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1.
Marit Seifert Stephanie Hesse Vasken Kabrelian Dieter Klemm 《Journal of polymer science. Part A, Polymer chemistry》2004,42(3):463-470
For the modification of medically useful biomaterials from bacterially synthesized cellulose, fleeces of Acetobacter xylinum have been produced in the presence of 0.5, 1.0, and 2.0% (m/v) carboxymethylcellulose (CMC), methylcellulose (MC), and poly(vinyl alcohol) (PVA), respectively, in the Hestrin-Schramm culture medium. The incorporation of the water-soluble polymers into cellulose and their influence on the structure, crystal modifications, and material properties are described. With IR and solid-state 13C NMR spectroscopy of the fleeces, the presence of the cellulose ethers and an increase in the amorphous parts of the cellulose modifications (NMR results) have been detected. The incorporation is represented by a higher product yield, too. As demonstrated by scanning electron microscopy, a porelike cellulose network structure forms in the presence of CMC and MC. This modified structure increases the water retention ability (expressed as the water content), the ion absorption capacity, and the remaining nitrogen-containing residues from the culture medium or bacteria cells. The water content of bacterial cellulose (BC) in the never dried state and the freeze-dried, reswollen state can be controlled by the CMC concentration in the culture solution. The freeze-dried, reswollen BC-CMC (2.0%) contains 96% water after centrifugation, whereas standard BC has only 73%. About 98% water is included in a BC-MC composite in the wet state, and about 93% is included in the reswollen state synthesized in the presence of 0.5, 1.0, or 2.0% MC. These biomaterial composites can be stored in the dried state and reswollen before use, reaching a higher water absorption than pure, never dried BC. The copper ion capacity of BC-CMC composites increases proportionally with the added amount of CMC. BC-CMC (0.5%) can absorb 3 times more copper ions than original BC. In the case of 0.5 and 1.0% PVA additions to the culture solution, this polymer cannot be detected in the cellulose fleeces after they are washed. Nevertheless the presence of PVA in the culture medium effects a decreased product yield, a retention of nitrogen-containing residues in the material during purification, a reduced water absorption ability, and a slightly higher copper ion capacity in comparison with original BC. The water content of freeze-dried, reswollen BC-PVA (0.5%) is only 62%. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 463–470, 2004 相似文献
2.
Active flow-separation control is an effective and efficient mean for drag reduction and unsteady load alleviation resulting
from locally or massively separated flow. Such a situation occurs in configurations where the aerodynamic performance is of
secondary importance to functionality. The performance of heavy transport helicopters and aeroplanes, having a large, and
almost flat, aft loading ramp suffer from the poor aerodynamics of the aft body. Hence, a combined experimental and numerical
investigation was undertaken on a generic transport aeroplane/helicopter configuration. The experimental study provided surface
pressures, direct drag measurements, surface and smoke flow visualization. The baseline flow was numerically analyzed, using
finite volume solutions of the RANS equations. The baseline flow around the model was insensitive to the Reynolds number in
the range it was tested. The flow separating from the aft body was characterized by two main sources of drag and unsteadiness.
The first is a separation bubble residing at the lower ramp corner and the second is a pair of vortex systems developing and
separating from the sides of the ramp. As the model incidence is decreased, the pair of vortex systems also penetrates deeper
towards the centerline of the ramp, decreasing the pressure and increasing the drag. As expected, the ramp lower corner bubble
was highly receptive to periodic excitation introduced from four addressable piezo-fluidic actuators situated at the ramp
lower corner. Total drag was reduced by 3–11%, depending on the model incidence. There are indications that the flow in the
wake of the model is also significantly steadier when the bubble at the lower ramp corner is eliminated. The vortex system
is tighter and steadier when the ramp-corner bubble is eliminated. 相似文献
3.
F. Jancik B. Seifert C. K. Laird W. Czysz J. S. Dunnett W. Schmidt und P. Bachhausen 《Fresenius' Journal of Analytical Chemistry》1987,327(7):747-755
Ohne Zusammenfassung 相似文献
4.
Ohne Zusammenfassung 相似文献
5.
Golodnitsky Diana Ulus Avi Ishay Jacob S. 《Journal of Thermal Analysis and Calorimetry》2003,74(1):85-96
In this paper we try to establish a link between the microclimate in the wasp nest and the structure and thermal stability
of vespid silk. We suggest that there are at least two types of water that is absorbed by the silk of Oriental hornets, namely,
surface water and intrinsic structural water. The release of both types of water was found to be reversible. The enthalpy
values of the endothermic peaks associated with the release of water from different silk samples do not differ substantially
and are in the range of 106 to 130 J g-1 for the Vespa orientalis male larvae silk (sample #1), Paravespula germanica (yellowjacket) worker larvae silk (#3) and Vespa orientalis nest envelope(#4). For the Vespa orientalis worker larvae silk (sample #2), however, it is twice as large (228 J g-1). This is in agreement with the increased total amount of absorbed water. The silk studied has a fibrilar structure with
interconnecting surfaces overlying entire regions. It is assumed that the initial water loss stems from water evaporation
from the coat of the fibers - a daily occurrence in the hornets' nest. Heating to above 70°C may result in structural changes
in the silk core.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
6.
7.
Hay DN Rickert PG Seifert S Firestone MA 《Journal of the American Chemical Society》2004,126(8):2290-2291
The synthesis and purification of a poly(N-isopropylacrylamide)-lipid conjugate and its use in the preparation of a thermoresponsive lipid mesophase is described. Specifically, poly(N-isopropylacrylamide) with a single carboxyl group at one end was activated with dicyclohexylcarbodiimide/N-hydroxysuccinimide to form an active ester. This N-hydroxysuccinimide ester was then used to form a dimyristoyl-sn-glycero-3-phosphoethanolamine conjugate with poly(N-isopropylacrylamide) via an amide bond, rendering the conjugate amphiphilic. Quaternary phases comprising the conjugate, a phosopholipid, dimyristoylphosphatidylcholine, and a cosurfactant, N,N-dimethyldodecylamine-N-oxide, dispersed in water were found to self-assemble at room temperature to form liquid crystalline gels, adopting an expanded lamellar structure. A modest increase in temperature triggered the reversible conversion of the aggregate to a collapsed lamellar structure, while a modest reduction in temperature resulted in its conversion to a nonlamellar phase. The phases were characterized by polarized optical microscopy and small-angle X-ray scattering (SAXS). 相似文献
8.
The Systems AI/CoI2 (A = Alkali Metal, Tl, Ag) and the Crystal Chemistry of the Double Halides AnCoX(n+2) with X = Cl, Br, I The systems AI/CoI2 (A = Cs, Rb, K, Tl, Na, Ag) were investigated by differencethermal analysis. The systems of NaI and AgI are found to be eutectical. A compound A2CoI4 always exists in the other systems. Cs2CoI4 crystallizes in the β-K2SO4 type with a coordination number (C.N.) for Cs equal to 9/10. Results obtained with single crystal technique reveal for the first time that among the double halides Rb2CoI4 is of the monoclinic Sr2GeS4 type (C. N. for Rb = 6(+2)). The compounds K2CoJ4, Tl2CoJ4, T-K2CoBr4, and T-Tl2CoBr4 are isotypic. Both structure groups are characterized by isolated CoX42? tetrahedra. Reflectance spectra and magnetic susceptibilities can be explained on the basis of crystal field theory. – Our results close presently existing gaps in the knowledge on systems of CoBr2 and CoCl2 too. 相似文献
9.
Aviñó A Cubero E González C Eritja R Orozco M 《Journal of the American Chemical Society》2003,125(51):16127-16138
The structural, dynamical, and recognition properties of antiparallel DNA triplexes formed by the antiparallel d(G#G.C), d(A#A.T), and d(T#A.T) motifs (the pound sign and dot mean reverse-Hoogsteen and Watson-Crick hydrogen bonds, respectively) are studied by means of "state of the art" molecular dynamics simulations. Once the characteristics of the helix are defined, molecular dynamics and thermodynamic integration calculations are used to determine the expected stabilization of the antiparallel triplex caused by the introduction of 8-aminopurines. Finally, oligonucleotides containing 8-aminopurine derivatives are synthesized and tested experimentally using several approaches in a variety of systems. A very large stabilization of the triplex is found experimentally, as predicted by simulations. These results open the possibility for the use of oligonucleotides carrying 8-aminopurines to bind single-stranded nucleic acids by formation of antiparallel triplexes. 相似文献
10.
The interplay between electronic and geometric structure is investigated for covalently bonded phosphorus clusters. We use a modification of the molecular dynamics/ density functional (MD/DF) method of Car and Parrinello, describing the electronic structure by a simplified linear combination of atomic orbitals (LCAO) approach. The results show clearly the tendency of phosphorus to threefold coordination, and substantial variations in bond angles lead to a large variety of isomers. 相似文献