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Complete Transformation (complete + transformation)
Selected AbstractsStructure and Phase Transitions of Poly(heptamethylene p,p,-bibenzoate): Time-Resolved Synchrotron WAXS and DSC StudiesMACROMOLECULAR CHEMISTRY AND PHYSICS, Issue 14 2003Ernesto Pérez Abstract Time-resolved wide-angle X-ray scattering (WAXS), as well as differential scanning calorimetry (DSC) and polarisation microscopy studies, were applied to investigate the structure and phase transitions of poly(heptamethylene p,p,-bibenzoate). Temperature dependencies of several structural parameters were determined. Complete transformation from an isotropic melt to a smectic phase was suggested whereas the transition from a smectic to crystalline phase is only partial (around 30%), although it takes place from the ordered SCA phase. Crystals are formed within the SCA domains with nearly the same coherent length. On the basis of the analysis of the position and the profile of the diffuse wide-angle X-ray scattering and mesogenic layer spacing, it was assumed that either crystallisation modifies the smectic structure, or mesophase losses its positional order because of the lack of mobility of the spacers at low temperatures. WAXS scattering profiles corresponding to P7MB: a) cooling from the isotropic melt at 2,°C,·,min,1; b) subsequent melting at 12,°C,·,min,1. [source] Activator-Controlled High Temperature In-Situ Ligand Synthesis for the Formation of Rare Earth Thiolate Amide Coordination Polymers,EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 17 2009Alexander Zurawski Abstract 1D rare earth thiolate amide coordination polymers can be obtained by an activator-controlled in situ ligand synthesis in reactions of molten 2-mercaptobenzimidazole with lanthanide metals. Catalytic amounts of mercury activate the metals and induce a C,S bond cleavage that in situ forms 2,2,-bibenzimidazole. The latter then reacts as a linker ligand to give 1,[Ln2(Mbim)4(Bbim)], Mbim, = C7H5SN2,, Bbim2,, C14N4H82,. In addition cinnabar is formed. Stoichiometric amounts of mercury cause the complete transformation of 2-mercaptobenzimidazole to 2,2,-bibenzimidazole. Instead of thiolate coordination polymers the reaction then yields sulfur-free monomeric bibenzimidazolate complexes (BimH2)+[Ln(BbimH)4],, (BimH2)+ = C7H6N2+, BbimH, = C14N4H9,. Thus the amount of Hg also controls the dimensionality of the products by defining the reaction path.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) [source] A Chemical Route to BiNbO4 CeramicsINTERNATIONAL JOURNAL OF APPLIED CERAMIC TECHNOLOGY, Issue 2 2009Oleg A. Shlyakhtin The liquid phase sintering of fine BiNbO4 powders allows to obtain dense ceramics with excellent microwave dielectric properties (,=44,46; Q×f=16,500,21,600 GHz) at T,700°C. The thermal decomposition of freeze-dried precursors results in the crystallization of a metastable ,,-BiNbO4 polymorph that transforms into a stable orthorhombic ,-modification at T,700°C. The dependence of sinterability on the powder synthesis temperature shows the maximum at 600°C, corresponding to the formation of crystalline BiNbO4 powders with a grain size 80,100 nm. Sintering temperature reduction to 700°C prevents the deterioration of silver contacts during co-firing with BiNbO4 ceramics. In situ scanning electron microscopy observation of the morphological evolution during sintering shows that the intense shrinkage soon after the appearance of a CuO,V2O5 eutectics-based liquid phase is accompanied by complete transformation of the ensemble of primary BiNbO4 particles. [source] Influence of land use on plant community composition and diversity in Highland Sourveld grassland in the southern Drakensberg, South AfricaJOURNAL OF APPLIED ECOLOGY, Issue 5 2005T. G. O'CONNOR Summary 1Biodiversity conservation of grasslands in the face of transformation and global climate change will depend mainly on rangelands because of insufficient conservation areas in regions suited to agriculture. Transformed vegetation (pastures, crops and plantations) is not expected to conserve much biodiversity. This study examined the impact of land use on the plant diversity and community composition of the southern Drakensberg grasslands in South Africa, which are threatened with complete transformation to pastures, crops and plantations. 2The main land uses in this high rainfall region are: ranching or dairy production under private tenure using indigenous grassland, pastures (Eragrostis curvula, kikuyu and ryegrass) and maize; plantation forestry; communal tenure (maize and rangelands); and conservation. 3Plant diversity and composition were assessed using Whittaker plots. Transformed cover types were depauperate in species and ranged from kikuyu (1·4 species m,2) and ryegrass (2·9), to pine plantation (3·1), E. curvula pasture (3·1), commercial maize (3·2) and communal maize (7·8). With the exception of pine plantations, these communities supported mostly exotic (50 of 70 species) or ruderal indigenous species and made little contribution to plant species conservation. Abandoned communal cropland reverted to an indigenous grassland almost devoid of exotic species within c. 20 years. 4It was predicted that frequently cultivated sites (maize and ryegrass) would support less diversity than long-lived pastures (kikuyu and E. curvula). This was contradicted by the relatively high species diversity of communal maize fields, which was attributed to a lack of herbicides, and the depauperate communities of kikuyu and of E. curvula pasture, which were attributed, respectively, to a dense growth form and a severe mowing regime. 5Pine plantations harboured fourfold more indigenous species per plot (27) than other transformed types. Species were mostly shade-tolerant grassland relics that had persisted for 12 years since planting, and some forest colonizers. Indigenous species were unlikely to be maintained because of aggressive invasion by the exotic Rubus cuneifolius and severe disturbance associated with tree harvest and replanting. 6The richness of indigenous grasslands was expected to differ in response to grazing pressure but they differed only in composition. Grasslands were dominated by grasses, despite the richness of herbaceous species. The dominance of Themeda triandra was reduced under livestock grazing in favour of more grazing-tolerant species. Exotic species were inconspicuous except for the dicotyledon Richardia brasiliensis, a subdominant under communal grazing. 7Southern Drakensberg grasslands are probably now stocked with livestock six- to 35-fold higher than during pre-settlement times. A grassland protected for c. 50 years supported twofold greater richness (101 species plot,1) than grazed grasslands, suggesting that a 150-year history of increased mammalian grazing had already reduced plant diversity. 8Synthesis and applications. Land acquisition is costly, thus conservation of plant diversity in the southern Drakensberg requires a policy that inhibits transformation of rangelands. This can be achieved by enhancing their economic viability without changing the vegetation composition. Their inherent value must be recognized, such as for water production. The viability of commercial ranches can be improved by increasing their size. Conservation efforts need to be focused on plant taxa that only occur on unprotected rangelands. [source] The irreversible cattle-driven transformation of a seasonally flooded Australian savannaJOURNAL OF BIOGEOGRAPHY, Issue 5 2003Ben R. Sharp Abstract Aim ,Anecdotal historical and photographic evidence suggests that woody vegetation is increasing dramatically in some northern Australian savanna habitats. Vegetation change in savannas has important implications for pastoral land-use, conservation management, and landscape-scale carbon storage, and informs theoretical debates about ecosystem function. This study seeks to determine the nature, extent and cause(s) of woody vegetation change in a seasonally flooded alluvial savanna habitat. Location ,The study area is located within the seasonally inundated alluvial zone of the tidal portion of the Victoria River, Northern Territory, Australia. The study area has been grazed by domestic stock since c. 1900, prior to which the area was inhabited and more likely regularly burnt by Aboriginal people for thousands of years. Methods ,Digital georeferenced aerial photographic coverages were used to examine and quantify woody vegetation change between 1948 and 1993. Transect surveys of woody and herbaceous vegetation were carried out to ground-truth air-photo results and determine the nature and causes of observed vegetation changes. Results ,There has been a dramatic increase in woody vegetation cover throughout the study area. Vegetation change patterns are roughly uniform across the full range of edaphic habitat variation and are unrelated to the depositional age of fluvial sediments. Two woody species, Eucalyptus microtheca and Excoecaria parvifolia, are predominantly responsible for observed increases. Demographic analyses reveal that woody invasions have been episodic and indicate that in most locations peak woody species establishment occurred in the mid-1970s. Grasses are almost absent in a majority of habitats within the study area. Instead, large areas are covered by scalded soil, dense invasive weed populations, and unpalatable forbs and sedges. What grasses do occur are predominantly of very low value for grazing. The condition of the herbaceous layer renders most of the study area almost completely non-flammable; what fires do burn are small and of low intensity. Main conclusions ,Multiple working hypotheses explaining observed patterns of woody vegetation increase were considered and rejected in turn. The only hypothesis consistent with the evidence is as follows: (1) observed changes are a direct consequence of extreme overgrazing by cattle, most likely when stocking rates peaked in the mid-1970s; (2) prolonged heavy grazing effected the complete transformation of much of the herbaceous vegetation to a new state that is not flammable; and (3) in the absence of regular fire mortality, woody vegetation increased rapidly. The relatively treeless system that existed in 1948 was apparently stable and resilient to moderate grazing levels, and perhaps also to episodic heavy grazing events. However, grazing intensity in excess of a sustainable threshold has forced a transition that is irreversible in the foreseeable future. Stable-state transitions such as this one inform debates at the heart of ecological theory, such as the nature of stability, resilience, equilibrium and carrying capacity in dynamic savanna ecosystems. [source] Processing-induced phase transitions of theophylline,implications on the dissolution of theophylline tabletsJOURNAL OF PHARMACEUTICAL SCIENCES, Issue 5 2007Jaidev S. Tantry Abstract Aqueous wet massing of stable anhydrous theophylline (A) with polyvinylpyrrolidone (PVP) resulted in its complete transformation to theophylline monohydrate (M). Drying at 45°C, resulted in the formation of metastable anhydrous theophylline (A*) which then transformed to A. PVP, a known crystallization inhibitor, was effective in inhibiting the A*,,,A transition. The higher molecular weight polymer, PVP K90, was more effective in inhibiting the A*,,,A transition as compared to PVP K17. The disappearance of M, and the formation of A* and A was simultaneously monitored by XRD. An increase in the drying temperature from 45 to 55°C accelerated the A*,,,A transition. In granules prepared by the high-shear process, approximately 50% of theophylline existed as A and the rest as A*. In contrast, the fluid-bed granulation process yielded granules containing only A. Thus, the physical form of theophylline in tablets was influenced by the molecular weight of the binding agent, the granulation method, and the drying temperature. Using A as the starting material, tablets were manufactured by high-shear aqueous wet granulation process and the A* content was quantified. These tablets were stored under various relative humidity (RH) conditions at 25°C for 2 weeks. Storage at RH,,,33% caused complete A*,,,A conversion accompanied by a pronounced decrease in the initial dissolution rate indicating that phase transitions during processing and storage can have a significant influence on product performance. © 2007 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 96: 1434,1444, 2007 [source] Variable-Temperature Powder X-ray Diffraction of Aromatic Carboxylic Acid and Carboxamide CocrystalsCHEMISTRY - AN ASIAN JOURNAL, Issue 4 2007L. Sreenivas Reddy Abstract The effect of temperature on the cocrystallization of benzoic acid (BA), pentafluorobenzoic acid (FBA), benzamide (BAm), and pentafluorobenzamide (FBAm) is examined in the solid state. BA and FBA formed a 1:1 complex 1 at ambient temperature by grinding with a mortar and pestle. Grinding FBA and BAm together resulted in partial conversion into the 1:1 adduct 2 at 28,°C and complete transformation into the product cocrystal at 78,°C. Further heating (80,100,°C) and then cooling to room temperature gave a different powder pattern from that of 2. BAm and FBAm hardly reacted at ambient temperature, but they afforded the 1:1 cocrystal 3 by melt cocrystallization at 110,115,°C. Both BA+FBAm (4) and BA+BAm (5) reacted to give new crystalline phases upon heating, but the structures of these products could not be determined owing to a lack of diffraction-quality single crystals. The stronger COOH and CONH2 hydrogen-bonding groups of FBA and FBAm yielded the equimolar cocrystal 6 at room temperature, and heating of these solids to 90,100,°C gave a new crystalline phase. The X-ray crystal structures of 1, 2, 3, and 6 are sustained by the acid,acid/amide,amide homosynthons or acid,amide heterosynthon, with additional stabilization from phenyl,perfluorophenyl stacking in 1 and 3. The temperature required for complete transformation into the cocrystal was monitored by in,situ variable-temperature powder X-ray diffraction (VT-PXRD), and formation of the cocrystal was confirmed by matching the experimental peak profile with the simulated diffraction pattern. The reactivity of H-bonding groups and the temperature for cocrystallization are in good agreement with the donor and acceptor strengths of the COOH and CONH2 groups. It was necessary to determine the exact temperature range for quantitative cocrystallization in each case because excessive heating caused undesirable phase transitions. [source] | |||||||||||||