Corinne A. Stone

Enhanced Stability of Cu-BTC MOF via Perfluorohexane Plasma-Enhanced Chemical Vapor Deposition

Metal organic frameworks (MOFs) are a leading class of porous materials for a wide variety of applications, but many of them have been shown to be unstable toward water. Cu-BTC (1,3,5 benzenetricarboxylic acid, BTC) was treated with a plasma-enhanced chemical vapor deposition (PECVD) of perfluorohexane creating a hydrophobic form of Cu-BTC. It was found that the treated Cu-BTC could withstand high humidity and even submersion in water much better than unperturbed Cu-BTC. Through Monte Carlo simulations it was found that perfluorohexane sites itself in such a way within Cu-BTC as to prevent the formation of water clusters, hence preventing the decomposition of Cu-BTC by water. This PECVD of perfluorohexane could be exploited to widen the scope of practical applications of Cu-BTC and other MOFs.

Reversible water uptake by a stable imine-based porous organic cage

A crystalline porous organic cage molecule, CC3, is shown to adsorb up to 20.1 wt% water reversibly. This was confirmed by both gravimetric sorption and by crystallographic analysis. Crystals of CC3 are stable in boiling water for at least 4 h. The surprising chemical and supramolecular stability of these imine-based molecular crystals suggests scope for practical applications in humid environments.

Charge assisted tailoring of chemical functionality at electrospun nanofiber surfaces

Electrospinning using positive and negative polarity applied voltages is used to produce polyamide nanofibers with tailored surface functionality. The surface free energy of the resultant nanofibers is characterized from individual nanofiber wetting experiments. The polar contribution to the total nanofiber surface energy is seen to vary with the polarity of the applied voltage used. A mechanism to describe the change in the nanofiber surface free energy with electrospinning polarity is proposed, based on the formation of either positive or negative charges at the liquid jet–air interface during the electrospinning process. These charges at the liquid jet–air interface cause molecular orientation of chemical functional groups of the polymer chains during electrospinning and the mechanism is supported by subsequent grazing angle X-ray photoelectron spectroscopy (XPS) of the nanofiber surfaces. A one-step electrospinning process is therefore demonstrated to tailor specific chemical functionalities at polymer nanofiber surfaces.

Wetting Hierarchy in Oleophobic 3D Electrospun Nanofiber Networks.

Wetting behavior between electrospun nanofibrous networks and liquids is of critical importance in many applications including filtration and liquid-repellent textiles. The relationship between intrinsic nanofiber properties, including surface characteristics, and extrinsic nanofibrous network organization on resultant wetting characteristics of the nanofiber network is shown in this work. Novel 3D imaging exploiting focused ion beam (FIB) microscopy and cryo-scanning electron microscopy (cryo-SEM) highlights a wetting hierarchy that defines liquid interactions with the network. Specifically, small length scale partial wetting between individual electrospun nanofibers and low surface tension liquids, measured both using direct SEM visualization and a nano Wilhelmy balance approach, provides oleophobic surfaces due to the high porosity of electrospun nanofiber networks. These observations conform to a metastable Cassie-Baxter regime and are important in defining general rules for understanding the wetting behavior between fibrous solids and low surface tension liquids for omniphobic functionality.

Computational chemistry studies of phenolic resin

Different computational techniques have been used to study phenolic resins with the objective of developing methods for predicting the high temperature physics and chemistry of resins with sufficient accuracy to produce data for the engineering modelling codes used to design thermal protection systems. The building of structural models of the resin using MOPAC ® and the validation of these models through comparison of measured and calculated thermochemical, structural and spectroscopic properties is described. Using these structural models, high temperature resin chemistry is simulated using the reactive molecular dynamics programme ReaxFF. Preliminary results are presented for early stage gas evolution and final stage char production during pyrolysis.

The influence of humidity on the protective performance of a membrane based on poly(vinyl alcohol)

A solid-state NMR study has been used to rationalize the way humidity affects the barrier properties of a polyvinyl alcohol-based membrane. Polymer blends composed of polyvinyl alcohol–polyethyleneimine (PVOH–PEI) and polyvinyl alcohol–polydiallyldimethyl ammonium chloride (PVOH–PDADMAC) were investigated. At 90% relative humidity the PVOH–PEI material retains a PVOH-rich component that is detected in a cross-polarization NMR experiment and is therefore assumed to be relatively immobile. The barrier properties of this material are also largely retained at this relative humidity. Under the same conditions of humidity no signal is detected in the cross-polarization NMR experiment for the PVOH–PDADMAC system and the barrier properties of the PVOH–PDADMAC system are compromised. The persistence of the more rigid domains in the PVOH–PEI material is used to explain the barrier properties of this blend at high relative humidity. For the PVOH–PDADMAC system the cross-polarization signal is recovered and the barrier properties of the blend are restored to their original level as the humidity is lowered.

Hydrolytic stability in hemilabile metal–organic frameworks

Highly porous metal–organic frameworks (MOFs), which have undergone exciting developments over the past few decades, show promise for a wide range of applications. However, many studies indicate that they suffer from significant stability issues, especially with respect to their interactions with water, which severely limits their practical potential. Here we demonstrate how the presence of ‘sacrificial’ bonds in the coordination environment of its metal centres (referred to as hemilability) endows a dehydrated copper-based MOF with good hydrolytic stability. On exposure to water, in contrast to the indiscriminate breaking of coordination bonds that typically results in structure degradation, it is non-structural weak interactions between the MOF’s copper paddlewheel clusters that are broken and the framework recovers its as-synthesized, hydrated structure. This MOF retained its structural integrity even after contact with water for one year, whereas HKUST-1, a compositionally similar material that lacks these sacrificial bonds, loses its crystallinity in less than a day under the same conditions.The promise shown by metal–organic frameworks for various applications is somewhat dampened by their instability towards water. Now, an activated MOF has shown good hydrolytic stability owing to the presence of weak, sacrificial coordination bonds that act as a ‘crumple zone’. On hydration, these weak bonds are cleaved preferentially to stronger coordination bonds that hold the MOF together.