222 research outputs found
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
Self-assembled aggregates in the gravitational field: growth and nematic order
The influence of the gravitational field on the reversible process of
assembly and disassembly of linear aggregates is focus of this paper. Even the
earth gravitational field can affect the equilibrium properties of heavy
biological aggregates such as microtubules or actin filaments. The gravity
gives rise to the concentration gradient which results in the distribution of
aggregates of different lengths with height. Strong enough gravitational field
induces the overall growth of the aggregates. The gravitational field
facilitates the isotropic to nematic phase transition reflecting in a broader
transition region. Coexisting phases have notedly different length
distributions and the phase transition represent the interplay between the
growth in the isotropic phase and the precipitation into nematic phase. The
fields in an ultracentrifuge can only reinforce the effect of gravity, so the
present description can be applied to a wider range of systems
General model of phospholipid bilayers in fluid phase within the single chain mean field theory
Coarse-grained model for saturated (DCPC, DLPC, DMPC, DPPC, DSPC) and
unsaturated (POPC, DOPC) phospholipids is introduced within the Single Chain
Mean Field theory. A single set of parameters adjusted for DMPC bilayers gives
an adequate description of equilibrium and mechanical properties of a range of
saturated lipid molecules that differ only in length of their hydrophobic tails
and unsaturated (POPC, DOPC) phospholipids which have double bonds in the
tails. A double bond is modeled with a fixed angle of 120 degrees, while the
rest of the parameters are kept the same as saturated lipids. The thickness of
the bilayer and its hydrophobic core, the compressibility and the equilibrium
area per lipid correspond to experimentally measured values for each lipid,
changing linearly with the length of the tail. The model for unsaturated
phospholipids also fetches main thermodynamical properties of the bilayers.
This model is used for an accurate estimation of the free energies of the
compressed or stretched bilayers in stacks or multilayers and gives reasonable
estimates for free energies. The proposed model may further be used for studies
of mixtures of lipids, small molecule inclusions, interactions of bilayers with
embedded proteins
Self-Consistent Field Theory of Brushes of Neutral Water-Soluble Polymers
The Self-Consistent Field theory of brushes of neutral water-soluble polymers
described by two-state models is formulated in terms of the effective Flory
interaction parameter that depends on both temperature, T
and the monomer volume fraction, . The concentration profiles,
distribution of free ends and compression force profiles are obtained in the
presence and in the absence of a vertical phase separation. A vertical phase
separation within the layer leads to a distinctive compression force profile
and a minimum in the plot of the moments of the concentration profile vs. the
grafting density. The analysis is applied explicitly to the Karalstrom model.
The relevance to brushes of Poly(N-isopropylacrylamide)(PNIPAM) is discussed.Comment: Accepted for publication in the Journal of Chemical Physic
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