Synthesis of Low-Density Microcellular Materials

This section describes and gives examples of processes developed at the DOE laboratories to make LDMMs. In general, these processes fall under one of two schemes: (1) phase separation of polymer or polymer-like solutions, or (2) replication of sacrificial pore formers. Both schemes produce a liquid-filled precursor. The LDMM can be obtained only if this precursor can be “dried” without collapsing the underlying structure. This key synthetic step is rarely a trivial task, and because of its importance, this section starts with a description of LDMM drying technologies.Most liquid-filled LDMM precursors will collapse into a high-density mass if subjected to evaporative drying. This occurs because large capillary forces are generated when the liquid meniscus moves through the small cells and pores of the precursor. Only very strong and relatively large-celled precursor structures (e.g., inverse emulsions) can withstand these forces.Some LDMM precursors of moderate strength can be dried by evaporation if the precursor is exchanged into a liquid of low surface tension. Liquified carbon dioxide is an example — at room temperature, liquid CO2 has a surface tension of 1–2 dynes/cm, compared to ˜70 dynes/cm for water, and 20–40 dynes/cm for most organic solvents. CO2 evaporative drying should not be confused with C02 supercritical drying discussed later.