Membranes with through-thickness porosity prepared by unidirectional freezing

Directional freezing is a simple and environmentally friendly method for producing aligned porous materials. Porous structures of uniaxially aligned nanoparticles can be produced by unidirectional freezing a concentrated nanoparticle dispersion and subsequent freeze-drying. However, mechanically strong through-thickness membranes have seldom been reported. The film prepared by directional freezing and subsequent freeze-drying is usually too weak to be a free-standing membrane. By using precise control of freezing rate and direction, we successfully produced free-standing films (with 20-200 μm thickness and 40-60 vol% through-thickness porosity) from inorganic particles/polymer (polytetrafluoroethylene and polyvinylidene fluoride) composites. The pore size could be conveniently controlled by freezing rate and dispersion concentration. The use of composite materials, emulsion states, and post-annealing processes facilitated the preparation of free-standing two-dimensional particulate networks due to enhanced interparticulate coherence. This method could provide novel porous networks with controlled morphology, reduced tortuosity, and enhanced mechanical properties, which have broad applications such as separation/purification, fuel cells, nanocomposites, catalysts, tissue engineering, controlled delivery, and other medical applications.