A new method could lead to the faster, better way of fabricating soft materials called diblock polymers, speeding up production from five months down to three minutes.
Researchers from Carnegie Mellon University in Pittsburgh and the University of Minnesota in Minneapolis are behind the new study. Presenting their findings in the report "Shear-Modulated Rates of Phase Transitions in Sphere-Forming Diblock Oligomer Lyotropic Liquid Crystals," they published their paper in the latest ACS Macro Letters.
Working with Diblock Polymers
Carnegie's chemical engineering Ph.D. student Connor Valentine and chemical engineering professor Lynn Walker are working with diblock polymers. These materials are molecular chains in which one end is hydrophobic (repels water) and the other is hydrophilic (attracts water). Diblock polymers are used in soap, whereas the hydrophobic side grabs dirt and oil from the surface, while the hydrophilic side keeps these molecules dissolved in water.
However, when these substances are placed in water reaching certain levels of concentrations, they start to form clusters where the hydrophilic parts clump together at the center of the ball as a natural response against moisture. On the other hand, their hydrophobic ends extend outward from the central clumps, protecting the other end where these chains are connected.
Now, adding more of these polymers into the water, space starts running out, and the clusters of diblock polymers start to stack themselves spontaneously and intelligently. This optimizes the space available to them to stack clusters above and between them. They start arranging themselves in a loose analog to a lattice of molecules extending in the same pattern, virtually in every direction, which leads to this order of molecular clusters being called a 'crystal.'
Crystals built from diblock polymers are found across nature, such as metals, gemstones, and polymer materials, with people taking advantage of the repetitive and consistent gaps to be used as polymeric membranes. These are used for filtering water, gases, and specific substances. These structures also have potential applications in other fields, such as biomedical implants, food packaging, industrial adhesives, condiments, and a variety of beauty products.
Developing a More Efficient Production Method
One challenge in employing the crystalline structures from diblock polymers is in creating them precisely. Materials scientists and engineers have to consistently produce crystals with the intended arrangement and sizes to make the materials usable.
In order to control the fabrication process, the people working on them must understand the forces that drive and affect their formation in the first place.
More importantly, the wrong temperature setting, formulation, or even mixing speed could cause unwanted effects, such as the sudden formation of crystals, forming different shapes and sizes, premature degradation, and more. These unintended outcomes can damage or jam material mixers or other equipment and ultimately end in waste.
This problem led the Carnegie Mellon researchers to work with colleagues from the University of Minnesota, discovering that the heating and cooling rates can result in the formation of intermediate crystal structures that retain coherence for months on end. Then, they continued to investigate these discoveries by observing the effect of shear processing on these structures.
"Shear processing can help with the dynamics, the speed, and the rates of structural change, not just the final result, which is something people don't really think about," Valentines said in a press release from Carnegie. "They often think when you shear these materials, it's going to change the structure into something different, but that's not necessarily true."
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