Bottlebrush polymers filter water

U. BUFFALO (US) — A newly developed nanomembrane with pores large enough for water to flow freely, but small enough to keep bacteria out may be the answer to the global water problem.

To the naked eye, water molecules and germs are both invisible—but at the microscopic level, differ greatly in size. A single water molecule is less than a nanometer wide, while some of the most diminutive bacteria are a couple hundred.

Working with a special kind of polymer called a block copolymer, researchers synthesized a new kind of nanomembrane containing pores about 55 nanometers in diameter—the largest to date.

Block copolymers have special properties that ensure pores will be evenly spaced, says Javid Rzayev, assistant professor of chemistry at the University at Buffalo.

“These materials present new opportunities for use as filtration membranes,” he says. “Commercial membranes have limitations as far as pore density or uniformity of the pore size. The membranes prepared from block copolymers have a very dense distribution of pores, and the pores are uniform.”

Details of the research are reported online in the journal Nano Letters.

“There’s a lot of research in this area, but what our research team was able to accomplish is to expand the range of available pores to 50 nanometers in diameter, which was previously unattainable by block-copolymer-based methods,” Rzayev says.

“Making pores bigger increases the flow of water, which will translate into cost and time savings. At the same time, 50 to 100 nm diameter pores are small enough not to allow any bacteria through. So, that is a sweet spot for this kind of application.”

The new nanomembrane owes its special qualities to the polymers that scientists used to create it. Block copolymers are made up of two polymers that repel one another but are “stitched” together at one end to form the single copolymer.

When many block copolymers are mixed together, their mutual repulsion leads them to assemble in a regular, alternating pattern. The result of that process, called self-assembly, is a solid nanomembrane comprising two different kinds of polymers.

To create evenly spaced pores in the material, the scientists simply removed one of the polymers. The pores’ relatively large size was due to the unique architecture of the original block copolymers, which were made from bottle-brush molecules that resemble a round hair brush, with molecular “bristles” protruding all the way around a molecular backbone.

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