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Magnesium borohydride (Mg(BH4)2, abbreviated here MBH) has received tremendous attention as a promising onboard hydrogen storage medium due to its excellent gravimetric and volumetric hydrogen storage capacities. While the polymorphs of MBH-alpha (α), beta (β), and gamma (γ)-have distinct properties, their synthetic homogeneity can be difficult to control, mainly due to their structural complexity and similar thermodynamic properties. Here, we describe an effective approach for obtaining pure polymorphic phases of MBH nanomaterials within a reduced graphene oxide support (abbreviated MBHg) under mild conditions (60-190 °C under mild vacuum, 2 Torr), starting from two distinct samples initially dried under Ar and vacuum. Specifically, we selectively synthesize the thermodynamically stable α phase and metastable β phase from the γ-phase within the temperature range of 150-180 °C. The relevant underlying phase evolution mechanism is elucidated by theoretical thermodynamics and kinetic nucleation modeling. The resulting MBHg composites exhibit structural stability, resistance to oxidation, and partially reversible formation of diverse [BH4]- species during de- and rehydrogenation processes, rendering them intriguing candidates for further optimization toward hydrogen storage applications. more...

In this article, the capabilities of soft and hard X-ray techniques, including X-ray absorption (XAS), soft X-ray emission spectroscopy (XES), resonant inelastic soft X-ray scattering (RIXS), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD), and their application to solid-state hydrogen storage materials are presented. These characterization tools are indispensable for interrogating hydrogen storage materials at the relevant length scales of fundamental interest, which range from the micron scale to nanometer dimensions. Since nanostructuring is now well established as an avenue to improve the thermodynamics and kinetics of hydrogen release and uptake, due to properties such as reduced mean free paths of transport and increased surface-to-volume ratio, it becomes of critical importance to explicitly identify structure-property relationships on the nanometer scale. X-ray diffraction and spectroscopy are effective tools for probing size-, shape-, and structure-dependent material properties at the nanoscale. This article also discusses the recent development of in-situ soft X-ray spectroscopy cells, which enable investigation of critical solid/liquid or solid/gas interfaces under more practical conditions. These unique tools are providing a window into the thermodynamics and kinetics of hydrogenation and dehydrogenation reactions and informing a quantitative understanding of the fundamental energetics of hydrogen storage processes at the microscopic level. In particular, in-situ soft X-ray spectroscopies can be utilized to probe the formation of intermediate species, byproducts, as well as the changes in morphology and effect of additives, which all can greatly affect the hydrogen storage capacity, kinetics, thermodynamics, and reversibility. A few examples using soft X-ray spectroscopies to study these materials are discussed to demonstrate how these powerful characterization tools could be helpful to further understand the hydrogen storage systems. more...

Solid-state metal hydrides are prime candidates to replace compressed hydrogen for fuel cell vehicles due to their high volumetric capacities. Sodium aluminum hydride has long been studied as an archetype for higher-capacity metal hydrides, with improved reversibility demonstrated through the addition of titanium catalysts; however, atomistic mechanisms for surface processes, including hydrogen desorption, are still uncertain. Here, operando and ex situ measurements from a suite of diagnostic tools probing multiple length scales are combined with ab initio simulations to provide a detailed and unbiased view of the evolution of the surface chemistry during hydrogen release. In contrast to some previously proposed mechanisms, the titanium dopant does not directly facilitate desorption at the surface. Instead, oxidized surface species, even on well-protected NaAlH4 samples, evolve during dehydrogenation to form surface hydroxides with differing levels of hydrogen saturation. Additionally, the presence of these oxidized species leads to considerably lower computed barriers for H2 formation compared to pristine hydride surfaces, suggesting that oxygen may actively participate in hydrogen release, rather than merely inhibiting diffusion as is commonly presumed. These results demonstrate how close experiment-theory feedback can elucidate mechanistic understanding of complex metal hydride chemistry and potentially impactful roles of unavoidable surface impurities. more...

Mg(BH4)2 is a promising solid-state hydrogen storage material, releasing 14.9 wt% hydrogen upon conversion to MgB2. Although several dehydrogenation pathways have been proposed, the hydrogenation process is less well understood. Here, we present a joint experimental-theoretical study that elucidates the key atomistic mechanisms associated with the initial stages of hydrogen uptake within MgB2. Fourier transform infrared, X-ray absorption, and X-ray emission spectroscopies are integrated with spectroscopic simulations to show that hydrogenation can initially proceed via direct conversion of MgB2 to Mg(BH4)2 complexes. The associated energy landscape is mapped by combining ab initio calculations with barriers extracted from the experimental uptake curves, from which a kinetic model is constructed. The results from the kinetic model suggest that initial hydrogenation takes place via a multi-step process: molecular H2 dissociation, likely at Mg-terminated MgB2 surfaces, is followed by migration of atomic hydrogen to defective boron sites, where the formation of stable B-H bonds ultimately leads to the direct creation of Mg(BH4)2 complexes without persistent BxHy intermediates. Implications for understanding the chemical, structural, and electronic changes upon hydrogenation of MgB2 are discussed. more...

Mg(BH4)2 is a promising solid-state hydrogen storage material, releasing 14.9 wt% hydrogen upon conversion to MgB2. Although several dehydrogenation pathways have been proposed, the hydrogenation process is less well understood. Here, we present a joint experimental-theoretical study that elucidates the key atomistic mechanisms associated with the initial stages of hydrogen uptake within MgB2. Fourier transform infrared, X-ray absorption, and X-ray emission spectroscopies are integrated with spectroscopic simulations to show that hydrogenation can initially proceed via direct conversion of MgB2 to Mg(BH4)2 complexes. The associated energy landscape is mapped by combining ab initio calculations with barriers extracted from the experimental uptake curves, from which a kinetic model is constructed. The results from the kinetic model suggest that initial hydrogenation takes place via a multi-step process: molecular H2 dissociation, likely at Mg-terminated MgB2 surfaces, is followed by migration of atomic hydrogen to defective boron sites, where the formation of stable B-H bonds ultimately leads to the direct creation of Mg(BH4)2 complexes without persistent BxHy intermediates. Implications for understanding the chemical, structural, and electronic changes upon hydrogenation of MgB2 are discussed. more...

Intermeshing counter-rotating twin screw extruders play an important role in polymer processing, especially for the extrusion of profiles and pipe, largely from polyvinyl chloride. There has been little effort on flow modeling of these machines and most of this has involved representing the machine as a 'leaky' positive displacement pump and estimating leakages. In recent years, M.H. Hong and the authors have developed more general methods of simulation of flow in this machine and both applied and experimentally verified it for a few designs. Here we extend these efforts to a broader range of screw designs, especially with deeper screw channels where transverse shearing induced by the flights is important. Calculations are done for isothermal power law fluids. The results are compared with experiment. POLYM. ENG. SCI., 51:37-42, 2011. [C]2010 Society of Plastics Engineers, INTRODUCTION The flow of polymer melts in intermeshing counter-rotating twin screw extruder (Fig. 1) has been subjected to only limited study. The machine seems first described in the 19th century [...] more...

We describe the development of phase morphology in polypropylene (PP)/ethylene-butene copolymer (EBM) blends in both extrusion through dies and in subsequent melt spinning to form filaments. This immiscible blend has a very low interfacial tension around 0.69 dynes/cm. In the die extrusion process, at low extrusion rates, the dispersed PP phase was sheared into fibrils; at higher extrusion rates, the PP fibrils formed an encapsulation layer near the die wall first, then it broke into droplets because of both interfacial tension and long residence time. These droplets agglomerated together to form a network. In melt spinning, the dispersed phase was also drawn down into fibrils, which coalesced into a continuous layer on the outer surface of the filaments at higher drawdown rates. POLYM. ENG. SCI., 50:1969-1977, 2010. [C] 2010 Society of Plastics Engineers, INTRODUCTION Polymer blends play an important role in the polymer industry and much attention has been addressed to their phase morphology and its variation with flow and processing conditions (1-13). [...] more...

A basic study on crystal structure and orientation behavior of transversely compressed ethylene-1-octene copolymer with different 1-octene contents was described. All polymers were first melt spun under different spinline stress and subsequently transversely compressed. For the melt-spun filaments, an orthorhombic crystal structure was found for all polymers, but a pseudo-hexagonal mesophase was also found for polymers with the highest 1-octene level (13.3 mol%). For the transversely compressed filaments, several reflection peaks from a monoclinic unit cell were found for polyethylene without octene. For those with higher octene levels, the reflection peaks from monoclinic became fainter and disappeared for the one with the highest 1-octene level. After being transversely compressed, the (110) and (200) peaks of orthorhombic crystal structures became oriented along the meridian direction, which is the fiber axis direction. The reason for this appears to be that the compression deformation of the filament induces elongation along its width direction and shrinkage along its length and thickness direction, and in this change the polymer chain orients. POLYM, ENG., 48:2297-2304, 2008. © 2008 Society of Plastics Engineers, INTRODUCTION Polyethylene has become an important commercial thermoplastic, since its low density form (LDPE) was first produced in the 1930s by ICI (1). A high density linear form (HDPE) was [...] more...

We have made a study of the development of phase morphology of an immiscible blend(75/25)(polypropylene-polyamide-6) for different types of continuous mixers including (i) Buss Kneader, (ii and iii) modular intermeshing corotating and counter-rotating twin screw extruders, and (iv) NEX-T Kobelco Continuous Mixer. Comparisons are made using different screw configurations for each machine. Generally, in comparison of the different machines, the intermeshing counter-rotating twin screw extruder produced the finest dispersed morphology. Using a droplet breakup kinetic model, we interpreted the blend dispersed phase droplet breakdown rate and coalescence rate. In comparison with our earlier study of the continuous mixing of agglomerates of CaC[O.sub.3] particles the polymer droplet breakup rate was smaller than that of the particle agglomerates and the coalescence rates of droplets were many times greater than the particle reagglomerates rates. POLYM. ENG. SCI., 48:756-766, 2008. © 2008 Society of Plastics Engineers, INTRODUCTION One of the key factors in achieving the desired final properties of polymer blends is the control of the control of phase morphology. The properties of blended polymers can [...] more...

Syndiotactic polystyrene (s-PS) and atactic polystyrene (a-PS) were melt-spun into filaments. The s-PS filaments exhibited increasing amounts of crystallinity and orientation with increasing drawdown ratio and spinline stress. The a-PS filaments were amorphous but exhibited birefringence. The birefringence and Hermans orientation factors for a-PS were proportional to this spinline stress. In ice water and at low drawdown ratios, the s-PS is glassy or mesomorphic. At higher drawdown ratios and spinline stresses, it crystallized. The crystalline form was the zigzag TTTT hexagonal [alpha]-form. The birefringence and orientation factors of the s-PS filaments were higher than those of the a-PS filaments and the difference of the birefringence increased with increasing spinline stress. Mechanical testing results showed that the Young's modulus and tensile strength generally increased with increasing spinline drawdown ratio for both a-PS and s-PS filaments. The elongation to break was enhanced for both materials by increased chain orientation. Polym. Eng. Sci. 44:2141-2147, 2004. [c] 2004 Society of Plastics Engineers., INTRODUCTION Syndiotactic polystyrene (s-PS) was first announced in 1986 by Idemitsu Kosan (1), who commercialized it in succeeding years. Today it is produced by both Idemitsu and Dow Chemical. s-PS [...] more...

We investigated the crystallization and orientation development in melt spinning and tubular blown film extrusion of several different types of polypropylenes, including conventional high tacticity isotactic polypropylenes (iPP) and metallocene catalyst low tacticity iPPs and syndiotactic polypropylenes (sPP). The fiber and film samples were characterized by wide-angle X-ray diffraction (WAXD), birefringence and differential scanning calorimetry (DSC). In melt spinning iPP, we found that the mesomorphic structure of iPP is more readily formed in lower tacticity fibers, and significant amounts of hexagonal β-form crystals are found in low tacticity iPP fibers spun at high draw-down ratios. Low tacticity iPP fibers exhibited a significant decrease in the crystalline chain-axis orientation at high draw-down ratios, resulting from increased epitaxially branched lamellae. Melt-spun sPP fibers exhibit Form I helical structure at low spinning speeds and Form III zigzag all trans structure at high spinning speeds. We found that the level of spinline stress is the governing factor for this structural change. Melt-spun sPP fibers exhibit much higher chainaxis (c-axis) orientation factors ([f.sub.c]) and lower birefringence than iPP fibers spun at the same spinline stresses. In tubular blown sPP films, the a-axis of Form I unit cell tends to orient perpendicular to the film surface, while the b-axis of monoclinic α unit cell does so in iPP blown films., INTRODUCTION Stereoregular crystalline polypropylene was first synthesized in 1955 by Natta and his co-workers (1, 2). It was the isotactic form (iPP), which has asymmetric carbon atoms with all the [...] more...

Free radical grafting with methyl methacrylate onto molten polypropylene was investigated in both an internal mixer and a modular co-rotating twin-screw extruder. There has been little open literature on melt free radical grafting copolymerization of methyl methacrylate. There is also little information on the evolution of grafting reaction with respect to reaction time in an internal mixer and along the screw axes with methyl methacrylate. The influence of residence time on the degree of grafting in an internal mixer and a twin-screw extruder was studied through measuring reaction yields with respect to reaction time in a mixer and evolution of reaction yield along the screw axis. The degree of grafting increased with initial monomer and peroxide concentration. The grafting reactions with three different peroxides were also investigated. The grafting levels were similar to maleic anhydride and suggested that only an individual methyl methacrylate unit be grafted. The melt viscosity was dramatically reduced with addition of peroxide. A kinetic scheme of our reaction system for methyl methacrylate was proposed and compared with the experimental results., INTRODUCTION In recent years reactive extrusion has become an important area of polymer processing. There have been two distinct thrusts in reactive extrusion. One has involved polymerization of monomer to [...] more...

Previous experimental studies of melting of pellets in an intermeshing counter-rotating twin-screw extruder have shown that melting is initiated both between the screws and at the barrel. Models are developed [...] more...

A process dynamics model of transient output composition variations in a moduar self-wiping twin-screw extruder caused by input disturbances from feeders is developed. The changes of output mass flow rate and weight fraction of additives in the output stream were predicted for various time dependent input modes into a second feeder and other ports part way along the length of the machine., INTRODUCTION Modular intermeshing co-rotating twin-screw extruders including screw and kneading disc block elements have become increasingly important continuous processing machines widely used in blending, compounding, devolatilization, and reactive extrusion, including [...] more...

The synthesis of polyetheramide, containing poly(tetramethylene ether) (PTMEG) as the soft segment phase and polylactams as the hard phase was carried out in a Leistritz modular intermeshing counter-rotating twin-screw extruder in a one-step, solvent-free process. No by-product such as carbon dioxide or [H.sub.2]O is formed during the reaction procedure. Isocyanate-terminated telechelic PTMEG was premixed with caprolactam, and these were fed into the twin-screw extruder to form the polyetheramide triblock copolymer. This triblock copolymer has not been previously polymerized in a twin-screw extruder or continuous mixer. Subsequent to polymerization, the copolymer and PA6 homopolymer were also melt-spun into fibers. Molecular weights and molecular weight distributions were determined by GPC on trifluroacetylated samples. Thermal, rheological, mechanical, and structural properties were investigated. It appears that phase separation (arising from immiscibility between the polyether segment and the polyamide segm ent as the reaction proceeds) did not hinder the chemical reaction between the two reactants because of the intense mixing in the twin-screw extruder. The new triblock copolymer should have application as a rubber-toughened thermoplastic or thermoplastic elastomer depending upon the elastomer content., INTRODUCTION Various authors (1-6) have described the synthesis of block copolymers by polymerization of ζ-caprolactam in the presence of isocyanate-terminated poly(propylene oxide) and poly(butadiene) polymeric activators. The immiscibility between hard [...] more...

A comparative study of the crystallization and orientation development in injection molding isotactic and syndiotactic polypropylenes was made. The injection molded samples were characterized using wide angle X-ray diffraction (WAXD) techniques and birefringence. The injection molded isotactic polypropylene samples formed well-defined sublayers (skin, shear and core zones) and exhibited polymorphic crystal structures of the monoclinic α-form and the hexagonal β-form. Considerable amounts of β-form crystal were formed in the shear and core zones, depending on the injection pressure or on the packing pressure. The isotactic polypropylene samples had relatively high frozen-in orientations in the skin layer and the shear zone. The injection molded syndiotactic polypropylene exhibited the disordered Form I structure, but it did not appear to crystallize during the mold-filling stage because of its slow crystallization rate and to develop a distinct shear zone. The core zone orientation was greatly increased by application of high packing pressure. The isotactic polypropylene samples exhibited much higher birefringence than the syndiotactic polypropylene samples at the skin and shear layers, whereas both materials exhibited similar levels of crystalline orientation in these layers., INTRODUCTION Injection molding is one of the major processing operations for isotactic polypropylene. The morphology. crystallization and orientation development in injection molded isotactic polypropylene have been extensively studied in the [...] more...

The development of orientation of extrusion cast polyamide 11 films in the biaxial stretching process was studied with birefringence and wide angle X-ray diffraction (WAXD) pole figures. White-Spruiell biaxial orientation factors of the crystalline phase were calculated with the pole figure data. Both biaxially stretched films were developed. Planarity of hydrogen bonding planes with respect to the film surface was observed from WAXD pole figures. Mechanical properties of the films were studied. Tensile strength and elongation at break were successfully correlated with the out-of-plane birefringences., INTRODUCTION As an Important engineering thermoplastic polymer, polyamide 11 has been widely used for molding processes. Its unique properties such as excellent gas barrier, chemical resistance, and mechanical properties should [...] more...

The processability of single bubble polyamide 11 and polyamide 12 films was investigated. The development of crystalline structure and chain orientation of polyamide 11 and polyamide 12 films in single [...] more...

The differences in behavior of isotactic polypropylene (iPP) and syndiotactic polypropylene (sPP) in tubular film extrusion are qualitatively described. The crystalline form and orientation in the films were characterized using [...] more...