polymer physics – Softbites

2020-11-052021-03-10

For the People, By the People: Early career researchers organize virtual polymer physics symposium

Symposium Website: 2020 Virtual Polymer Physics Symposium

In these unprecedented and fluid times, conferences and symposia have gone virtual as STEM collectively settles into a new normal. Many large meetings, like the formerly “in-person only” American Physical Society (APS) and American Chemical Society (ACS) national meetings, have been canceled or transitioned to virtual-only participation this year. The 2021 Spring APS meeting will go virtual as well. I love big in-person meetings and have shied away from virtual alternatives thinking they would not provide the same feeling of community with my fellow scientists. However, the isolation of quarantine and the desire to get comfortable with the “new normal” motivated me to step out of my comfort zone and into the world of virtual science meetings this summer. So, when the opportunity to attend the 2020 Virtual Polymer Physics Symposium (VPPS) arose in July, I jumped at the chance to participate.

The 2020 VPPS was a two-day virtual event organized to fill the void left by the cancellation of the 2020 Polymer Physics Gordon Research Seminar . The event was organized by two early career researchers (ECRs), Konane Bay from Princeton, and Whitney Loo from UC Berkeley, the current co-chairs of the Polymer Physics Gordon Research Seminar (GRS).

The Polymer Physics GRS is held biennially; the last seminar was in 2018 and, due to the pandemic, the next one won’t occur until 2022. “Two years feels like a lifetime to ECRs and we know many of our colleagues will be at different institutions and career stages in 2022, so we wanted to create a space for them to share and discuss their recent research,” explains Konane. The 2020 VPPS connected more than 100 scientists, including 20 ECRs presenting their work across four oral presentation sessions during the two-day event. In the spirit of the Polymer Physics GRS, this new virtual event also incorporated professional development and discussion sessions, including a Mentorship Panel and the “Dispersity and Diversity Hour” discussion focused on how to increase diversity, equity, and inclusion (DEI) in the field of polymer physics and the broader scientific community.

The Mentorship Panel included researchers at different career stages working in academia, industry, or government research. The discussion was focused on steps students and postdocs can take to prepare for future careers in polymer physics as we adapt to a global pandemic. The panelists emphasized the quarantine-proof nature of computational work since it can often be done anywhere including at home and encouraged experimentalists to broaden their computational skill set. Both Nate Lynd, assistant professor at UT Austin, and Michelle Sing, an engineer at Braksem USA, suggested that experimentalists should become familiar with Python as a first step. Debra Audus, a scientist at NIST, highlighted lab work automation and strategic experiment planning to maximize “in-lab” time during the transition to shift-style lab work that many American universities have adopted as a way to overcome the challenges related to performing socially distanced science.

Participating in the “Dispersity and Diversity Hour” required some homework. The organizers asked all attendees to prepare for the discussion by reading about the experience of Black researchers in STEM (links below). Ben Yavitt, a Stony Brook University postdoc, opened the event by emphasizing that the goal of the discussion was to brainstorm potential solutions to address the issues spotlighted by national movements such as #BLM, #ShutdownSTEM, and #BlackintheIvory. More than 70 attendees participated in small group discussions across 15 breakout rooms led by volunteer discussion leaders. This event was an important first step in raising awareness of the necessary academic culture shift required to empower more scientists of color to pursue careers in soft matter and polymer physics.

Overall, the meeting events were very well moderated and designed for maximum virtual engagement during both the science presentations and the discussion sessions. For me, attending this event clarified the current trajectory of the polymer physics field as it transitions from fundamental studies to applied research focused on exploiting polymer physics for advanced technology. This transition in research focus was evident in some of the science presented during the symposium. While I was apprehensive about the shift to virtual meetings and conferences, attending the 2020 VPPS has won me over. I highly recommend taking advantage of the proliferation of virtual science conferences and symposia to stay engaged, learn about new science and perspectives, and do some networking along the way!

Diversity and Diversity Hour Resources
Science Is For Everyone — Until It’s Not
Your Black Colleagues May Look Like They’re Okay — Chances Are They’re Not
AIP Team-Up Report Executive Summary

Disclosure: I am acquainted with Konane Bay, one of the event co-organizers. She was a graduate student in my department, UMass Amherst Department of Polymer Science and Engineering. However, I was not involved in any way with the organization of the VPPS.

2019-05-08

Slithering Like A Snake and Beyond: Microscopy of Polymer Dynamics

Original paper: Entangled polymer dynamics beyond reptation

Scientists often draw inspiration from biological organisms to describe phenomena, even when they are studying outside the realm of biology. Physicist Pierre-Gilles de Gennes[1] was no exception. In 1971, after being inspired by the movement of snakes, he proposed reptation theory, or the reptation model, which has since been widely used to describe motions of polymers[2]. As the name “reptation” suggests, de Gennes assumed polymer chains move like snakes. As shown in Figure 1, the model describes a polymer chain’s motion in an environment that is highly populated by other chains (shown in gray) by assuming that the chain is confined in a virtual tube (shown in red) formed by surrounding polymer chains. According to reptation theory, the chain wiggles through this tube, similar to a snake slithering through the woods. As one might imagine, directly imaging the snake-like slithering of polymers is a challenging affair; however, in today’s study, Maram Abadi and coworkers from King Abdullah University of Science and Technology were able to do just that with DNA chains – an example of a polymer – and compared their results to prevailing theory.

**Figure 1.** A schematic of the reptation model. In a crowded and entangled polymer environment, a long and linear polymer chain (black) is located in a virtual tube (red), which traces the chain trajectory. Surrounding polymers are shown in gray. (Adapted from the Wikipedia page for reptation.)

While reptation theory has done fairly well in describing experimental observations of polymers, there are some shortcomings to both the experiments supporting this theory as well as to the theory itself. Namely, previous experiments mostly considered the overall motion of the chain; but local chain motion, such as motion at the ends of a polymer chain, have not been thoroughly studied. In addition, the theory was only designed for polymers with two ends, known as linear polymers. Thus, it does not account for the dynamics of polymers with different geometries, such as those that form rings, known as cyclic polymers. Given these observations, Abadi and coworkers realized that there was more work to be done in the studies of polymer dynamics.

To scrutinize the movements of the polymer chains, the authors used super-resolution fluorescence localization microscopy[3], which lets them monitor the movements beyond the typical microscopy resolution of ~200 nm. This technique allowed Abadi and coworkers to not only observe whole-chain dynamics but also local dynamics. To test the predictions of reptation theory, they chose both linear and cyclic DNAs with fluorescent dyes attached as model polymers for their study.

**Figure 2.** Fluorescent images of a linear DNA chain collected at different time points (indicated in the figure), overlapped for comparison. Insets show the enlarged views of the highlighted areas. (Adapted from the original paper.)

First, linear DNAs were used to confirm what has been known from reptation theory in great detail. Shown above in Figure 2 are images of a linear DNA as a function of time. Their results were consistent with theory. First, polymer chains traveled along virtual tubes that followed the contour of the chain (shown in white boxes in Figure 2A). Second, most of the polymer chain’s displacements were within the confinements of the virtual tubes, which had a diameter around 51–95 nm (shown in red boxes in Figure 2B). Further, they occasionally saw displacements of the DNA that exceeded the size of tube diameter (shown in cyan boxes in Figure 2C), known as constraint release in reptation theory. Finally, Abadi and coworkers observed that the chain-ends were able to move farther than the centers of the chains, which in turn creates a new tube for further DNA reptation (shown in green boxes in figure 2C). In reptation theory, this is called contour-length fluctuation.

However, there was one particular deviation from the theory found in the authors’ results. While the chain-ends were expected to move more freely than other parts of the chains, the chain-end motions were a lot faster than what is predicted by reptation theory. Therefore, the authors concluded that the motions at the chain-ends were beyond the scope of the reptation theory. These unexpectedly fast movements were not observed in previous experiments, in which only the chain as a whole was considered.

Fluorescent images of cyclic DNAs collected at different time points — **Figure 3.** Both rows are fluorescent images of cyclic DNAs collected at different time points (indicated in the figure), overlapped for comparison. 3A shows amoeba-like motion, and 3B shows contracting of an open structure. More details can be found in the main text below. (Adapted from the original paper.)

The authors also observed cyclic DNAs using the same methods. As they are not linear, reptation theory fails to accurately explain their movements. The authors observed diverse motion of the cyclic DNAs. You may notice in Figure 3A that the cyclic DNA has a loop-like region, shown in the white boxes. They found that cyclic DNAs repeatedly contract and extend this region, resembling the motions of amoeba. In addition, as shown in the first panel of Figure 3B, some cyclic DNA molecules may start with an open structure. However, as time progresses, these open DNAs may contract into more linear forms and expand back into the open shape again. Thus, Abadi and coworkers were able to show two phenomena that cannot be explained by reptation theory, thus requiring it to be further refined.

The results of this paper support many of the conclusions of reptation theory; however, it does suggest that there is still a need to expand this otherwise well-accepted theory. By considering different geometries and shorter timescales, this theory will be more powerful as a predictor or explainer of novel polymeric material dynamics. Furthering the understanding of polymer dynamics will then help us understand polymer properties for use in a variety of applications that we see in our lives every single day.

[1] This 1991 Nobel laureate in Physics is also the one who popularized the term “soft matter”.

[2] Polymers are molecules that are consist of repeating chemical structures.

[3] Super-resolution microscopy is a technique that lets us observe things that are smaller than the diffraction limit of ~200 nm, which is the limit that is imposed by the physics of light.

2018-08-23

“Precise” Polymers Promote Fast Proton Transport

Original paper: Self-Assemble Highly Ordered Acid Layers in Precisely Sulfonated Polyethylene Produce Efficient Proton Transport

Global climate change has necessitated the development of ways to harvest electricity from renewable sources, such as the wind and the sun. However, because the wind isn’t always blowing and the sun isn’t always shining, we need to store some of the harvested energy for later use. We can store this energy by converting it into fuels such as methanol and hydrogen, but we need a way to convert it back into electricity when it’s needed. One device that allows us to do this is the fuel cell.

508px-Solid_oxide_fuel_cell_protonic — Figure 1: Example proton exchange membrane fuel cell. Hydrogen gas mixed with water vapor enters on the left-hand side of the device and reacts to produce protons and electrons. The protons drift rightward through the center of the device to react with oxygen to produce water. [1]

Fuel cells are electrochemical devices that can convert a stored simple fuel, like hydrogen or methanol, directly into electricity. Because these fuels can be stored and easily converted near homes, vehicles, and businesses, fuel cells can be used to produce energy on demand. One type of fuel cell is the proton exchange membrane (PEM) fuel cell, which uses hydrogen and oxygen gases to make electricity and water. Hydrogen gas reacts at the anode (the negatively charged electrode) to lose its electrons, forming protons (denoted H+ in Figure 1). The electrons flow first through the fuel cell’s load (e.g. a home or business), powering it, and subsequently to the fuel cell’s cathode. The protons must drift across the proton exchange membrane to meet the electrons and oxygen to form water.

One of the greatest challenges of making PEM fuel cells is designing the membrane after which they are named. Engineers would love to have a membrane that transports protons quickly and efficiently. Although there are polymeric materials (e.g. Nafion) that can do this fairly well, there is still a need for faster transport. This week’s paper investigates the role of a polymer’s “precise” structure in facilitating fast proton transport.

Designing polymers, which are large molecules consisting of repeating units called monomers, with a given role relies on interspersing functional groups along the polymer chain. These functional groups are atoms that may contain some important features, such as electrical charge, necessary for the role of polymer. Under most synthetic schemes, the spacing between these functional groups is random. In the case of a typical proton exchange membrane, such as Nafion, negatively charged groups known as sulfonic acids love to interact with each other and with water present in the fuel cell, forming channels that facilitate proton transport, as shown in Figure 2.

Figure 2: Structure of (left panel) disordered and (right panel) ordered polymer materials. Black lines are the polymer backbone, yellow spheres are sulfonic acid groups, blue spheres are water molecules, and green spheres are water molecules with protons attached. Adapted from current work.

However, because the locations of these groups along the polymer chain are random, each chain cannot organize uniformly, forming disordered channels (Figure 2 left panel). In contrast, Edward Trigg and coauthors were able to attach these sulfonic acids at controlled, repeating locations along the polymer chain. Because the sulfonic acid groups are uniformly dispersed along the polymer (every twenty one carbon atoms to be exact), the chains can fold precisely in the same way, forming a uniformly layered structure and ordered channels filled with protons and water and lined with sulfonic acid (Figure 2 right panel).

The authors of this work first compared this material with Nafion, a widely used PEM polymer that is currently one of the best performing materials available. Like Nafion, this material readily absorbs water into the channels formed by the sulfonic acid groups from the air. The water widens the channels and transports protons more quickly as a result. At high levels of humidity, the new polymer performs as well as Nafion. However, as mentioned earlier, Nafion’s structure is amorphous and disordered because of the random placement of its functional groups (see Figure 2 left panel). Why then is a membrane structure so different from Nafion able to transport protons just as well?

The authors used simulation to answer this question. Specifically, they examined the dynamics of water contained in channels. Faster proton transport is facilitated by faster water dynamics. They compared the ordered and disordered versions of their polymer. The water dynamics in the ordered structure were faster than those in the disordered structure, suggesting slower proton transport in the disordered material. They attributed the slower water dynamics to the nonuniform size and the poor connectivity of the water channels in the disordered structure. These findings suggest that PEM membrane materials like Nafion can be further improved by creating ordered, rather than disordered, channels.

In short, Edward Trigg and his colleagues opened a new potential path to better performing fuel cells through the design of a well-ordered polymeric proton exchange membrane. They demonstrated that the ordered polymeric structure within their PEM leads to faster proton transport than in a disordered version of the structure. With further refinement of the synthesis techniques, membranes like these may yield faster proton transport than is currently achievable, leading to exceptional performance in PEM fuel cells. With better performance, PEM fuel cells may be more readily available to quickly convert stored energy for use in domestic and industrial applications when renewable sources are not immediately available.

[1] https://en.wikipedia.org/wiki/Fuel_cell