64 research outputs found
Solvent evaporation of spin cast films: crust effects
When a glassy polymer film is formed by evaporation, the region near the free
surface is polymer rich and becomes glassy first, as noticed long ago by
Scriven et al. We discuss the thickness of this "crust" and the time interval
where it is present -before freezing of the whole film. We argue that the crust
is under mechanical tension, nd should form some cracks. This may be the source
of the roughness observed on the final, dry films, when the solvent vapor
pressure is high (and leads to thin crusts).Comment: 8 pages, 2 EPS figures. submitted to Europ. Phys. J. (E
Instabilities during the evaporation of a film: non glassy polymer + volatile solvent
We consider solutions where the surface tension of the solvent is smaller
than the surface tension of the polymer. In an evaporating film, a plume of
solvent rich fluid, then induces a local depression in surface tension, and the
surface forces tend to strengthen the plume. We give an estimate (at the level
of scaling laws) for the minimum thickness required to obtain this instability.
We predict that the thickness a) is a decreasing function of the solvent vapor
pressure b) should be very small (<1 micron) provided that the initial solution
is rather dilute. The overall evaporation time for the film should be much
longer than the growth time of the instability. The instability should lead to
distortions of the free surface and may be optically observable. It should
dominate over the classical Bernard-Marangoni instability induced by cooling.Comment: 8 pages, 2 EPS figures. submitted to Eur. Phys. J. (E
A spatial interpretation of emerging superconductivity in lightly doped cuprates
The formation of domains comprising alternating 'hole rich' and 'hole poor'
ladders recently observed by Scanning Tunneling Microscopy by Kohsaka et al.,
on lightly hole doped cuprates, is interpreted in terms of an attractive
mechanism which favors the presence of doped holes on Cu sites located each on
one side of an oxygen atom. This mechanism leads to a geometrical pattern of
alternating hole-rich and hole-poor ladders with a periodicity equal to 4 times
the lattice spacing in the CuO plane, as observed experimentally. To cite this
article: G. Deutscher, P.-G. de Gennes, C. R. Physique 8 (2007).Comment: 4 pages, 3 figuer
Damping of acoustic waves in dilute polymer solutions
The shear viscosity eta_s(w) of dilute polymer solutions (of flexible coils)
has been classically measured and interpreted in terms of the Zimm modes. We
point out that the longitudinal viscosity eta_l(w) is the sum of two components
eta_l=4*eta_s/3+eta_p, where eta_p is due to an effect of pressure on the coil
size. We discuss eta_p for a crude model involving a dependence of both the
Kuhn length a and the Flory parameter chi on pressure. We find that eta_p is
comparable to eta_s for good solvent conditions, but that eta_p should dominate
over eta_s near the theta point.Comment: 5 page
A Scaling Theory of the Competition between Interdiffusion and Cross-Linking at Polymer Interfaces
We study theoretically situations where competition arises between an
interdiffusion process and a cross-linking chemical reaction at interfaces
between pieces of the same polymer material. An example of such a situation is
observable in the formation of latex films, where, in the presence of a
cross-linking additive, colloidal polymer particles initially in suspension
come at contact as the solvent evaporates, and, optimally, coalesce into a
continuous coating. We considered the low cross-link density situation in a
previous paper (A. Aradian, E. Raphael, P.-G. de Gennes, Macromolecules 33,
9444 (2000)), and presented a simple control parameter that determines the
final state of the interface. In the present article, with the help of simple
scaling arguments, we extend our description to higher cross-link densities. We
provide predictions for the strength of the interface in different favorable
and unfavorable regimes, and discuss how it can be optimized.Comment: 19 pages, 5 figures. To appear in Macromolecule
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