6,376 research outputs found
On the Helix-Coil transition in grafted chains
The helix-coil transition is modified by grafting to a surface. This
modification is studied for short peptides capable of forming -helices.
Three factors are involved: (i) the grafting can induced change of the boundary
free energy of the helical domain (ii) the van der Waals attraction between the
helices and (iii) the crowding induced stretching of the coils. As a result the
helix-coil transition acquires ``all or nothing'' characteristics. In addition
the transition temperature is elevated and the transition itself sharpens as
the grafting density increases.Comment: 6 pages, 1 figures, europhys.sty and euromacro.sty Submitted to
Europhys. Let
Concentration Dependence of the Flory Chi Parameter within Two-State Models
The Flory chi parameter is typically assumed to depend only on the
temperature, T. Experimental results often require the replacement of this
chi(T) by chieff, that depends also on the monomer volume fraction, phi,
chieff(phi,T). Such chieff(phi,T) can arise from two state-models, proposed for
polyetheleneoxide (PEO) and other neutral water-soluble polymers. The predicted
phi dependence of chibar=chieff-(1-phi)\partial \chieff/\partial phi,
obtainable from colligative properties, differs qualitatively between the
various models: (i) The model of Karlstrom (J. Phys. Chem. 1985, 89, 4962)
yields \partial chibar/\partial phi > 0 while the model of Matsuyama and Tanaka
(Phys. Rev. Lett. 1990, 65, 341) and of Bekiranov et al (Phys. Rev. E 1997, 55,
577) allows for \partial chibar/\partial phi <0 (ii) chibar(phi) as calculated
from the Karlstrom model, utilizing the parameters used to fit the phase
diagram of PEO, agrees semiquantitatively with the experimental values. On the
other hand, chibar(phi) similarly calculated from the model of Bekiranov et al.
differs qualitatively from the measured results. Altogether, chibar(phi)
provides useful measure for the performance of a model.Comment: 8 pages, 5 figures, Macromolecules, in pres
Signatures of a Concentration Dependent Flory chi Parameter: Swelling and Collapse of Coils and Brushes
The quality of solvents of polymers is often described in terms of the Flory
chi parameter typically assumed to depend only on the temperature, T. In
certain polymer-solvent systems fitting the experimental data enforces the
replacement of chi(T) by a concentration dependent chieff. In turn, this
modifies the swelling and collapse behavior. These effects are studied, in the
framework of a mean-field theory, for isolated coils and for planar brushes.
The phi dependence of chieff gives rise to three main consequences: (i) Shift
in the cross-over between Gaussian and self-avoidance regimes; (ii) A
possibility of first-order collapse transition for isolated flexible coils;
(iii) The possibility of a first-order phase transition leading to a vertical
phase separation within the brush. The discussion relates these effects
directly to thermodynamic measurements and does not involve a specific
microscopic model. The implementation for the case of
Poly(N-isopropylacrylamide) (PNIPAM) is discussed.Comment: 12 pages, 9 figure
HVAC modifications and computerized energy analysis for the Operations Support Building at the Mars Deep Space Station at Goldstone
The key heating, ventilation, and air-conditioning (HVAC) modifications implemented at the Mars Deep Space Station's Operation Support Building at Jet Propulsion Laboratories (JPL) in order to reduce energy consumption and decrease operating costs are described. An energy analysis comparison between the computer simulated model for the building and the actual meter data was presented. The measurement performance data showed that the cumulative energy savings was about 21% for the period 1979 to 1981. The deviation from simulated data to measurement performance data was only about 3%
Grafted Rods: A Tilting Phase Transition
A tilting phase transition is predicted for systems comprising rod like
molecules which are irreversibly grafted to a flat surface, so that the non
interacting rods are perpendicularly oriented. The transition is controlled by
the grafting density . It occurs as increases as a result of the
interplay between two energies. Tilt is favoured by the van-der-Waals
attraction between the rods. It is opposed by the bending elasticity of the
grafting functionality. The role of temperature is discussed, and the tilting
mechanism is compared to other tilting transitions reported in the literature.Comment: 21 pages, 2 figures, to appear in Journal de Physique I
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