ABSTRACT
It has been demonstrated that a micropatterned surface can decrease the resistance of anion-exchange membranes (AEMs) and can induce desirable flow properties in devices, such as mixing. Previously, a model that related the resistance of flat and patterned membranes with the same equivalent thickness was proposed, which used the patterned area and thickness ratio of the features to describe the membrane resistance. Here, we explored the validity of the parallel resistance model for a variety of membrane surface designs and area ratios. We demonstrated that the model can predict the resistance of a wide range of patterned AEMs. We showed that the resistance is independent of the spatial ordering of the design by examining random patterns, which is relevant for applications that require, for example, increased mixing in multilayered devices. Some experimental values of resistance obtained for patterned membranes presented deviations from the model. Scanning electron microscopy (SEM) images of the patterned membranes revealed resolution variations and pattern replication errors due to the stereolithographic process. A geometric correction of the target ratios improved the fit of the modeled data to the experimental values, showing that light bleeding during curing was a source of error. Two additional experimental factors were not accounted for in the model: a distinct interface between the bottom and top layer and overcuring of the bottom layer during successive steps. These sources of error were investigated by examining the resistance of single- and double-layered membranes, as well as single-layer membranes with different curing times. The differences obtained in the resistances for control samples demonstrated that both the interface and the overcuring influenced the resistance of the membrane. The results obtained in this study enlighten the discussion relating membrane-surface morphology and transport properties, as well as the optimization of 3D-printed membranes using a stereolithography process.
ABSTRACT
In situ measurement of hydrocarbons in water is critical for assuring the safety and quality of drinking water and in environmental remediation activities such as the cleanup of oil spills. Thus, effective detection methods of hydrocarbons in aqueous environments are important and several methods have been used for this type of sensing, including spectroscopic techniques, fiber optic sensors, and chromatography. However, under aqueous conditions, small amounts of hydrocarbons are difficult to detect due to their low concentration in water and robust sensing of these types of compounds in an aqueous environment remains a challenging analytical task. Hydrophobic polymer coatings have been widely used to concentrate hydrocarbons for attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) detection at the surface of an ATR crystal by preventing water molecules from penetrating into the polymer coating while absorbing hydrocarbons. However, in typical coating designs only thin films (<5 µm) can be applied onto the ATR sensor due to the decrease in detection limit and sensitivity to hydrocarbons with increasing film thickness. This paper demonstrates that a semi-crystalline linear low-density polyethylene (LLDPE) polymer coating with thicker thickness (40 µm) can be applied effectively for in situ ATR-FTIR detection of hydrocarbons in aqueous solution. The ATR signal is enhanced by the polymer coating which swells in response to the hydrocarbons and prevents water accumulation at the IR detection interface. Coating the ATR element with a LLDPE film (crystallinity = 12%) reduced the detection time for various hydrocarbons, including toluene, benzene and chloroform. The detection limits and kinetics of the ATR-FTIR detection were not significantly altered when the thickness of the LLDPE coating was increased to improve its mechanical properties which represents a significant improvement from coatings published in the literature. The LLDPE coating described in this research has the potential to be applied as a sensor coating for rapid detection of hydrocarbon-based substances or non-polar biomolecules in aqueous environments.
