Thinner Plastic Blocks Gas Better

Plastic packaging can't be used for bottling products such as beer because it's permeable to oxygen and carbon dioxide. Although these gas molecules are blocked by crystalline regions in plastic film, they readily slip through amorphous zones between the crystalline regions. Researchers now report the counterintuitive finding that making a plastic film super-thin reduces its permeability to oxygen by two orders of magnitude. Anne Hiltner of Case Western Reserve University, in Cleveland, and colleagues coextruded poly(ethylene oxide) and poly(ethylene-co-acrylic acid). The researchers passed the assembly through a series of dies that split and spread the combined layers. The layers were then stacked on others and the process was repeated to create films with thousands of ever-thinner layers. When the researchers tested the performance of the resulting films, they were startled to find that oxygen and carbon dioxide permeability fell as the polymer layers became thinner. They discovered that when poly(ethylene oxide) is confined to layers just 20-nm thick, it forms large ordered regions that resemble giant single crystals (Science 2009, 323, 757).