CO2-Blown Nanocellular Foams

Polymeric nanocellular foams are broadly defined as having cell size below one micron. However, it is only when cell size reaches 100 nm or less that unique thermal conductivity, dielectric constant, optical, or mechanical properties are expected due to gas confinement in the cells or polymer confinement in the cell walls. Producing such materials with low density by physical foaming with CO2 requires the controlled nucleation and growth of 10(15)-10(16) cells/cm(3). This is a formidable challenge that necessitates new foaming strategies. This review provides a description of processes, conditions, and polymer systems that have been employed over the past 15 years to achieve increasingly higher cell densities and expansion ratio, culminating with the recent development of low density nanofoams and of nanostructured polymers in which nucleation can be precisely controlled. Remaining barriers to scale-up are summarized. (C) 2014 Wiley Periodicals, Inc.