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  • 1
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] Parallel processes for patterning densely packed nanometre-scale structures are critical for many diverse areas of nanotechnology. Thin films of diblock copolymers can self-assemble into ordered periodic structures at the molecular scale (∼5 to 50 nm), and have been used as templates ...
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  • 2
    ISSN: 1573-904X
    Keywords: trehalose ; phase diagram ; glass transition ; viscosity ; molar volume ; cryoprotection
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Purpose. To address the lack of fundamental thermophysical data for trehalose and its aqueous systems by measuring equilibrium and non-equilibrium properties of such systems. Methods/Results. Differential scanning calorimetry (DSC) and dynamic mechanical analysis were used to measure glass transition temperatures of trehalose and its solutions. X-ray diffractometry was used to verify the structure of amorphous trehalose. Controlled-stress rheometry was used to measure viscosity of several aqueous trehalose systems at ambient and sub-ambient temperatures. Over this temperature range, the density of these solutions was also measured with a vibrating tube densimeter. The equilibrium phase diagram of aqueous trehalose was determined by measuring the solubility and freezing point depression. Conclusions. Our solubility measurements, which have allowed long times for attainment of chemical equilibrium, are substantially different from those reported earlier that used different techniques. Our measurements of the glass transition temperature of trehalose are higher than reported values. A simple model for the glass transition is presented to describe our experimental observations.
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  • 3
    ISSN: 1573-904X
    Keywords: protein stabilization ; cryopreservation ; lyophilization ; trehalose ; borate
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Purpose. The purpose of this work was to investigate the effects of trehalose and trehalose/sodium tetraborate mixtures on the recovery of lactate dehydrogenase (LDH) activity following freeze-thawing and centrifugal vacuum-drying/rehydration. The storage stability of LDH under conditions of either high relative humidity or high temperature was also studied. Methods. LDH was prepared in buffered aqueous formulations containing trehalose alone and trehalose/'borate' mixtures. Enzymatic activity was measured immediately following freeze-thawing and vacuum-drying/rehydration processes, and also after vacuum-dried formulations were stored in either high humidity or high temperature environments. Also, glass transition temperatures (Tg) were measured for both freeze-dried and vacuum-dried formulations. Results. The Tgvalues of freeze-dried trehalose/borate mixtures are considerably higher than that of trehalose alone. Freezing and vacuum-drying LDH in the presence of 300 mM trehalose resulted in the recovery of 80% and 65% of the original activity, respectively. For vacuum-dried mixtures, boron concentrations below 1.2 mole boron/ mole trehalose had no effect on recovered LDH. After several weeks storage in either humid (100% relative humidity) or warm (45°C) environments, vacuum-dried formulations that included trehalose and borate showed greater enzymatic activities than those prepared with trehalose alone. We attribute this stability to the formation of a chemical complex between trehalose and borate. Conclusions. The high Tgvalues of trehalose/borate mixtures offer several advantages over the use of trehalose alone. Most notable is the storage stability under conditions of high temperature and high relative humidity. In these cases, formulations that contain trehalose and borate are superior to those containing trehalose alone. These results have practical implications for long-term storage of biological materials.
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  • 4
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Physics
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  • 5
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 108 (1998), S. 9905-9911 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A Monte Carlo simulation study has been conducted to assess the ability of recently proposed force fields to predict orthobaric densities, second virial coefficients, and P-V-T data for short and long alkanes. A new, modified force field is proposed that provides good agreement with experimental phase equilibrium and second virial coefficient data over wide ranges of temperature and chain length. © 1998 American Institute of Physics.
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  • 6
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 2911-2923 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Pseudo-ensemble simulations and Gibbs–Duhem integrations are formulated within the framework of the expanded grand canonical ensemble. Pseudo-isobaric–isothermal simulations are proposed in which volume moves are replaced by fluctuations in the number of molecular segments. For large systems of dense athermal polymers, this pseudo-isobaric–isothermal method is shown to achieve mechanical equilibration faster than both conventional volume moves and the recently proposed slab volume moves. Pseudo-ensembles are also discussed for Gibbs ensemble simulations and canonical simulation (of the chemical potential). It is shown that coexistence curves for pure homopolymers and polymer mixtures can be traced by performing a numerical integration of the Gibbs–Duhem equation based on (expanded) grand canonical simulations. The validity of the methods is demonstrated by tracing the vapor–liquid coexistence curve of pure square-well chains and the liquid–liquid binodal curve of a blend of square-well chains. © 1997 American Institute of Physics.
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  • 7
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 9858-9868 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Monte Carlo simulations of mesogenic polymers have been performed for single and binary systems in the bulk and for pure components in constrained environments. An expanded ensemble formalism was employed to facilitate fluctuations of the number of particles in grand canonical, Gibbs, and pseudo-Gibbs ensemble simulations. Phase diagrams were generated for pure systems of athermal 8-mers and 16-mers with varying degrees of flexibility, for a binary mixture of rigid and semiflexible 16-mers (at constant pressure and variable flexibility), and for a bidisperse mixture of rigid chains. Simulations of the partitioning of rigid 12-mer chains between a bulk and a slit pore revealed that, for a certain range of slit gaps, the preferential coplanar orientation of molecules at the proximity of the walls induces the formation of a nematic phase within the pore. A similar induction of a nematic phase was observed for anisotropic molecules dispersed in a stretched, swollen gel. For the bulk systems, our simulation results were compared to predictions of various theories. © 1997 American Institute of Physics.
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  • 8
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: This note presents an analysis of the viscosity of linear and branched polymer melts at low shear rates and compares the results of several different nonequilibrium molecular dynamics methods. The new, more precise results presented here indicate that earlier calculations of the zero shear viscosity of linear polymers are consistent with Green–Kubo estimates. For branched chains, however, these results suggest that earlier extrapolation of viscosities to zero shear rate overpredicted the effect of branching. © 1997 American Institute of Physics.
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  • 9
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 793-810 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Results of Monte Carlo simulations are presented for the equilibrium swelling of athermal (hard-core) polymeric gels. The networks investigated in this study are defect-free and exhibit strand lengths that range from small to moderate. It is shown that at equilibrium, increasing pressure and strand length leads to higher solvent holdups by the gel. Computer experiments of swelling of gels in binary solvent mixtures indicate that the gel absorbs preferentially the component having the smaller molecular excluded volume. A simple mean-field theory is also presented to describe the swelling of athermal networks. Agreement between theory and simulations is good. © 1997 American Institute of Physics.
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  • 10
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 105 (1996), S. 4391-4394 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A novel formalism is presented for simulation of polymers in expanded grand canonical and expanded Gibbs ensembles. Molecular creation and destruction attempts are replaced by transition attempts between states of a tagged chain of variable length. Results are presented for expanded grand canonical simulations of hard-core chain fluids in the bulk and in a slit pore and for expanded Gibbs ensemble simulations of vapor–liquid equilibria for square-well chains. © 1996 American Institute of Physics.
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