Tech Briefs

Films can be tailored to reduce permeability or to enable filtration.

Several advances have been made in research in the art of tailoring polymeric films to be, variously, (1) much more effective as barriers to diffusion of liquids and gases than they would otherwise be or (2) porous, with pore sizes suitable for filtration of selected gases or liquids. The part of the research addressing the barrier problem has provided a scientific basis for reducing the permeabilities of paints and packaging films to small fractions of the permeabilities of corresponding paints and packaging materials now in common use. In this part of the research, consideration was given to two strategies, described below, that can be used separately or together.

This Electron Micrograph shows a polystyrene film containing nanopores. The film was made by solution casting and rapid drying of a styrene/lactic acid copolymer, followed by etching with a base to remove the poly(lactic acid).
One strategy is to incorporate thin, impermeable flakes into a polymer film, oriented substantially in the plane of the film, in order to force diffusing molecules to take long, tortuous paths through the film. This strategy reduces both the lag (the time before a significant amount of penetration of the film by a diffusing substance) and the steady-state rate of diffusion during long exposure. Early in this research, it was found, consistently with observations by other researchers, that incorporation of properly aligned flakes of mica or clay into a polymer film is effective in reducing the permeability of the film. Later, it was found that membranes can be made from block copolymers that, under the proper process conditions, align themselves so as to form flake-like structures within the membranes. If one of the polymer blocks in a given membrane is a glass (which is typically impermeable), then permeability of the membrane as a whole is correspondingly reduced. If the other polymer block is a rubber, then the membrane remains flexible. Thus, the membrane is effectively a composite material having flexibility dominated by the rubber and permeability dominated by the glass.

The other diffusion-barrier strategy is to incorporate a sacrificial scavenger material that reacts chemically with diffusing molecules to trap them. This strategy can increase the lag by a large factor. Of course, this strategy does not inhibit steady-state diffusion because it ceases to be effective once all the scavenger material has been consumed. The effectiveness of this strategy was demonstrated in experiments in which iron nanoparticles were incorporated as a scavenger material into polyethylene films like those used as barriers in landfills. The lag in diffusion of chlorinated hydrocarbons through the iron-containing films was 300 times that through non-iron-containing films.

The portion of the research addressing porosity has been motivated by the prospect of using films containing nanoscopic pores for filtering water. Several approaches to utilization of the tendency of block copolymers to assemble themselves into aligned structures have been investigated. The most promising approach is based on the observation that when a diblock copolymer is cast rapidly from a highly volatile solvents, under some circumstances, the polymer blocks segregate themselves so as to form cylinders of one polymer, oriented perpendicularly to the surface, within a matrix of the other polymer. Then the cylinder polymer can be etched away, leaving a porous film consisting of the matrix polymer (see figure).

This work was done by Edward L. Cussier of the University of Minnesota for the Air Force Research Laboratory.

AFRL-0052

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Films Containing Nanostructures and/or Reactive Scavengers (reference AFRL-0052) is currently available for download from the TSP library.

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