Jeanne M. Hossenlopp

Aluminum-containing layered double hydroxides: the thermal, mechanical, and fire properties of (nano)composites of poly(methyl methacrylate)

Hydrotalcite-like anionic clays or layered double hydroxides (LDHs) of the general formula [MII1−xMIIIx(OH)2]intra[(CH2CH(CH2)8COO−)x·nH2O]inter, with MIII = Al and MII = Co, Ni, Cu and Zn, have been prepared by the co-precipitation method and used to prepare nanocomposites with poly(methyl methacrylate) (PMMA). One goal of this work was to compare the morphology, thermal, fire and mechanical properties with those of the well known PMMA–montmorillonite system. The thermal properties of these systems are greatly improved relative to the virgin PMMA, with ZnAl2 and CoAl2 being the best systems when 50% mass loss is the point of comparison. NiAl2, on the other hand, is more thermally stable when 10% mass loss was the point of comparison. The mechanical properties, such as Youngs Modulus and elongation, were not significantly impacted by nanocomposite formation. The cone calorimetric results showed that the PMMA–CoAl2 system gives the best reduction (41%) in peak heat release rate (PHRR); this value is significantly larger than that seen for the PMMA–montmorillonite system. Small improvements were observed for the nickel-containing LDH with the same polymer. XRD of the char produced in the cone calorimeter, and after heating to 1000 °C, suggest the formation of a mixture of the spinel and the MII oxide for Zn, Cu and Ni systems while only the spinel was identified in the case of PMMA–CoAl2 systems.

Removal of 2,4-dichlorophenoxyacetic acid by calcined Zn–Al–Zr layered double hydroxide

The adsorption equilibrium, kinetics, and thermodynamics of removal of 2,4-dichlorophenoxy-acetic acid (2,4-D) from aqueous solutions by a calcined Zn-Al layered double hydroxide incorporated with Zr(4+) were studied with respect to time, temperature, pH, and initial 2,4-D concentration. Zr(4+) incorporation into the LDH was used to enhance 2,4-D uptake by creating higher positive charges and surface/layer modification of the adsorbent. The LDH was capable of removing up to 98% of 2,4-D from 5 to 400 ppm aqueous at adsorbent dosages of 500 and 5000 mg L(-1). The adsorption was described by a Langmuir-type isotherm. The percentage 2,4-D removed was directly proportional to the adsorbent dosage and was optimized with 8% Zr(4+) ion content, relative to the total metals (Zr(4+)+Al(3+)+Zn(2+)). Selected mass transfer and kinetic models were applied to the experimental data to examine uptake mechanism. The boundary layer and intra-particle diffusion played important roles in the adsorption mechanisms of 2,4-D, and the kinetics followed a pseudo-second order kinetic model with an enthalpy, ΔH(ads) of -27.7±0.9 kJ mol(-1). Regeneration studies showed a 6% reduction in 2,4-D uptake capacity over six adsorption-desorption cycles when exposed to an analyte concentration of 100 ppm.

ATR-FTIR spectroscopic analysis of sorption of aqueous analytes into polymer coatings used with guided SH-SAW sensors

Attenuated total internal reflectance Fourier transform infrared (ATR-FTIR) spectroscopy was used for the investigation of sorption of aqueous solutions of analytes into polymer coatings. A series of simple model polymers, such as poly(dimethylsiloxane), poly(epichlorhydrin), and poly(isobutylene), and films and analytes, such as aqueous solutions of ethylbenzene, xylenes, toluene, and nitrobenzene, were used to evaluate the use of ATR-FTIR spectroscopy as a screening tool for sensor development. The ratios of integrated infrared absorption bands provided a simple and efficient method for predicting trends in partition coefficients. Responses of polymer-coated guided shear horizontal surface acoustic wave (SH-SAW) sensor platforms to the series of analytes, using polymer coatings with similar viscoelastic properties, were consistent with ATR-FTIR predictions. Guided SH-SAW sensor responses were linear in all cases with respect to analyte concentration in the tested range. Comparison of ATR-FTIR data with guided SH-SAW sensor data identifies cases where mass loading is not the dominant contribution to the response of the acoustic wave sensor. ATR-FTIR spectra of nitrobenzene, coupled with computational chemistry, provided additional insight into analyte/polymer interactions.

Applications of Acoustic Wave Devices for Sensing in Liquid Environments

Abstract Acoustic wave devices such as thickness shear mode (TSM) resonators and shear horizontal surface acoustic wave (SH‐SAW) devices can be utilized for characterizing physical properties of liquids and for chemical sensor applications. Basic device configurations are reviewed and the relationships between experimental observables (frequency shifts and attenuation) and physical properties of liquids are presented. Examples of physical property (density and viscosity) determination and also of chemical sensing are presented for a variety of liquid phase applications. Applications of TSMs and polymer‐coated guided SH‐SAWs for chemical sensing and uncoated SH‐SAWs for “electronic tongue” applications are also discussed.

Adsorption kinetics, isotherms and thermodynamics of atrazine removal using a banana peel based sorbent

Atrazine removal from water by treated banana peels was studied. The effect of pH, contact time, initial atrazine concentration, and temperature were investigated. Batch experiments demonstrated that 15 g L(-1) adsorbent dosage removed 90-99% of atrazine from 1-150 ppm aqueous solutions. The removal was both pH and temperature dependent with the most atrazine removed between pH 7 and 8.2 and increased with increasing temperature. Equilibrium data fitted well to the Langmuir and Redlich-Peterson models in the concentration and temperature ranges investigated, with a maximum adsorption capacity of 14 mg g(-1). Simple mass transfer models were applied to the experimental data to examine the adsorption mechanism and it was found that both external mass transfer and intraparticle diffusion played important roles in the adsorption mechanisms. The enthalpy of atrazine adsorption was evaluated to be 67.8 ± 6.3 kJ mol(-l) with a Gibbs free energy of -5.7 ± 1.2 kJ mol(-1).

Thermal stability and degradation kinetics of polystyrene/organically- modified montmorillonite nanocomposites

Organically-modified montmorillonite (MMT) clays have been prepared using ammonium salts containing quinoline, pyridine, benzene, and styrenic groups. The nanocomposites were prepared by melt blending and the formation of nanocomposites was characterized using X-ray diffraction (XRD) and transmission electron microscopy (TEM). Thermal stability and flammability were evaluated by thermogravimetric analysis (TGA) and cone calorimetry measurements, respectively. The presence of modified MMT at 5% loading resulted in significant improvement in thermal stability compared to the virgin polymer. Effective activation energies for mass loss were determined via a model-free isoconversional approach from TGA data obtained under N2 and under air. The additives served to raise the activation energy, with a more significant impact observed under pyrolysis conditions. The onset temperature of degradation and temperature of maximum decomposition rate are increased, while the peak heat release rate and mass loss rates are significantly reduced in the presence of three of the modified clays. No reduction in the total heat released is observed.

Design considerations for high sensitivity guided SH-SAW chemical sensor for detection in aqueous environments

Guided shear horizontal surface acoustic wave (guided SH-SAW) devices coated with a polymer waveguiding layer and/or chemically sensitive layer have been investigated for the detection of analytes in liquid environments in our previous work. Design considerations for optimizing these devices for liquid phase detection is the focus of the current work. Using dual delay line geometry on LiTaO/sub 3/, guided SH-SAW sensors are designed and analyzed. The reference line, used to correct for changes in environmental conditions such as temperature fluctuations, is coated with a waveguiding layer of poly(methylmethacrylate) (PMMA). The sensing line is coated either with a polymer that functions as both the waveguiding layer and the chemically sensitive layer (3-layer model) or with a PMMA waveguiding layer below the chemically sensitive layer (4-layer model). Experimental measurements show the 3-layer model provides higher sensitivity than the 4-layer model. Increased sensitivity when using the 4-layer model can only be achieved through rigorous selection of the guiding polymer layer and chemically sensitive layer, considering both mass loading and viscoelastic effects. Appropriate selection of the partially selective chemical layer to optimize sensitivity is also a critical design factor, particularly in sensing polar analytes in aqueous sensing applications. A methodology based on attenuated total internal reflectance Fourier transform infrared spectroscopy (ATR-FTIR) for screening the potential effectiveness of new polymer coatings for these devices has been developed and used in our work. The ATR-FTIR methodology provides an accurate determination of trends for partitioning of analytes from water into polymer coatings.

Design of a portable guided SH-SAW chemical sensor system for liquid environments

Following successful application in gas sensing, acoustic wave liquid sensors attracted considerable attention due to the need for real-time, rapid and direct detection where the device is in direct contact with the solution. More importantly, there is a need for field measurement capability with portable devices. Challenges include a physical layout of the RF circuitry to minimize parasitic and spurious noise, continuous and realtime measurements capability, and obtaining vector network analyzer (VNA) performance in a portable RF unit, especially since the sensor signal noise dictates the limit of detection (LOD). Polymer-guided shear horizontal surface acoustic wave (SH-SAW) sensor platforms operating around 105 MHz on 36deg rotated Y-cut LiTaO 3 are investigated as portable detectors in liquid environments. The described system is self-contained including RF signal source, sensor input/output signal conditioning, and sensor signal amplitude and phase measuring capabilities. Amplitude and phase signals from the sensor are differentially compared with concomitant signals available directly from the RF signal source. The two primary outputs from the system are voltages related to the detected amplitude and phase changes that are caused by the sensors response to analyte sorption by the coated device. Several devices, coated with chemically sensitive polymers, are investigated in the detection of low concentrations (10-60 ppm) of ethylbenzene and xylenes in water using the RF portable unit. The units were tested for both reproducibility and repeatability, and the results matched very well with VNA measurements

Rapid detection of organophosphates in aqueous solution using a hybrid organic/inorganic coating on SH-SAW devices

Rapid detection of organophosphates pesticides (OPs) in groundwater is necessary to allow for real-time monitoring and cleanup. Detection of OPs in the liquid phase has already been demonstrated using poly(epichlorohydrin) [PECH] and polyurethane as the sensing layer. However, the response times are relatively long, on the order of hours. In this work, a hybrid organic/inorganic chemically sensitive layer [bisphenol A-hexamethyltrisiloxane (BPA-HMTS)] is synthesized and investigated for the rapid detection and analysis of organophosphate pesticides. Direct chemical sensing in aqueous solutions is performed using the guided shear horizontal surface acoustic wave sensor platform on 36° rotated Y-cut LiTaO3. It is shown that, for the same coating thickness, a 60% reduction in sensor response time is achieved without a significant reduction in sensitivity when compared with PECH. Considering the glass transition temperature, Tg, for the polymers, it is seen that the faster sensor response exhibited by the BPA-HMTS coating is due to the porous siloxane backbone, HMTS. Furthermore, sensor signal analysis in the form of the extended Kalman filter (EKF) is employed on-line during the detection process. This allows for the steady-state sensor response and absorption time constant to be extracted on-line well before equilibrium, thus further reducing the time required for analyte identification and quantification. 500 µg/L of parathion has been detected and a limit of detection of 20 µg/L (ppb) for parathion and 100 µg/L (ppb) of paraoxon is reported for the present non-optimized sensor.

Determination of Chemical Homogeneity of Fire Retardant Polymeric Nanocomposite Materials by Near-Infrared Multispectral Imaging Microscopy

Polymer nanocomposites containing layered double hydroxide (LDH) additives offer great potential for improving polymer physical properties. Of particular interest is the possibility of improving the fire retardancy and thermal stability of polymers using low loadings of this emerging class of nano-additives. Understanding the relationship between the quality of additive dispersion in the polymer matrix (i.e., chemical homogeneity) and selected flammability properties is a key question for optimizing LDHs for use in fire retardant formulations. We have demonstrated, for the first time, that the near infrared multispectral imaging (NIR-MSI) microscope can be successfully used to characterize the chemical homogeneity of a model system containing a magnesium aluminum hydroxide LDH modified with interlayer undecenoate anions mixed with poly(ethylene). The NIR-MSI is suited for this task because it can simultaneously record spectral and spatial information of a sample with high sensitivity (single pixel resolution) and high spatial resolution (∼0.9 μm/pixel). At 20% added, LDH was found to distributed inhomogeneously in a poly(ethylene) nanocomposite sample on the micron scale.