Session M26: Enabling Early Career Polymer Physics Researchers

Reflections on Enabling Early Career Polymer Physicists and Session Motivation

The field of polymer physics has led to the discovery of foundational principles that have positively impacted society in applications ranging from human health through advanced electronics. While this work has occurred in multiple institutions and in laboratories across the globe, many research and training efforts have occurred in academic institutions within the United States. Given the support systems associated with these laboratories, federal funding is crucial to the initial and sustained success of these endeavors. Thus, federal funding agencies play a critical role in the intellectual ecosystem of the polymer physics community. This session is dedicated to highlighting how the support for early-career researchers has enabled their initial success and helped launch impressive careers that have shaped the current state of polymer physics and, in turn, DPOLY. In this talk, we will provide an overview of this support landscape and set the stage for the distinguished researchers who will provide the technical talks of the session with a specific focus on the support and mentorship that has enabled early career development.   

Macromolecular Engineering of Sustainable Polymers

Sustainability plays a critical role in promoting economy and societal development. The growth of polymers has largely relied on commodity materials such as polyethylene, polypropylene, polystyrene, and polyvinyl chloride, which aggravate the global plastics crisis. The last two decades have overseen a rapidly evolving field of sustainable polymers. We have spearheaded a research program on the transformation of diverse biomass into sustainable intermediates, monomers, and polymers. Our contribution lies in the designs of macromolecular structures and compositions that significantly enhance thermal and mechanical properties of bioplastics. In this presentation, we will describe the use of macromolecular engineering approaches to tackling one of the field's fundamental challenges—chain entanglements—by devising various architectures, such as ultra-high molecular weight polymers, pentablock architectures, supramolecular entanglement, and dynamic crosslinking.   

Hierarchial Polymeric Materials Inspired by Nature

Natural materials, such as spider silk, wood, and seed pods, are excellent models for the design of polymeric systems that respond to complex and interacting environments, and that exhibit controlled and modular mechanical behavior under low energy conditions and with a limited set of chemical building blocks. These bio-inspired strategies provide a rich landscape for innovation, mentorship, and outreach via multidisciplinary, collaborative team science initiated by NSF CAREER-funding in the POL division.  In this context, I will highlight a few examples of how natural systems inspired the design of shape memory materials, injectable gels, and water-responsive composites utilizing principles, such as hierarchy, interfaces, orientation, and dynamics. Collectively, these vignettes offer a framework that provides insight of the interplay macromolecular design, molecular engineering, and robust manufacturing.   

Journey in polymer physics with the NSF Polymers Program

We will introduce silicone elastomers as flexible substrates to adjust the density of small-molecule modifiers and polymeric grafts. We will also show how these substrates can regulate the steepness of density gradients of small-molecule assemblies. We will then present the formation of random copolymers with tuned chemical composition and co-monomer sequences by brominating polystyrenes in solvents with selected solubility. We will describe the formation of “random” and “random-blocky” copolymers and study the kinetics of the formation of such copolymers, their adsorption on surfaces, and the stability of films made of such structures. Next, we will discuss the spontaneous degrafting of polymer grafts from surfaces as a function of graft density, molecular weight, charge density and charge distribution, and solution pH. Introducing the “on-demand” degrafting will illustrate the method of determining the grafting density of uncharged polymeric grafts and the formation of spatially varying densities of polymers. Finally, we will introduce the one-pot synthesis method to form surface-anchored polymer networks made by co-reacting sulfonyl-amide organosilanes commodity polymers. We will describe the formation of surface-anchored gel networks with gradually varying crosslink densities using temperature and UV light.   

Polymerizations with Particles at Fluid-Fluid Interfaces

When emulsions are combined with polymerization strategies, architected materials can be prepared, including foams by polymerization of the continuous phase, particles by polymerization of the discontinuous phase, and capsules by interfacial polymerization. The use of solid particles as emulsion stabilizers offers an intriguing opportunity to not only tailor the composition of the emulsion by controlling particle wettability, but also to integrate the particles into the structures that are produced. This presentation will highlight the use of 2D particles as surfactants to produce: (i) Janus particles by grafting from polymerization on one face of the nanosheets, (ii) nanosheet-armored polymer particles by polymerization of the droplets, (iii) porous foams coated with nanosheets by polymerization of the continuous phase, and (iv) capsules by interfacial polymerization. The impact and potential of controlling both composition and architecture on performance-related properties will be briefly addressed.   

Symmetry Breaking and Polymer Self-Assembly in the Presence of Liquid Crystals

The presence of mesogens attached to block copolymers (BCPs), or blended with BPCs, can result in a rich interplay of self-assembly on multiple length scales, and provides new opportunities to control nanostructure development. We explore the self-assembly and directed self-assembly of liquid crystalline (LC) BCPs, block co-oligomers and BCP-analogous macromolecules containing mesogens. These systems display complex phase behavior, including the formation of highly persistent domains, gyroid morphologies and strongly asymmetric phase diagrams, and we encounter systems with structural periodicities as small as ~6 nm. The stimuli responsiveness of LC mesophases represents a useful handle to control ordering processes, and we examine this in the context of a photoresponsive system in which cis-trans isomerization can be used to stimulate rapid ordering transitions under ambient conditions. We address the phase behavior and magnetic field alignment of LC BCPs in the presence of labile mesogens. The surface anchoring of the mesogens provides control over the orientation of the BCP structures, and volumetric swelling by the labile mesogens leads to order-order transitions. We observe a transition from hexagonal cylinders to FCC spheres beyond a critical mesogen concentration. Despite the isometric nature of the cubic lattice, this system aligns with its [100] axis parallel to an applied magnetic field, resulting in a degenerate, fiber-like texture. This response originates from symmetry breaking due to the action of the field, and shares features in common with magnetic metallic systems that undergo structural phase transformations associated with magnetic ordering.   

Understanding penetrant transport in dense, dynamic polymer networks

Polymer networks are routinely used in applications involving mass and ion transport. In this talk, dense polymer networks crosslinked by dynamic covalent bonds where the mesh size can approach 1 nm are investigated with a range of fluorescent dyes up to 2.8 nm in length. With increased crosslink density, the glass transition (Tg) increases and dye diffusion rapidly slows. Large dyes which are effectively slowed in permanent networks show a large enhancement of diffusion coefficients when the networks contain fast exchanging dynamic bonds. Deconvolution of mesh size effects and segmental dynamics is pursued through a combined experimental-theory-simulation approach and the role of the mesh confinement is a much weaker effect than the slowing segmental dynamics. Penetrants which can reversibly bind with the network are also analyzed, and when the reaction is slow relative to the diffusive hopping time a massive slowing of diffusion is observed. Finally, the role of network topology will be discussed comparing networks with regular dynamic bond distributions versus clustered bonds.   

Responsive polymer network interfaces: From mechanical surface instabilities to ionic heterojunctions

Stimuli-responsive polymer networks offer potential for materials with a wide variety of tunable surface properties including optical characteristics, electronic properties, bioactivity, and adhesivity. Through a series of early- and mid-career federally-funded projects, our group has explored the fundamental responses, and corresponding changes in properties, of soft polymer interfaces in a variety of contexts. Following a brief summary of our early work on mechanical surface instability modes, this talk will transition to recent findings related to ionic heterojunctions at the interface between single-ion conducting polymer networks of opposite polarity.      

Measurement of Real Contact Area and Its Consequences on Adhesion and Friction

The nature of the contact interface has a profound influence on electrical conductivity, adhesion, friction, and wetting. This problem has remained unresolved due to the complexity of probing the contact separation with sub-nm resolution. In addition, these interfaces are buried within two materials (solids or liquids). Here, we present the use of surface-sensitive infrared-visible sum frequency generation spectroscopy (SFG) to study the shift in OH frequency to probe the real contact area between two solid surfaces. The position of the OH peak in the SFG spectra is sensitive to sub-nm changes in separation distance (acid-base interactions) and provides a spectroscopic ruler to study the contact interface in static and dynamic conditions that are encountered during adhesion and friction. The influence of this contact as a function of chemistry, roughness, and the presence of confined fluids (water) will be discussed.   

Polyelectrolyte Multilayers and Complexes: Glass Transitions and Dynamics

Polyelectrolyte multilayers and complexes, both made from the assembly of oppositely charged polymers, bear many similarities. This talk will compare the glass transition behavior of multilayers and complexes that contain strong and weak polyelectrolytes, different salt types and concentrations, and different amounts of water. The experimental glass transition decreases with increasing salt and/or water content, which is well-described by the ratio of the number of intrinsic pairs to water molecules. Similarly, the dynamic mechanical behavior and the associated relaxation time are also well-described by the ratio of the number of intrinsic pairs to water molecules. To understand this behavior for individual polyelectrolyte chains within the complex, the polyelectrolyte’s diffusion coefficient is estimated using fluorescence recovery after photobleaching (FRAP) and correlated to the salt concentration of assembly. Taken together, these results indicate the importance of water’s role in lubricating or modifying the strength of the intrinsic ion pair for a larger scale relaxation of the multilayer or complex.   

Controlling Polymer Assembly with Precise Design

While the development of synthetic techniques to produce sequence-controlled polymers holds the promise of making polymers with the complexity and functionality of proteins, the design strategies for engineering these kinds of hierarchical architectures are not yet sufficiently sophisticated.  We combine experimental and molecular dynamics simulation techniques to understand how the monomer sequence of polypeptoids impacts polymer chain shape and assembly. To explore these effects, we have studied the conformation of polypeptoid chains in solution via neutron scattering and double electron-electron resonance spectroscopy to understand how the number and location of hydrophobic and chiral monomers leads to changes in radius of gyration and end-to-end distance.   We also use structural control enabled by sequence-defined polypeptoids to demonstrate that water behavior can be tuned near polymeric surfaces.  This system simultaneously offers both a route to control polymer chain shape and functional group position as well as an unique opportunity to map water diffusivity at different locations within the assembly.    

Nanoporous Materials from Ordered and Disordered Block Polymers

Block polymers are marvelous self-assembling materials that have found uses from the ordinary (e.g., pressure sensitive adhesives) to the fantastic (e.g., templates for nanolithography). By incorporating blocks that can undergo subsequent chemical transformations, the set of self-assembled morphologies that block polymers adopt can be modified to take advantage of the nanoscopic dimensions and narrow feature size distributions to generate new functional materials that have technological potential in wide ranging applications. In this presentation I will present our work over the past two decades on the design and development of block polymers that contain an etchable component for the generation of nanoporous materials from block polymer precursors. I will emphasize the preparation of both bulk and thin film nanoporous materials and the production of nanoporous materials from both ordered and disordered block polymers.   

Quantifying the effects of local structure and dynamics on ion transport in polymer electrolytes

Polymer electrolytes can boost the performance and safety of lithium-ion batteries relative to liquid electrolytes. However, the presence of interfaces between microphase-separated domains can introduce complexities in the local ion transport, as competing effects (e.g., interfacial segmental mixing vs. chain stretching) can increase or decrease local mobility. We present a framework to account for the effects of polymer architecture, segmental mixing, chain stretching, and confinement on the dynamics of poly(oligo-oxyethylene methacrylate) (POEM)-based electrolytes, and we validate this framework through techniques such as nuclear magnetic resonance spectroscopy measurements on solid‑state electrolyte samples. Notably, we found that a mobility-onset temperature that captures the heterogeneous dynamics along the POEM side chain is a better predictor of segmental mobility than the POEM thermal glass transition temperature. Additionally, our framework explains the mobility gradient across nanostructured domains when we combine segmental mixing effects with chain stretching and confinement information, especially at the higher segregation strengths. This quantitative link between local and global dynamics can facilitate the design of next‑generation electrolytes.