Synthesis of Low-Density Microcellular Materials
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acting as a thermal insulator and a résistance to vapor transport away from the interface. The only requirement in freeze-drying LDMM precursors is that the liquid always remain thoroughly frozen. In principle, a well-insulated frozen precursor should not require additional cooling because the process of evaporation is endôthermic. In practice, however, freeze-drying is facilitated by the use of programmable refrigerated Systems, especially for diluents with a relatively low melting point. Freeze-drying must be used with care in processing LDMMs. The process of
freezing can damage weak precursors by the formation of large solvent crystals at the freezing front.
Supercritical Fluid Processing (SCFP) Supercritical fluid processing (SCFP) is the least d a m a g i n g technique for eliminating the diluent from délicate LDMM precursors. In this process, the precursor is placed in a pressure vessel and heated above the supercritical point of the diluent. After a dwell period, the pressure vessel is slowly vented to atmosphère to exchange air with the supercritical fluid in the cells of the LDMM.
homogeneous
solid
Polymer Concentration Figure 3.1 Typical température vs. concentration phase diagramfora polymer/solvent System showing the binodal (heavy Une), spinodal (light Une), and freezing point (heavy solid and broken Unes) curves. Nucleated liquid phase séparation takes place between the binodal and spinodal curves (région I), and instantaneous phase séparation below the spinodal curve (région II). Arrow Une shows a typical quench
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