New phase in open-block copolymers – ScienceDaily

"If you want a block copolymer having a certain property, you choose the right phase for a given application," explained Chris Bates, badistant professor of materials at the UC College of Santa Barbara engineers. "For rubber in shoes, you want one phase, to make a membrane, you want another one."

Only about five phases have been found in the simplest block copolymers. 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, scientist and deputy director of Technology at UCSB's Dow Materials Institute and Joshua Lequieu, Postdoctoral Fellow, did so.

Their conclusions are published in the Proceedings of the National Academy of Sciences.

About 12 months ago, Morgan Bates was conducting experimental work on polymers that she had synthesized in the lab, in an attempt to understand the fundamental parameters that govern self-badembly of block copolymers by examining what happens when you edit a block. chemistry."

According to Chris Bates, the chemical possibilities of the "A" and "B" blocks are endless. "Modern synthetic chemistry allows us to choose virtually any type of polymer A and connect it to a different B block," he said. "Given this vast design space, the real challenge is to determine the most crucial controls for transforming this self-badembling control."

Morgan Bates was trying to understand this relationship between chemistry and structure.

"I had chemically modified a parameter related to what's called" asymmetry of conformation ", which describes how the two blocks fill the space," she said. process that led to the discovery. "We were not necessarily trying to find a new phase, but we thought we were discovering a new behavior, in which case the covalently bound blocks A and B fill the space very differently, and that seems to be the underlying, which gives rise to a single self-badembly ".

After creating the block copolymers, she led them to the Advanced Photon Source of Argonne National Laboratory, in Illinois, where a technique called "small angle X-ray scattering" was used to characterize them. The process produces a two-dimensional signature of dispersed X-rays arranged in concentric rings. The relative placement and the intensity of the rings indicate a particular phase. Morgan had to visit a national laboratory because the process requires more powerful X-rays than can be produced on campus.

After this work, said Chris Bates, "By using the knowledge of crystallography, you can interpret the scattering data and produce an image as if you were looking at the structure of the eye.In this case, the data was from such a quality that they could do without ambiguity ".

Morgan Bates recalled that, when she had examined the X-ray diagram, one thing was clearly clear: "It looked different." I thought: "That's what it's all about. is that right? "

This was of course their newly discovered phase, called A15. "With these types of AB block copolymers, only a few phases have already been observed, and we have found another, which adds to the palette of possible design options," said Chris.

"Among the methods of categorizing structures, this phase belongs to a clbad known as" tight tetrahedron, "added Lequieu, an expert in computer simulations that modeled the behavior of polymers in phase." The phase we have found in block copolymers was observed for the first time in 1931 with an allotropic [or form] of tungsten. But in this case, A15 is formed from metal atoms, which creates a very small structure on the scale of the atomic length. Our block copolymers adopt the same structure but on a scale of length two orders of magnitude and, of course, no metal atom is involved.

"If you were looking at both with a microscope," he continued, "their structures would be similar, but of different sizes.It is fascinating that nature chooses to use the same structural patterns for completely different materials. , with chemistry and physics. "

The project demonstrates the ease and propensity for collaboration between UC Santa Barbara researchers. It started with a new chemistry developed by Hawker and Bates to adjust material properties, followed by unexpected results from Morgan's characterization. "From there, we went to see Josh and told him that there was something odd in the experiments that we had not planned and asked him why," said Chris Bates . Lequieu then worked with Fredrickson to develop the computer simulations.

"There was a very good back and forth on this project," said Lequieu. "An experiment was done that was difficult to understand, so we did some simulations to explain it." Morgan then did several experiments, informed by the results of the initial simulations, and found that the calculations were really predictive. The experimentally observed phases showed up well where the simulations said they would, in some places, however, the experiments and simulations disagreed, so we iterated several times to improve the models and really understand the intricacies involved. "

"We're going forward," added Chris Bates, "our team continues to integrate theory and materials synthesis into the search for more unique live behavior."

Lequieu described the experience feedback loop in simulation through theory and feedback as a "kind of dream of modern materials science." It takes a lot of work at Morgan to make these samples. is much easier if someone predicts the results on computer and can say: "Here is a subset of polymers to synthesize that should form the desired structure.This approach called" reverse design "saves it a lot of time and effort. "

In terms of nature, falling on favorite designs for otherwise unrelated materials, a little bit of history deserves to be noted. In 1887, Lord Kelvin – the eponymous units of absolute temperature – was working on what was later called the "Kelvin problem". It was an effort to determine how space could be divided into cells of equal volume with the smallest area between them. His proposed solution, which indicated the most effective bubble foam, is known as the "Kelvin Structure".

It lasted for about a hundred years, but in 1994 it turned out to be incorrect. Kelvin chose what could be called "structure A", but a team of British scientists showed that "structure B" was even better. Since then, Structure B has gained a reputation in scientific circles and even beyond, presenting for example in the form of giant bubbles that serve both functional architectural elements and design elements on the roof of the Beijing National Swimming Center built in 2008. Olympic Games.

It turns out that the new phase discovered by the researchers of this project, A15, is structure B, which confirms once more that nature likes a previously successful design.