Polypeptoid Monomer Sequence and Chemical Composition as Independent Controls of Interfacial Tension and Elasticity at Air/Fluid Interfaces.

We use sequence-specific polypeptoids to characterize the impact of the monomer sequence on the adsorption of surface-active polymers at fluid/fluid interfaces. Sets of 36 repeat unit polypeptoids with identical chemical composition, but different sequences of hydrophobic moieties along the oligomer chain (taper, inverse taper, blocky, and evenly distributed), are designed and characterized at air/water interfaces. Polypeptoids are driven to the interfaces by decreasing the solvent quality of the aqueous solution. In situ processing of the adsorbed layers causes a collapse of polypeptoids and the formation of irreversibly adsorbed, solvent-avoiding layers at interfaces. Differences in thermodynamic properties, driven by solubility, between the collapsed structures at interfaces are studied with measurements of interfacial tension. The dilatational modulus of polypeptoid-coated interfaces is used as a proxy to probe the extent of the coil-globule collapse at the interface. The role of hydrophobicity is investigated by comparing four sequences of polypeptoids with an increased size of the hydrophilic side chains. In each set of polypeptoids, the composition of molecules, not the sequence, controls the surface concentration. The molecules are described in terms of the distribution of the hydrophobic monomers on the backbone of the polymer. Inverse taper (IT) and blocky (B) sequences of hydrophobic moieties favor the formation of highly elastic interfaces after processing, while taper (T) and distributed (D) showed lower elasticity after processing, which is achieved by replacing good solvent with poor solvent and then nonsolvent. These structures allow for the study of the impact of the chemical composition and sequence of monomers on the properties of polymer-coated interfaces.

Diffusely Charged Polymeric Zwitterions as Loosely Hydrated Marine Antifouling Coatings.

Polymeric zwitterions exhibit exceptional fouling resistance through the formation of a strongly hydrated surface of immobilized water molecules. While being extensively tested for their performance in biomedical, membrane, and, to a lesser extent, marine environments, few studies have investigated how the molecular design of the zwitterion may enhance its performance. Furthermore, while theories of zwitterion antifouling mechanisms exist for molecular-scale foulant species (e.g., proteins and small molecules), it remains unclear how molecular-scale mechanisms influence the micro- and macroscopic interactions of relevance for marine applications. The present study addresses these gaps through the use of a modular zwitterion chemistry platform, which is characterized by a combination of surface-sensitive sum frequency generation (SFG) vibrational spectroscopy and marine assays. Zwitterions with increasingly delocalized cations demonstrate improved fouling resistance against the green alga Ulva linza. SFG spectra correlate well with the assay results, suggesting that the more diffuse charges exhibit greater surface hydration with more bound water molecules. Hence, the number of bound interfacial water molecules appears to be more influential in determining the marine antifouling activities of zwitterionic polymers than the binding strength of individual water molecules at the interface.

Sequence Modulates Polypeptoid Hydration Water Structure and Dynamics.

We use molecular dynamics simulations to investigate the effect of polypeptoid sequence on the structure and dynamics of its hydration waters. Polypeptoids provide an excellent platform to study small-molecule hydration in disordered polymers, as they can be precisely synthesized with a variety of sidechain chemistries. We examine water behavior near a set of peptoid oligomers in which the number and placement of nonpolar versus polar sidechains are systematically varied. To do this, we leverage a new computational workflow enabling accurate sampling of polypeptoid conformations. We find that the hydration waters are less dense, are more tetrahedral, and have slower dynamics compared to bulk water. The magnitude of these shifts increases with the number of nonpolar groups. We also find that shifts in the water structure and dynamics are strongly correlated, suggesting that experimental insight into the dynamics of hydration water obtained by Overhauser dynamic nuclear polarization (ODNP) also contains information about water structural properties. We then demonstrate the ability of ODNP to probe site-specific dynamics of hydration water near these model peptoid systems.

Polycation radius of gyration in a polymeric ionic liquid (PIL): the PIL melt is not a theta solvent.

The conformation of the polycation in the prototypical polymeric ionic liquid (PIL) poly(3-methyl-1-aminopropylimidazolylacrylamide) bis(trifluoromethylsulfonyl)imide (poly(3MAPIm)TFSI) was probed using small-angle neutron scattering (SANS) and ultra-small-angle neutron scattering (USANS) at 25 °C and 80 °C. Poly(3MAPIm)TFSI contains microvoids which lead to intense low q scattering that can be mitigated using mixtures of hydrogen- and deuterium-rich materials, allowing determination of the polycation conformation and radius of gyration (Rg). In the pure PIL, the polycation adopts a random coil conformation with Rg = 52 ± 0.5 Å. In contrast to conventional polymer melts, the pure PIL is not a theta solvent for the polycation. The TFSI- anions, which comprise 48% v/v of the PIL, are strongly attracted to the polycation and act like small solvent molecules which leads to chain swelling analogous to an entangled, semi-dilute, or concentrated polymer solution in a good solvent.

Light-Switchable and Self-Healable Polymer Electrolytes Based on Dynamic Diarylethene and Metal-Ion Coordination.

Self-healing polymer electrolytes are reported with light-switchable conductivity based on dynamic N-donor ligand-containing diarylethene (DAE) and multivalent Ni2+ metal-ion coordination. Specifically, a polystyrene polymer grafted with poly(ethylene glycol-r-DAE)acrylate copolymer side chains was effectively cross-linked with nickel(II) bis(trifluoromethanesulfonimide) (Ni(TFSI)2) salts to form a dynamic network capable of self-healing with fast exchange kinetics under mild conditions. Furthermore, as a photoswitching compound, the DAE undergoes a reversible structural and electronic rearrangement that changes the binding strength of the DAE-Ni2+ complex under irradiation. This can be observed in the DAE-containing polymer electrolyte where irradiation with UV light triggers an increase in the resistance of solid films, which can be recovered with subsequent visible light irradiation. The increase in resistance under UV light irradiation indicates a decrease in ion mobility after photoswitching, which is consistent with the stronger binding strength of ring-closed DAE isomers with Ni2+. 1H-15N heteronuclear multiple-bond correlation nuclear magnetic resonance (HMBC NMR) spectroscopy, continuous wave electron paramagnetic resonance (cw EPR) spectroscopy, and density functional theory (DFT) calculations confirm the increase in binding strength between ring-closed DAE with metals. Rheological and in situ ion conductivity measurements show that these polymer electrolytes efficiently heal to recover their mechanical properties and ion conductivity after damage, illustrating potential applications in smart electronics.

End-to-End Distance Probability Distributions of Dilute Poly(ethylene oxide) in Aqueous Solution.

We introduce a powerful, widely applicable approach to characterizing polymer conformational distributions, specifically the end-to-end distance distributions, P(Ree), accessed through double electron-electron resonance (DEER) spectroscopy in conjunction with molecular dynamics (MD) simulations. The technique is demonstrated on one of the most widely used synthetic, disordered, water-soluble polymers: poly(ethylene oxide) (PEO). Despite its widespread importance, no systematic experimental characterization of PEO's Ree conformational landscape exists. The evaluation of P(Ree) is particularly important for short polymers or (bio)polymers with sequence complexities that deviate from simple polymer physics scaling laws valid for long chains. In this study, we characterize the Ree landscape by measuring P(Ree) for low molecular weight (MW: 0.22-2.6 kDa) dilute PEO chains. We use DEER with end-conjugated spin probes to resolve Ree populations from ∼2-9 nm and compare them with full distributions from MD. The P( Ree)'s from DEER and MD show remarkably good agreement, particularly at longer chain lengths where populations in the DEER-unresolvable range (<1.5 nm) are low. Both the P(Ree) and the root-mean-square R̃ee indicate that aqueous PEO is a semiflexible polymer in a good solvent, with the latter scaling linearly with molecular weight up to its persistence length (lp ∼ 0.48 nm), and rapidly transitioning to excluded volume scaling above lp. The R̃ee scaling is quantitatively consistent with that from experimental scattering data on high MW (>10 kDa) PEO and the P(Ree)'s crossover to the theoretical distribution for an excluded volume chain.

The Role of Backbone Polarity on Aggregation and Conduction of Ions in Polymer Electrolytes.

The usual understanding in polymer electrolyte design is that an increase in the polymer dielectric constant results in reduced ion aggregation and therefore increased ionic conductivity. We demonstrate here that in a class of polymers with extensive metal-ligand coordination and tunable dielectric properties, the extent of ionic aggregation is delinked from the ionic conductivity. The polymer systems considered here comprise ether, butadiene, and siloxane backbones with grafted imidazole side-chains, with dissolved Li+, Cu2+, or Zn2+ salts. The nature of ion aggregation is probed using a combination of X-ray scattering, electron paramagnetic resonance (in the case where the metal cation is Cu2+), and polymer field theory-based simulations. Polymers with less polar backbones (butadiene and siloxane) show stronger ion aggregation in X-ray scattering compared to those with the more polar ether backbone. The Tg-normalized ionic conductivities were however unaffected by the extent of aggregation. The results are explained on the basis of simulations which indicate that polymer backbone polarity does impact the microstructure and the extent of ion aggregation but does not impact percolation, leading to similar ionic conductivity regardless of the extent of ion aggregation. The results emphasize the ability to design for low polymer Tg through backbone modulation, separately from controlling ion-polymer interaction dynamics through ligand choice.

Light-Controllable Ionic Conductivity in a Polymeric Ionic Liquid.

Polymeric ionic liquids (PILs) have attracted considerable attention as electrolytes with high stability and mechanical durability. Light-responsive materials are enabling for a variety of future technologies owing to their remote and noninvasive manipulation, spatiotemporal control, and low environmental impact. To address this potential, responsive PIL materials based on diarylethene units were designed to undergo light-mediated conductivity changes. Key to this modulation is tuning of the cationic character of the imidazolium bridging unit upon photoswitching. Irradiation of these materials with UV light triggers a circa 70 % drop in conductivity in the solid state that can be recovered upon subsequent irradiation with visible light. This light-responsive ionic conductivity enables spatiotemporal and reversible patterning of PIL films using light. This modulation of ionic conductivity allows for the development of light-controlled electrical circuits and wearable photodetectors.

Controlling inelastic light scattering quantum pathways in graphene.

Inelastic light scattering spectroscopy has, since its first discovery, been an indispensable tool in physical science for probing elementary excitations, such as phonons, magnons and plasmons in both bulk and nanoscale materials. In the quantum mechanical picture of inelastic light scattering, incident photons first excite a set of intermediate electronic states, which then generate crystal elementary excitations and radiate energy-shifted photons. The intermediate electronic excitations therefore have a crucial role as quantum pathways in inelastic light scattering, and this is exemplified by resonant Raman scattering and Raman interference. The ability to control these excitation pathways can open up new opportunities to probe, manipulate and utilize inelastic light scattering. Here we achieve excitation pathway control in graphene with electrostatic doping. Our study reveals quantum interference between different Raman pathways in graphene: when some of the pathways are blocked, the one-phonon Raman intensity does not diminish, as commonly expected, but increases dramatically. This discovery sheds new light on the understanding of resonance Raman scattering in graphene. In addition, we demonstrate hot-electron luminescence in graphene as the Fermi energy approaches half the laser excitation energy. This hot luminescence, which is another form of inelastic light scattering, results from excited-state relaxation channels that become available only in heavily doped graphene.

Surface Structure and Hydration of Sequence-Specific Amphiphilic Polypeptoids for Antifouling/Fouling Release Applications.

Amphiphilic polypeptoids can be designed with specific sequences of hydrophilic and hydrophobic units, which determine their surface properties for antifouling/fouling release purposes. Although the sequence-dependent surface structures of polypeptoids have been extensively investigated, e.g., with X-ray spectroscopy, their molecular structures under the aqueous conditions relevant to marine fouling have not been studied. In this work, we applied sum frequency generation (SFG) vibrational spectroscopy to study the surface structures and hydration of a series of amphiphilic polypeptoid coatings with different sequences in air and water. SFG spectra, in agreement with X-ray spectroscopy studies, revealed that the surface coverage of the hydrophilic N-(2-methoxyethyl)glycine (Nme) units in air is affected by both the number and position of the hydrophobic N-(heptafluorobutyl)glycine (NF) units in the peptoid chain and is negatively correlated with the surface concentration of the fluorine element. Our ability to probe the SFG signals of water molecules at the peptoid surface provides new information on the hydrated film properties. From these SFG signals and the time evolution of water contact angles on the polymers, we see that the hydrated film properties are also dependent upon the peptoid sequence. This work indicates that the surface presence of the Nme groups and the ability of the polymers to order and strongly hydrogen bond with interfacial water molecules determine their antifouling properties, whereas the surface restructuring rate upon contact with water affects their fouling release behaviors.

Exploring the potential of fulvalene dimetals as platforms for molecular solar thermal energy storage: computations, syntheses, structures, kinetics, and catalysis.

A study of the scope and limitations of varying the ligand framework around the dinuclear core of FvRu2 in its function as a molecular solar thermal energy storage framework is presented. It includes DFT calculations probing the effect of substituents, other metals, and CO exchange for other ligands on ΔHstorage . Experimentally, the system is shown to be robust in as much as it tolerates a number of variations, except for the identity of the metal and certain substitution patterns. Failures include 1,1',3,3'-tetra-tert-butyl (4), 1,2,2',3'-tetraphenyl (9), diiron (28), diosmium (24), mixed iron-ruthenium (27), dimolybdenum (29), and ditungsten (30) derivatives. An extensive screen of potential catalysts for the thermal reversal identified AgNO3 -SiO2 as a good candidate, although catalyst decomposition remains a challenge.

Ultralow thermal conductivity in polycrystalline CdSe thin films with controlled grain size.

Polycrystallinity leads to increased phonon scattering at grain boundaries and is known to be an effective method to reduce thermal conductivity in thermoelectric materials. However, the fundamental limits of this approach are not fully understood, as it is difficult to form uniform sub-20 nm grain structures. We use colloidal nanocrystals treated with functional inorganic ligands to obtain nanograined films of CdSe with controlled characteristic grain size between 3 and 6 nm. Experimental measurements demonstrate that thermal conductivity in these composites can fall beneath the prediction of the so-called minimum thermal conductivity for disordered crystals. The measurements are consistent, however, with diffuse boundary scattering of acoustic phonons. This apparent paradox can be explained by an overattribution of transport to high-energy phonons in the minimum thermal conductivity model where, in compound semiconductors, optical and zone edge phonons have low group velocity and high scattering rates.

Salt-Screened Transition toward Bulk-Like Water Dynamics near Polymeric Zwitterions.

The superior antifouling performance of zwitterionic materials is commonly linked to their hydration structure, in which tight surface binding of water molecules inhibits solute adsorption. However, there is comparatively little direct experimental data on the hydration water structure and dynamics around zwitterionic moieties, including the longer-range behavior of the hydration shell that modulates the approach of solutes to the polymer surface. This work experimentally probes the dynamics of the diffusing hydration water molecules around a series of zwitterion chemistries using Overhauser dynamic nuclear polarization relaxometry. Surprisingly, water dynamics measured within ∼1 nm of the zwitterions were minimally inhibited compared to those near uncharged hydrophilic or cationic side chains. Specific dissolved ions further enhance the water diffusivity near the zwitterions, rendering the hydration shell bulk water-like. These results that the hydration of a zwitterion surface is nearly indistinguishable from bulk water suggest that these surfaces are "invisible" to biological constituents in a manner tunable by the ionic environment and the chemical design of the zwitterionic surface.

Nanoscale water-polymer interactions tune macroscopic diffusivity of water in aqueous poly(ethylene oxide) solutions.

The separation and anti-fouling performance of water purification membranes is governed by both macroscopic and molecular-scale water properties near polymer surfaces. However, even for poly(ethylene oxide) (PEO) - ubiquitously used in membrane materials - there is little understanding of whether or how the molecular structure of water near PEO surfaces affects macroscopic water diffusion. Here, we probe both time-averaged bulk and local water dynamics in dilute and concentrated PEO solutions using a unique combination of experimental and simulation tools. Pulsed-Field Gradient NMR and Overhauser Dynamic Nuclear Polarization (ODNP) capture water dynamics across micrometer length scales in sub-seconds to sub-nanometers in tens of picoseconds, respectively. We find that classical models, such as the Stokes-Einstein and Mackie-Meares relations, cannot capture water diffusion across a wide range of PEO concentrations, but that free volume theory can. Our study shows that PEO concentration affects macroscopic water diffusion by enhancing the water structure and altering free volume. ODNP experiments reveal that water diffusivity near PEO is slower than in the bulk in dilute solutions, previously not recognized by macroscopic transport measurements, but the two populations converge above the polymer overlap concentration. Molecular dynamics simulations reveal that the reduction in water diffusivity occurs with enhanced tetrahedral structuring near PEO. Broadly, we find that PEO does not simply behave like a physical obstruction but directly modifies water's structural and dynamic properties. Thus, even in simple PEO solutions, molecular scale structuring and the impact of polymer interfaces is essential to capturing water diffusion, an observation with important implications for water transport through structurally complex membrane materials.

Ion and Water Dynamics in the Transition from Dry to Wet Conditions in Salt-Doped PEG.

The influence of the water content on ion and water transport mechanisms in polymer membranes under low to moderate hydration conditions remains poorly understood. In this study, we combine ion and water diffusivity (PFG-NMR) measurements with atomistic molecular dynamics simulations to better understand transport processes in hydrated salt-doped poly(ethylene glycol). Above the water percolation threshold, the experimental and simulated diffusivities are in good agreement with the free volume transport models. At low hydration levels, unlike dry systems, ion diffusion cannot be described by polymer segmental dynamics alone. We rationalize such observations by the interplay between ion-water and ion-polymer solvation of cations and between ion-water and cation-anion interactions for anions. Further, we demonstrate that a two-state model combining ion-water solvation and free volume transport can describe water dynamics across the entire hydration range of interest. Our findings provide a more encompassing analysis of ion and water transport in hydrated polyelectrolytes, specifically in the low hydration regime.

Improved Mechanical Strength without Sacrificing Li-Ion Transport in Polymer Electrolytes.

Next-generation batteries demand solid polymer electrolytes (SPEs) with rapid ion transport and robust mechanical properties. However, many SPEs with liquid-like Li+ transport mechanisms suffer a fundamental trade-off between conductivity and strength. Dynamic polymer networks can improve bulk mechanics with minimal impact to segmental relaxation or ionic conductivity. This study demonstrates a system where a single polymer-bound ligand simultaneously dissociates Li+ and forms long-lived Ni2+ networks. The polymer comprises an ethylene oxide backbone and imidazole (Im) ligands, blended with Li+ and Ni2+ salts. Ni2+-Im dynamic cross-links result in the formation of a rubbery plateau resulting in, consequently, storage modulus improvement by a factor of 133× with the introduction of Ni2+ at rNi = 0.08, from 0.014 to 1.907 MPa. Even with Ni2+ loading, the high Li+ conductivity of 3.7 × 10-6 S/cm is retained at 90 °C. This work demonstrates that decoupling of ion transport and bulk mechanics can be readily achieved by the addition of multivalent metal cations to polymers with chelating ligands.

Crystallization-Induced Flower-like Superstructures via Peptoid Helix Assembly.

We report a unique method to construct hierarchical superstructures based on molecular programming of peptidomimetics. Chiral steric hindrance in the polymer backbone stabilizes peptoid helices that crystallize into nanosheets during solvent evaporation. The stacking of nanosheets results in flower-like superstructures. The helical peptoid, nucleated from chiral monomers, is characterized as locally stiffer and more extended than the unstructured peptoid. Molecular dynamics (MD) simulations further suggest a constraint on the dihedral angles and a preference toward the trans configuration, resulting in an extended chain structure. The nanosheet assemblies at various length scales indicate an extent of intermolecular ordering amplified by chiral steric hindrance. Such molecular programming and processing protocols will benefit the future design and controlled assembly of hierarchical peptidomimetics.

Thermoelectric power factor optimization in PEDOT:PSS tellurium nanowire hybrid composites.

The thermoelectric properties of a unique hybrid polymer-inorganic nanoparticle system consisting of tellurium nanowires and a conducting polymer, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), can be optimized by both controlling the shape of the nanoparticles and the loading and doping of the polymeric matrix with polar solvents. The mechanism for an observed improvement in power factor is attributed to the unique conducting nature of PEDOT:PSS, which exhibits a transition from a hopping transport-dominated regime to a carrier scattering-dominated regime upon doping with polar solvents. Near this transition, the electrical conductivity can be improved without significantly reducing the thermopower. Relying on this principle, the power factor optimization for this new thermoelectric material is experimentally carried out and found to exceed 100 µW m(-1) K(-2), which is nearly five orders of magnitude greater than pure PEDOT:PSS.

Integrated microfluidic test-bed for energy conversion devices.

Energy conversion devices require the parallel functionality of a variety of components for efficient operation. We present a versatile microfluidic test-bed for facile testing of integrated catalysis and mass transport components for energy conversion via water electrolysis. This system can be readily extended to solar-fuels generators and fuel-cell devices.

Morphology and thermodynamic properties of a copolymer with an electronically conducting block: poly(3-ethylhexylthiophene)-block-poly(ethylene oxide).

We report on the synthesis and morphology of a block copolymer, poly(3-(2'-ethylhexyl)thiophene)-b-poly(ethylene oxide) (P3EHT-b-PEO), that conducts both electrons and ions. We show that in the melt state the P3EHT-b-PEO chains self-assemble to produce traditional nanoscale morphologies such as lamellae and gyroid. This is in contrast to a majority of previous studies on copolymers with electronically conducting blocks wherein a nanofibrillar morphology is obtained. Our approach enables estimation of the Flory-Huggins interaction parameter, χ. The segregation strength between the two blocks is controlled through the addition of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). For the salt-free sample, the gyroid morphology, obtained in the melt state, is transformed into lamellae below the melting temperature of the P3EHT block. This is due to the "breaking out" of the crystalline phase. For the salt-containing sample, P3EHT-b-PEO has a lamellar morphology in both melt and crystalline states (confined crystallization).