Maria R. Coleman

Isomeric polyimides based on fluorinated dianhydrides and diamines for gas separation applications

Abstract The gas sorption and transport properties of two isomeric polyimides with hexafluoroisopropylidene moieties in the diamine and dianhydride monomers were characterized for a variety of gases at 35°C at pressures up to 60 atm. These materials have structural properties which inhibit intrasegmental rotational mobility and intersegmental chain packing. The effect of isomerism on the physical and gas separation properties of these rigid materials was investigated. The effect of isomerism on the hindrance to packing is reflected in the wide angle X-ray diffraction (WAXD) measurements of the average spacing between adjacent polymer chains. The para connected polyimide showed significant increases in permeability relative to a series of polyimides studied earlier with less packing-disruptive substituents on the polymer backbone. The permeability of the higher flux material was 64 barrers for CO 2 and 16 barters for O 2 . The meta connected polyimide showed large decreases in permeability with corresponding increases in permselectivity when compared to its para counterpart. For example, the permselectivity of the meta material for O 2 relative to N 2 is 6.9 which is 50% greater than that of the para connected material. The differences in permeability and permselectivity are due to both penetrant solubility and diffusivity effects.

A hybrid functional nanomaterial: POSS functionalized carbon nanofiber

A hybrid functional nanomaterial was synthesized by functionalizing carbon nanofibers (CNF) with polyhedral oligomeric silsesquioxane (POSS). The reaction between CNF and the amine group of ocataminophenyl POSS was achieved using carbodiimide chemistry. The CNF-POSS hybrids were designed to increase the reactivity of CNF without affecting its inherent properties. The reactive amine groups of CNF-POSS were further modified with an oligomer of polyimide polymer to improve the interaction with the polymer matrix, thus forming a nanocomposite with enhanced multifunctional properties. Functionalization was characterized using thermal gravimetric analysis (TGA), x-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM).

Modification of commercial water treatment membranes by ion beam irradiation

The effect of ion beam irradiation on the performance of two commercial water treatment membranes was studied. The results of this testing indicate that the irradiation induced structure modifications had a positive impact on the membranes application to wastewater treatment. Irradiation of the first membrane (TFC-S, Koch Membrane Systems, San Diego, CA) led to a slight decrease in selectivity, but this was outweighed by the positive effects of the irradiation. Not only did the irradiation improve both abiotic and biofouling resistance, it also doubled the membrane solvent mass transfer coefficient (MTC). Ion beam irradiation also improved the performance of the second membrane (NTR 7450, Hydranautics, San Diego, CA). Water quality testing revealed similar removal of contaminants, while fouling tests indicated an improvement in the membranes resistance to fouling, especially in the case of biofouling.

Preliminary investigation of gas transport mechanism in a H+ irradiated polyimide-ceramic composite membrane

Abstract Recent research by our group indicated that ion beam irradiation can simultaneously increase the gas permeability and permselectivity of polymeric membrane materials. The temperature dependence of the gas permeation properties of a H+ ion irradiated polyimide-ceramic composite membrane was investigated to address issues of changes in the gas transport mechanism in irradiated polymers. As was seen for glassy polymers, the temperature dependence of the permeation properties of the irradiated membrane followed an Arrhenius type relationship. Both the activation energy (Ep) for gas permeation and the pre-exponential factor (P0) of the irradiated polymer were greater than the values of the unmodified bulk polymer. Large increases in the pre-exponential factor of the irradiated sample for small size gas molecules (He, O2 and CO2) combined with the dominant contribution of the pre-exponential factor to the permselectivity for several gas pairs (He/CH4, O2/N2, and CO2/CH4) implied that the irradiated sample had a different permeation mechanism than the bulk material.

Impact of ion beam irradiation on microstructure and gas permeance of polysulfone asymmetric membranes

Abstract Ion beam irradiation has been widely used to modify the structure and properties of thin polymer surface layers. The transfer of energy from the ions to the polymer chains leads to significant evolution in the chemical structure, microstructure and transport properties of polymeric dense films. This paper focuses on the impact of energetic H + ions over a range of doses on the transport properties of polysulfone (PSF) asymmetric membranes. There was a large simultaneous decrease in permeance and permselectivity for the asymmetric membranes following H + irradiation over a range of fluences. These modifications in the transport properties of the asymmetric membranes were not consistent with the results for bulk polymer. At the energies used for this study, the ions penetrated well into the porous substrate of the polysulfone membranes, which caused a collapse of the intermediate porous substrate and formation of a thick non-selective resistance layer. Positron annihilation spectroscopy (PAS) and scanning electron microscopy (SEM) confirmed the existence of a compacted region in porous support.

Ion implant-induced change in polyimide films monitored by variable energy positron annihilation spectroscopy

6FDA-pMDA polyimide membranes were implanted with 140 keV N+ ions to fluences between 2 × 1014 and 5 × 1015 cm−2. Variable energy positron annihilation spectra were taken and spectral features compared to previously reported changes in gas permeability and permselectivity of these membranes as a function of ion fluence. Positron data corroborate the explanation of these changes in terms of molecular damage caused by the implant: for fluences up to about 1 × 1015 cm−2, the concentration of irradiation-induced defects merely increases with implant fluence; while fluences exceeding this threshold value create a second type of positron annihilation site, thereby marking a distinct change in the structure of the polymer, which is responsible for the vast improvement of gas permselectivity data found at the same threshold fluence. PACS codes: 78.70.Bj—positron annihilation; 61.82.Pv—polymers, organic compounds; 61.72.Ww—doping and impurity implantation.

Development of Smart Membrane Filters for Microbial Sensing

Abstract Recent efforts aimed at minimizing membrane fouling have emphasized an increasing demand for on-line monitoring in an effort to accurately predict membrane performance. The development of an in-situ bacterial monitoring system that is integrated within the membrane can meet that need. Because the target is bacterial monitoring, organic matter fouling must be controlled to avoid interference/masking of bacterial sensing as well as to prevent permeability loses. Combining bacterial monitoring with a membrane designed for fouling control is a novel and unique concept. We have produced a fouling-resistant membrane by attaching a stimuli-responsive polymer film on the surface, which offers the potential to collapse or expand the polymer film. A temperature decrease can cause the film to expand into a hydrophilic state while a temperature increase causes a collapse into a hydrophobic state. By continuously triggering the phase transition, the non-equilibrium movement of the polymer film may offer better protection of the surface than at equilibrium. Increasing temperature to collapse the film and immediately decreasing temperature to expand it would create a sweeping motion at the molecular (nanometer) level along the surface. The surface of a cellulose acetate membrane was grafted with a thermally responsive hydroxypropyl cellulose (HPC) film layer. Aqueous solutions of HPC possess a lower critical solution temperature of approximately 40°C (while cross-linked structures had an LCST of 46°C): above this temperature the solution phase separates. When attached to the membrane surface, the film layer collapses upon increasing the temperature above the phase transition temperature and expands away from the surface when cooled. Biorecognition molecules targeting selected bacteria were covalently bound to specific moities originating from the polymer film for in situ detection. Typically, the biological recognition component consists of enzymes, receptors, nucleic acids, or antibodies specific to biological markers. In this proof-of-concept study, antibodies were used due to their simplicity, proven efficacy, and rapid response.

A Novel Approach to Improve the Barrier Properties of PET/Clay Nanocomposites

An investigation of oleic acid-modified clay versus plain clay with regard to the physical and barrier properties of PET/clay nanocomposites was performed. Montmorillonite (MMT) and Cloisite 30B nanoclays were modified by long-chain oleic acid and identified as ol-MMT and ol-30B, respectively. Fourier Transformed Infrared Spectroscopy and X-ray diffraction (XRD) results revealed that the fatty acid was associated with the clay surface and that the gallery spacing of the layered silicates was expanded. In the case of ol-MMT, a disordered structure of layered silicates was achieved. TGA results indicated that ol-MMT showed thermal stability and could survive PET processing temperature. The degradation of ol-30B, however, increased after modification because of the presence of oleic acid. PET/clay nanocomposites were prepared with modified ol-MMT and modified ol-30B by using a twin screw extruder. XRD indicated that there was a significant improvement on the dispersion of nanoclays modified with long-chain oleic acid into the PET matrix, and an exfoliated structure was achieved. DSC data also revealed that crystallization behaviors of nanocomposites prepared with oleic acid-modified clays are similar to that of extruded PET. Significant improvements in the mechanical and barrier properties of stretched PET/clay nanocomposites were also achieved.

Effect of H+ and N+ Irradiation on Structure and Permeability of the Polyimide Matrimid®

Abstract This paper presents a comparison of the impact of H+ and N+ ion irradiation on the chemical structure, microstructure, and gas permeation properites of the polyimide, Matrimid®. While irradiation with both ions resulted in evolution of chemical structure with loss of functional groups and crosslink formation, there was greater modification of polyimide structure following N+ irradiation at similar total deposited energy. Irradiation with N+ resulted in simultaneous large increases in permeance and permselectivity at comparatively low ion fluences or irradiation time. For example, irradiation at 4 × 1014 N+/cm2 resulted in a 2.5 fold increase in He permeance with a selectivity of He/CH4 of 340. Much higher H+ fluences were required to achieve similar total deposited energy and combined increases in permeance and permselectivity. The larger modification in chemical structure and gas permeation properties following N+ irradiation was attributed to the relatively large energy loss and damage from the nuclear energy relative to electronic energy loss.

High‐Throughput Continuous Production of Shear‐Exfoliated 2D Layered Materials using Compressible Flows

2D nanomaterials are finding numerous applications in next-generation electronics, consumer goods, energy generation and storage, and healthcare. The rapid rise of utility and applications for 2D nanomaterials necessitates developing means for their mass production. This study details a new compressible flow exfoliation method for producing 2D nanomaterials using a multiphase flow of 2D layered materials suspended in a high-pressure gas undergoing expansion. The expanded gas-solid mixture is sprayed in a suitable solvent, where a significant portion (up to 10% yield) of the initial hexagonal boron nitride material is found to be exfoliated with a mean thickness of 4.2 nm. The exfoliation is attributed to the high shear rates (γ˙ > 105 s-1 ) generated by supersonic flow of compressible gases inside narrow orifices and converging-diverging channels. This method has significant advantages over current 2D material exfoliation methods, such as chemical intercalation and exfoliation, as well as liquid phase shear exfoliation, with the most obvious benefit being the fast, continuous nature of the process. Other advantages include environmentally friendly processing, reduced occurrence of defects, and the versatility to be applied to any 2D layered material using any gaseous medium. Scaling this process to industrial production has a strong possibility of reducing the cost of creating 2D nanomaterials.