Researchers have developed a technique that uses low-cost magnets to precisely control the structure of self-assembled polymers. The discovery signals a huge savings boon in terms of magnetic materials; now scientists can achieve the same level of structural control using a $20 magnet purchased at a hobby store instead of a high-field magnet costing $200,000.
Self-assembled nanostructures formed by materials called block copolymers are critical to a broad range of applications, like high-performance filtration systems that make clean water more accessible. Magnetic fields offer potential for controlling the field alignment of block copolymers, but to date, their use has been costly and complex. Typically, magnetic fields with strength of about 5 Tesla are required. That’s more powerful than the fields used in a conventional MRI (which can reach about 3 Tesla).
But a new technique developed by Yale researcher Chinedum Osuji, associate professor of chemical and environmental engineering, offers the same structural control achieved by $20 permanent magnets as that of much larger and costlier superconducting electromagnets. Permanent magnets are solid materials that have sufficiently high resistance to demagnetizing fields and sufficiently high magnetic flux output to provide useful and stable magnetic fields.
A material or substance that undergoes an ordering transition tends to produce small crystals called grains with different orientations: For example, water that freezes into ice. Osuji has helped pioneer the use of high-intensity magnetic fields to uniformly align such grains in block copolymers to tailor existing properties or to create new useful properties for various applications. The response of the grains is similar to a compass needle’s alignment in a magnetic field.
For a particular type of liquid crystalline block copolymers, the researchers found that by adding small liquid crystal molecules known as mesogens into the system, they could significantly increase the size of the grains. As a result, this enabled the material to respond, or align, to much lower-strength magnetic fields.
“From past experience, we knew that these materials got softer when we add the mesogens, and that they responded faster to magnetic fields as a consequence,” says Manesh Gopinadhan, an associate research scientist at Yale and lead author of the paper detailing the study.
What the researchers didn’t know was that mesogen addition would enlarge the grains and reduce the need for high-intensity magnetic fields. The amount of the mesogen additive needed is small enough that it doesn’t otherwise change the nature of the material.
“So if you want to control the self-assembled structure to make a membrane, instead of using a $200,000 magnet and all the complexities entailed with that, you can potentially use a $20 magnet instead,” Osuji says.
In other words, the discovery has reduced the cost of conducting studies using low-cost permanent magnets by a factor of ten thousand.
“Doing these sorts of experiments has traditionally required big and costly magnets paired with other analytical tools,” says Osuji. “Such requirements limit the level of activity in the field as only a small number of people have access to the specialized high field magnets. Our hope is that other researchers will adopt the approach we developed once they realize that they can use it with their materials to explore new properties or applications.”
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