Explore mixed materials along compositional gradients

UPTON, NY – Blending is a powerful strategy for improving the performance of electronics, coatings, seals and other functional materials. For example, high-efficiency solar cells and light-emitting diodes were produced by optimizing mixtures of organic and inorganic components.

However, finding the optimal blend composition to produce the desired properties has traditionally been a time consuming and inconsistent process. Scientists synthesize and characterize large numbers of individual samples with different compositions one at a time, eventually compiling enough data to create a compositional ‘library’. An alternative approach is to synthesize a single sample with a compositional gradient so that all possible compositions can be explored simultaneously. Existing combinatorial methods for rapidly exploring compositions have been limited in terms of compatible material types, size of composition increments, or number of mixable components (often only two).

To overcome these limitations, a team from the Brookhaven National Laboratory of the United States Department of Energy (DOE), Yale University and the University of Pennsylvania recently built a one-of-a-kind automated tool for depositing films with compositions of finely controlled mixture composed of three components on single samples. The solutions of each component are loaded into syringe pumps, mixed according to a programmable “recipe” and sprayed as tiny electrically charged droplets onto the surface of a heated base material called a substrate. By programming the pump flows as a stage below the substrate changes position, users can achieve continuous gradients in the composition.

Now, the team has combined this electrospray deposition tool with the structural characterization technique of X-ray scattering. Together, these capabilities form a platform for probing how material structure changes in an entire compositional space. Scientists demonstrated this platform for a thin-film blend of three polymers – chains made of molecular blocks linked together by chemical bonds – designed to spontaneously arrange, or “self-assemble,” nanoscale (billionths of a meter) models. Their platform and demonstration are described in a paper published today in RSC Advances, a journal of the Royal Society of Chemistry (RSC).

“Our platform reduces the time it takes to explore complex compositional dependencies of mixed material systems from months or weeks to days,” said corresponding author Gregory Doerk, a staff scientist with the Electronic Nanomaterials Group at Brookhaven Lab’s Center. for Functional Nanomaterials (CFN).

“We built a morphological diagram with more than 200 measurements on a single sample, which is like making 200 samples conventionally,” said first author Kristof Toth, a PhD student in the University’s Department of Chemical and Environmental Engineering. by Yale. “Our approach not only reduces sample preparation time, but also sample-to-sample error.”

This diagram mapped how the morphologies, or shapes, of the blended polymer system changed along a 0 to 100 percent composition gradient. In this case, the system contained a widely studied self-assembling polymer composed of two distinct blocks (PS-b-PMMA) and the individual block constituents of this block copolymer, or homopolymers (PS and PMMA). Scientists programmed the electrospray deposition tool to consecutively create one-dimensional gradient “streaks” with all of the block copolymer at one end and all of the homopolymer blend at the other end.

To characterize the structure, the team performed grazing incidence small-angle X-ray scattering experiments at the complex materials scattering beam line (CMS), which is used at the National Synchrotron Light Source II (NSLS-II) of Brookhaven in collaboration with the CFN. In this technique, a high intensity X-ray beam is directed towards the surface of a sample at a very low angle. The beam reflects on the sample in a characteristic pattern, providing snapshots of nanoscale structures at different compositions along each five-millimeter strip. From these images it is possible to determine the shape, size and order of these structures.

“The synchrotron’s high-intensity X-rays allow us to take snapshots of each composition in seconds, reducing the overall time to map the morphological diagram,” said co-author Kevin Yager, leader of the CFN Electronic Nanomaterials Group.

The X-ray scattering data revealed the emergence of highly ordered morphologies of different types as the composition of the mixture varied. Normally, block copolymers self-assemble into cylinders. However, the mixing of very short homopolymers produced well-ordered spheres (increasing amount of PS) and vertical sheets (plus PMMA). The addition of these homopolymers also tripled or quadrupled the speed of the self-assembly process, depending on the ratio of PS to PMMA homopolymer. To further support their findings, the scientists performed imaging studies with a scanning electron microscope at the CFN Materials Synthesis and Characterization Facility.

Although the team focused on a self-assembling polymer system for their demonstration, the platform can be used to explore mixtures of a variety of materials such as polymers, nanoparticles and small molecules. Users can also study the effects of different substrate materials, film thicknesses, X-ray beam focal point sizes, and other processing and characterization conditions.

“This ability to examine a wide range of composition and processing parameters will inform the creation of complex nanostructured systems with improved or entirely new properties and functionality,” said co-author Chinedum Osuji, Presidential Professor of Chemical and Biomolecular Engineering Eduardo D. Glandt at the University of Pennsylvania.

In the future, the scientists hope to create a second generation of the instrument capable of creating samples with mixtures of more than three components and that is compatible with a range of characterization methods, including in situ methods for capturing morphological changes during electrospray deposition. processes.

“Our platform represents a huge advance in the amount of information that can be conveyed in a composition space,” said Doerk. “Within days, users can work with me at the CFN and dividing line staff alongside NSLS-II to create and characterize their mixed systems.”

“In many ways, this platform integrates standalone methods developed by CFN and NSLS-II scientists to identify trends in experimental data,” added Yager. “Pairing them together has the potential to greatly accelerate soft matter research.”

###

This work was funded by the DOE Office of Science and the National Science Foundation. The CFN and NSLS-II are DOE Office of Science user facilities.

CFN facilities are freely available to scientists from national universities, industries and laboratories around the world. If you are interested in using the new electrospray deposition tool for your research, please submit a proposal. The next deadline is January 31, 2021. If you have any questions about the CFN user agent, contact your CFN user agent administrator and outreach coordinator Grace Webster at (631) 344-3227 or [email protected]. For questions about using CFN facilities or collaborating with CFN scientists, contact Priscilla Antunez Assistant Director for Strategic Partnerships CFN at (631) 344-6186 or [email protected].

Brookhaven National Laboratory is supported by the US Department of Energy’s Office of Science. The Office of Science is the largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, visit https: //power.gov /science.

Follow @BrookhavenLab on Twitter or find us on Facebook.