Steven J. Pas

Degradation Studies of Polyolefins Incorporating Transparent Nanoparticulate Zinc Oxide UV Stabilizers

Coated and dispersed nanoparticulate zinc oxide is shown to improve ultra violet (UV) stability of polypropylene and high-density polyethylene without changing its characteristic absorption spectrum in the visible region (400–800-nm). The performance of these nanoparticulate UV stabilizers is compared to conventional hindered amine light stabilizers (HALS). QUV accelerated weathering is used to simulate long-term exposure. Positron annihilation lifetime spectroscopy (PALS) is used to provide an indication of physical and chemical changes due to accelerated weathering and is shown to have potential for detecting changes well before other techniques. Visual observation, optical microscopy, carbonyl index, yellowness index and PALS indicate that nanoparticulate zinc oxide gives superior resistance to UV degradation compared to organic HALS at appropriate loading levels.

Defect-assisted conductivity in organic ionic plastic crystals

In order to determine the role of defects (vacancies and extended lattice defects) in the conductivity mechanism of a well studied organic ionic plastic crystal electrolyte, conductivity and mean defect volumes were measured. The ionic conductivity of the salt showed a characteristic phase dependence. Defect volumes, as measured by positron annihilation lifetime spectroscopy, showed increasing rates of expansion with increasing rotational disorder. The dependence of ionic conductivity on defect volume was observed to be phase dependent. Increases in mean defect volume size below approximately 100 cm(3) mol(-1) did not always facilitate ionic conductivity. It was shown that the material undergoes a solid-solid phase transition to the most disordered phase (a plastic crystalline phase with the highest conductivity) when the mean defect volume becomes larger than the molar volume of either the rotating anionic or cationic species. Conductivity in this phase had the strongest dependence on defect volume. Critical volumes calculated from the free volume model of Cohen and Turnbull were unrealistically large.

Poly(m-xylene adipamide)-montmorillonite nanocomposites: effect of organo-modifier structure on free volume and oxygen barrier properties

It was shown that nanoparticle–polymer interactions that affect the free volume and oxygen barrier properties of poly(m-xylene adipamide)/clay nanocomposites can be tailored by the choice of organic modifier of montmorillonite clay. Three different organo-modified clay compounds based on montmorillonite (Cloisite 30B, 10A and 93A) were dispersed in the resin poly(m-xylene adipamide) at loading levels of 2 wt% clay. Samples were melt compounded and extruded using a laboratory scale twin screw micro compounder. Positron annihilation lifetime spectroscopy (PALS) was used to examine the free volume of the polymer nanocomposites. PALS results suggested that the Cloisite 10A additive should give the higher reduction in gas permeability as it results in the lowest free volume for the nylon resin when compared to all of the clay additives examined. Oxygen transmission rates (OTR) were measured on nanocomposite films and the Cloisite 10A additive was found to give the best oxygen barrier, showing a reduction of OTR of 66% compared to the neat resin. In all cases examined, PALS free volume data was found to have excellent correlation to the measured oxygen transmission rates. The addition of Cloisite 10A resulted in the highest crystallinity and an increase in glass transition temperature when compared to the neat resin. Results indicate that the improved barrier properties of the clay compounds is primarily due to an increase in the degree of crystallinity of the polymer, with the nanoparticles being more effective nucleating agents when favourable nanoparticle–polymer interactions are present.

Surprising effect of nanoparticle inclusion on ion conductivity in a lithium doped organic ionic plastic crystal

Doping lithium bis(trifluoromethanesulfonyl)amide (Li[NTf2]) into the N-ethyl,N′-methylpyrrolidinium bis(trifluoromethanesulfonyl)amide ([C2mpyr][NTf2]) plastic crystal material has previously indicated order of magnitude enhancements in ion transport and conductivity over pure [C2mpyr][NTf2]. Recently, conductivity enhancements in this ionic plastic crystal induced by SiO2nanoparticles have also been reported. In this work the inclusion of SiO2nanoparticles in Li ion doped [C2mpyr][NTf2] has been investigated over a wide temperature range by differential scanning calorimetry (DSC), impedance spectroscopy, positron annihilation lifetime spectroscopy (PALS), Raman spectroscopy, NMR spectroscopy and scanning electron microscopy (SEM). Solid state 1H NMR indicates that the addition of the nanoparticles increases the mobility of the [C2mpyr] cation and positron lifetime spectroscopy (PALS) measurements indicate an increase in mean defect size and defect concentration as a result of nanoparticle inclusion, especially with 10 wt% SiO2. Thus, the substantial drop in ion conductivity observed for this doped nanocomposite material was surprising. This decrease is most likely due to the decrease in mobility of the [NTf2] anion, possibly by its adsorption at the SiO2/grain boundary interface and concomitant decrease in mobility of the Li ion.

Positron annihilation lifetime spectroscopy (PALS) as a characterization technique for nanostructured self-assembled amphiphile systems.

Positron annihilation lifetime spectroscopy (PALS) has potential as a novel rapid characterization method for self-assembly amphiphile systems; however, a lack of systematic correlation of PALS parameters with structural attributes has limited its more widespread application. In this study, using the well-characterized phytantriol/water and the phytantriol/vitamin E acetate/water self-assembly amphiphile systems, the impact of systematic structural changes controlled by changes in composition and temperature on PALS parameters has been studied. The PALS parameters (orthopositronium (oPs) lifetime and intensity signatures) were shown to be sensitive to the molecular packing and mobility of the self-assembled lipid molecules in various lyotropic liquid crystalline phases, enabling differentiation between liquid crystalline structures. The oPs lifetime, related to the molecular packing and mobility, is correlated with rheological properties of the individual mesophases. The oPs lifetime links the lipid chain packing and mobility in the various mesophases to resultant macroscopic properties, such as permeability, which is critical for the use of these mesophase structures as diffusion-controlled release matrices for active liposoluble compounds.

Physical Absorption Of CO2 in Protic and Aprotic Ionic Liquids: An Interaction Perspective

The physical absorption of CO2 by protic and aprotic ionic liquids such as 1-ethyl-3-methyl-imidazolium tetrafluoroborate was examined at the molecular level using symmetry adapted perturbation theory (SAPT) and density functional techniques through comparison of interaction energies of noncovalently bound complexes between the CO2 molecule and a series of ionic liquid ions and ion pairs. These energies were contrasted with those for complexes with model amines such as methylamine, dimethylamine, and trimethylamine. Detailed analysis of the five fundamental forces that are responsible for stabilization of the complexes is discussed. It was confirmed that the nature of the anion had a greater effect upon the physical interaction energy in non functionalized ionic liquids, with dispersion forces playing an important role in CO2 solubility. Hydrogen bonding with protic cations was shown to impart additional stability to the noncovalently bound CO2···IL complex through inductive forces. Two solvation models, the conductor-like polarizable continuum model (CPCM) and the universal solvation model (SMD), were used to estimate the impact of solvent effects on the CO2 binding. Both solvent models reduced interaction energies for all types of ions. These interaction energies appeared to favor imidazolium cations and carboxylic and sulfonic groups as well as bulky groups (e.g., NTf2) in anions for the physical absorption of CO2. The structure-reactivity relationships determined in this study may help in the optimization of the physical absorption process by means of ionic liquids.

Lithium-functionalised silica nanoparticles for enhanced ionic conductivity in an organic ionic plastic crystal

Addition of silica nanoparticles functionalised with lithium propane sulfonate to the organic ionic plastic crystal N-ethyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)amide ([C2mpyr][NTf2]) results in a significant increase in ionic conductivity. Analysis of these nanocomposites by impedance spectroscopy, NMR, positron annihilation lifetime spectroscopy (PALS) and Raman spectroscopy suggests that this is the result of higher matrix mobility due to an increase in defect size and concentration. The effect of these functionalised nanoparticles is compared to that previously observed for unfunctionalised nanoparticles in the lithium-doped and pure plastic crystal.

Structure and transport properties in an N,N-substituted pyrrolidinium tetrafluoroborate plastic crystal system

Abstract The structure and transport of N -propyl- N -methylpyrrolidinium tetrafluoroborate (P 13 BF 4 ) has been investigated over a wide temperature range in consequence to exhibiting properties suitable for potential solid-state superionic electrolyte applications. Prior to melting, the organic salt, P 13 BF 4 , transforms into a plastic crystal phase. Intrinsic conductivity in this solid, phase I (45–65 °C), is comparable to that in the melt (∼10 −3 S cm −1 ). Ionic motion and transport properties were investigated by 1 H and 11 B nuclear magnetic resonance (NMR) spectroscopy. Pressure-induced plastic flow in this system may accommodate volume changes in device application and to this extent, X-ray diffraction (XRD) has been used. Scanning electron microscopy (SEM) revealed complex surface morphology and lattice imperfections associated with the strong orientational disorder of the plastic state.

Materials genome in action: Identifying the performance limits of physical hydrogen storage

The Materials Genome is in action: the molecular codes for millions of materials have been sequenced, predictive models have been developed, and now the challenge of hydrogen storage is targeted. Renewably generated hydrogen is an attractive transportation fuel with zero carbon emissions, but its storage remains a significant challenge. Nanoporous adsorbents have shown promising physical adsorption of hydrogen approaching targeted capacities, but the scope of studies has remained limited. Here the Nanoporous Materials Genome, containing over 850 000 materials, is analyzed with a variety of computational tools to explore the limits of hydrogen storage. Optimal features that maximize net capacity at room temperature include pore sizes of around 6 Å and void fractions of 0.1, while at cryogenic temperatures pore sizes of 10 Å and void fractions of 0.5 are optimal. Our top candidates are found to be commercially attractive as “cryo-adsorbents”, with promising storage capacities at 77 K and 100 bar with 30% enhancement to 40 g/L, a promising alternative to liquefaction at 20 K and compression at 700 bar.

Thermal and physical properties of an archetypal organic ionic plastic crystal electrolyte

The thermal and mechanical properties of the ionic plastic crystal N-methyl-N-propylpyrrolidinium hexafluorophosphate have been investigated and the effect of adding a miscible polymer on the mechanical properties is reported. The physical properties of the pure plastic crystal are discussed in detail and for the first time the change in volume with temperature for an organic ionic plastic crystal is reported. An increase in volume in conjunction with increased conductivity supports the hypothesis that ion conduction within the plastic crystal proceeds via defects. For phase I and melting, the magnitude of the volume increase does not appear to be in accord with the subtle change in conductivity. This is suggested to be due to the presence of layer defects, which allow for correlated ionic motion, which does not increase the conductivity. Addition of polymer to the plastic crystal significantly increases the mechanical strength, decreases the conductivity, but has little effect on the phase behaviour, further supporting the hypothesis of vacancy-mediated conduction.