Low-density, salt-loaded foams. [RbF] Page: 3 of 11
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solvent is evaporated to cause supersaturation (i.e., the equilibrium polymer-
rich phase grows into the solvent phase). Because spinoidal decomposition
occurs with diffuse boundaries (surfaces are not made), interpenetrating
networks are formed.
In cases where there is not enough supersaturation to cause phase
separation, solidification may occur by nucleation and growth. The nuclei are
supplied by the polymer-rich portion of the interpenetrating network. Growth
takes place to form columnar structures and continues until the growth rate
of the column exceeds the diffusion rate of the solvent. After a critical
columnar length is reached, the growth rate decreases until the solvent dif-
fusion rate reaches a high enough value to reinitiate the growth. However,
because more time is required for the solvent to reach the new layer of polymer,
the columns become longer and wider.
These general processes have been observed in the formation of both PAN
and CA foams. Spinoidal decomposition yields foams with uniform micrometer-
sized cells; nucleation and growth yields foams with columnar cells. Either
mechanism of polymer solidification can be selected by adjusting the solvent
system and the temperature of solidification.
EXPERIMENTAL PAN FOAMS
The process for making salt-loaded PAN foams consists of five basic steps:
(1) Preparation of the PAN solution.
(2) Molding or extrusion of a polymer gel rod.
(3) Soaking the gel rods in rubidium fluoride solutions.
(4) Solvent transfer to remove water.
(5) Freeze drying.
Each step is discussed below.
PAN Solution Preparation
For uniform, isotropic, micrometer-sized cells, the polymer solution must
be close to precipitation. That is, the polymer must be dissolved in a solvent
or solvent mixture that barely dissolves the polymer. When this solution is
extruded into a nonsolvent, the polymer phases out of solution rapidly
(spinoidal decomposition) and forms small, uniform, open cells. If the solvent
used to prepare the polymer solution is too good (i.e., readily dissolves the
polymer), columnar cells form by nucleation and growth.
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Rinde, J.A. Low-density, salt-loaded foams. [RbF], report, January 10, 1979; Livermore, California. (https://digital.library.unt.edu/ark:/67531/metadc1208676/m1/3/: accessed July 16, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.