO₂ isotherms on the composites, the BET SA or CO₂ uptake at STP was compared to the pure MOF values. We found that the CO₂ isotherms gave a much better prediction of the true MOF wt% when compared to TGA results. While we believe this phenomenon is mostly due to the cryogenic temperature of the N₂ isotherms measurement, it was not universal and showed that composite synthesis and morphology also impact the results. Our conclusion is that N₂ isotherms should not be used as a characterization method of MOF/polymer composites and measuring CO₂ isotherms at 0 °C was shown to be a facile, more accurate measurement of pore accessibility.

Acknowledgements:
This work was supported by the Defense Threat Reduction Agency under the Enhanced Synthetic Composites for Advanced Protective Equipment program.