920 research outputs found
Stretching necklaces
Polyelectrolytes in poor solvents show a necklace structure where collapsed
polymer pearls are linked to stretched strings. In the present paper the
elasticity of such chains is studied in detail. Different deformation regimes
are addressed. The first is the continuous regime, where many pearls are
present. A continuous force extension relation ship is calculated. The main
contribution comes from the tension balance and the electrostatic repulsion of
consecutive pearls. The main correction term stems from the finite size of the
pearls, which monitors their surface energy. For a finite amount of pearls
discontinuous stretching is predicted. Finally counterion effects are discussed
qualitatively.Comment: to appear in European Phys. Journal E (soft matter
Compression of finite size polymer brushes
We consider edge effects in grafted polymer layers under compression. For a
semi-infinite brush, the penetration depth of edge effects is larger than the natural height and the actual height
. For a brush of finite lateral size (width of a stripe or radius of a
disk), the lateral extension of the border chains follows the scaling law
. The scaling function is estimated within
the framework of a local Flory theory for stripe-shaped grafting surfaces. For
small , decays as a power law in agreement with simple arguments.
The effective line tension and the variation with compression height of the
force applied on the brush are also calculated.Comment: 6 pages, 7 figures, submitted to PCC
Microphase separation in thin block copolymer films: a weak segregation mean-field approach
In this paper we consider thin films of AB block copolymer melts confined
between two parallel plates. The plates are identical and may have a preference
for one of the monomer types over the other. The system is characterized by
four parameters: the Flory-Huggins chi-parameter, the fraction f of A-monomers
in the block copolymer molecules, the film thickness d, and a parameter h
quantifying the preference of the plates for the monomers of type A. In certain
regions of parameter space, the film will be microphase separated. Various
structures have been observed experimentally, each of them characterized by a
certain symmetry, orientation, and periodicity. We study the system
theoretically using the weak segregation approximation to mean field theory. We
restrict our analysis to the region of the parameter space where the film
thickness d is close to a small multiple of the natural periodicity. We will
present our results in the form of phase diagrams in which the absolute value
of the deviation of the film thickness from a multiple of the bulk periodicity
is placed along the horizontal axis, and the chi-parameter is placed along the
vertical axis; both axes are rescaled with a factor which depends on the
A-monomer fraction f. We present a series of such phase diagrams for increasing
values of the surface affinity for the A-monomers. We find that if the film
thickness is almost commensurate with the bulk periodicity, parallel
orientations of the structures are favoured over perpendicular orientations. We
also predict that on increasing the surface affinity, the region of stability
of the bcc phase shrinks.Comment: 35 pages, 20 figure
Theoretical Study of Comb-Polymers Adsorption on Solid Surfaces
We propose a theoretical investigation of the physical adsorption of neutral
comb-polymers with an adsorbing skeleton and non-adsorbing side-chains on a
flat surface. Such polymers are particularly interesting as "dynamic coating"
matrices for bio-separations, especially for DNA sequencing, capillary
electrophoresis and lab-on-chips. Separation performances are increased by
coating the inner surface of the capillaries with neutral polymers. This method
allows to screen the surface charges, thus to prevent electro-osmosis flow and
adhesion of charged macromolecules (e.g. proteins) on the capillary walls. We
identify three adsorption regimes: a "mushroom" regime, in which the coating is
formed by strongly adsorbed skeleton loops and the side-chains anchored on the
skeleton are in a swollen state, a "brush" regime, characterized by a uniform
multi-chains coating with an extended layer of non-adsorbing side-chains and a
non-adsorbed regime. By using a combination of mean field and scaling
approaches, we explicitly derive asymptotic forms for the monomer concentration
profiles, for the adsorption free energy and for the thickness of the adsorbed
layer as a function of the skeleton and side-chains sizes and of the adsorption
parameters. Moreover, we obtain the scaling laws for the transitions between
the different regimes. These predictions can be checked by performing
experiments aimed at investigating polymer adsorption, such as Neutron or X-ray
Reflectometry, Ellipsometry, Quartz Microbalance, or Surface Force Apparatus.Comment: 30 pages, 7 figures, to be published in Macromolecule
Cirurgia nas mamas: a experiência de mulheres que buscam a harmonia com seus corpos
TCC (graduação em Enfermagem) - Universidade Federal de Santa Catarina, Florianópoli
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