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Researchers discover a new phase in block copolymers

  Researchers discover a new phase in block copolymers
An artistic concept in which block polymers are packed together to form a new phase of the diblock copolymer in this case. Picture credits: Brian Long

All matter consists of one or more phases ̵

1; space regions with a uniform structure and uniform physical properties. The usual phases of H2O (solid, liquid and gaseous), also known as ice, water and steam, are well known. Similarly, although less familiar, polymeric materials can form different solid or liquid phases that determine their properties and ultimate utility. This is especially true for block copolymers which self-assemble macromolecules formed when a polymer chain of one type ("Block A") is chemically linked to that of another type ("Block B").

"If you need a block copolymer with a specific property, select the right phase for a particular application," said Chris Bates, assistant professor of materials at UC Santa Barbara College of Engineering. "For the rubber in shoes you want one phase, for the production of one membrane another."

In the simplest block copolymers, only about five phases were discovered. Finding a new phase is rare, but Bates and a team of other UC Santa Barbara researchers, including Professors Glenn Fredrickson (Chemical Engineering) and Craig Hawker (Materials), Morgan Bates, Research Associate and Deputy Director of Technology at the Dow Materials Institute at UCSB and postdoctoral fellow Joshua Lequieu did just that.

Their findings were published in the Proceedings of the National Academy of Sciences .

About 12 months ago, Morgan Bates was experimenting with polymers she synthesized in the laboratory, she said, "to understand the basic parameters that govern the self-assembly of block copolymers by studying what happens in the optimization of block chemistry . "

There are endless possibilities for the chemistry of the "A" and "B" blocks according to Chris Bates. "Modern synthetic chemistry allows us to basically select all types of A-polymers and connect them to another B-block," he said. "In the face of this huge design space, the real challenge is finding the key controls for the self-organization of the controller."

Morgan Bates attempted to understand this relationship between chemistry and structure.

"I had chemically optimized a parameter related to the so-called" conformational asymmetry "that describes how the two blocks fill the space," she recalled the process that led to the discovery. "We did not necessarily try to find a new phase but thought we could discover a new behavior, in which case the covalently linked blocks A and B fill out the space very differently, and that seems to be the reason parameters, leading to a unique self-organization. "

After making the block copolymers, she brought them to the Advanced Photon Source of the Argonne National Laboratory in Illinois, where a technique called" small-angle X-ray scattering "was used. was used to characterize it. The method provides a two-dimensional signature of scattered X-rays arranged in concentric rings. The relative placement and intensity of the rings indicate a particular phase. Morgan had to go to a national lab because the process requires x-rays that are more powerful than what can be produced on campus.

After this work, Chris Bates explained: "With crystallographic knowledge, you can interpret the scatter data and create a picture as if you were looking at the structure with your eye, in which case the data was of such high quality that we could do it clearly

Morgan Bates recalled that she was unequivocally clear when investigating the X. "It looked different. I thought, 'What's that?"

It was, of course, her newly discovered phase, known as the A15. "With these types of AB block copolymers, there are only a handful of phases that humans have previously observed, and we have found another that extends the range of possible design options," said Chris.

"When categorizing structures, this phase belongs to a class called" tetrahedrally packed "," added Lequieu, a computer simulation expert who modeled the phase behavior of polymers. "The phase we found in block copolymers was first observed with an allotrope [or form] by tungsten in 1931. In this case, A15 forms from metal atoms that form a very small structure on the atomic length scale." Our block copolymers take the same structure but on a length scale two orders of magnitude larger and, of course, no metal atoms are involved.

"If you look at them both with a microscope," he continued, "their structures would look the same, but only in different sizes , It is fascinating that nature uses the same structural motifs for completely different materials that have no relation whatsoever to chemistry and physics. "

The project demonstrates the ease and inclination of collaboration between UC Santa Barbara researchers, and it began with the development of a new chemistry by Hawker and Bates to optimize material properties, followed by Morgan's unexpected characterization results:" From there we went to Josh and told him the experiments contained something weird we did not expect and asked him why. "Chris Bates said: Lequieu then worked with Fredrickson to develop the computer simulations.

" There was a really nice hint and Her on this project, "said Lequieu." An experiment was done that was difficult to understand, so we performed simulations by explaining it. Morgan then carried out further experiments based on the results of the initial simulations and found that the calculations were indeed predictable. The experimentally observed phases were exactly where the simulations promised. However, in some places the experiments and simulations did not match, so we iterated several times to improve the models and really understand the intricacies. "

" Forward, "added Chris Bates," our team continues to integrate materials synthesis and theory in search of unique phase behavior. "

Lequieu described the feedback loop from experiment through simulation to theory and vice versa as" a kind of dream of modern materials science. "Morgan needs a lot of work to produce these samples Computer predicts and can say, "Here is a subset of the polymers to be synthesized to form the desired structure." This so-called 'inverse design' approach saves her a lot of time and effort. "

In relation A little history is worth mentioning when it comes to nature, using preferred designs for other unrelated materials. In the year 1887 Lord Kelvin stated: The unit of absolute temperature of the same name worked on what later became known as the "Kelvin problem." It was tried to determine how the space was divided into cells of equal volume with the least surface between them His proposed solution, which identified the most efficient bubble froth, became known as the "Kelvin structure."

It lasted for about a hundred years, but turned out to be incorrect in 1994. Kelvin had chosen what was called "structure." A team of British scientists has shown that "Structure B" is even better. Since then, Structure B has gained notoriety in scientific circles and even beyond, for example in the form of giant bubbles that perform both functions architectural elements and design elements on the roof of the Beijing National Aquatics Center, which was built for the 2008 Olympics.

The new phase that researchers have discovered in this project, A15, is Structure B, which once again confirms that nature may be a previously successful design.

Exotic discovery made of soft polymer

Further information:
Morgan W. Bates et al. Stability of the A15 Phase in Diblock Copolymer Melting, Method of the National Academy of Sciences (2019). DOI: 10.1073 / pnas.1900121116

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University of California – Santa Barbara

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Researchers discover a new phase in block copolymers (2019, July 30)
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