A US perspective on closing the carbon cycle to defossilize difficult-to-electrify segments of our economy

Electrification to reduce or eliminate greenhouse gas emissions is essential to mitigate climate change. However, a substantial portion of our manufacturing and transportation infrastructure will be difficult to electrify and/or will continue to use carbon as a key component, including areas in aviation, heavy-duty and marine transportation, and the chemical industry. In this Roadmap, we explore how multidisciplinary approaches will enable us to close the carbon cycle and create a circular economy by defossilizing these difficult-to-electrify areas and those that will continue to need carbon. We discuss two approaches for this: developing carbon alternatives and improving our ability to reuse carbon, enabled by separations. Furthermore, we posit that co-design and use-driven fundamental science are essential to reach aggressive greenhouse gas reduction targets.

Upcycling plastic waste into fully recyclable composites through cold sintering

Plastics have substantial societal benefits, but their widespread use has led to a critical waste management challenge. While mechanical recycling dominates the reuse of post-consumer plastics, it is limited in efficacy, especially for composites. To address this, we propose a direct reprocessing approach that enables the creation of hybrid, long-lasting, and durable composites from difficult-to-recycle plastics. This approach utilizes cold sintering, a process that consolidates inorganic powders through fractional dissolution and precipitation at temperatures far below conventional sintering; these temperatures are compatible with plastic processing. We show that this process can create inorganic-matrix composites with significant enhancements in tensile strength and toughness over pure gypsum, which is commonly found in construction waste. These composites can be recycled multiple times through direct reprocessing with the addition of only water as a processing promoter. This approach to recycling leads to composites with orders of magnitude lower energy demand, global warming potential, and water demand, when compared against common construction products. Altogether, we demonstrate the potential for cold sintering to integrate waste into high-performance recyclable composites.

Revealing Deformation Mechanisms in Polymer-Grafted Thermoplastic Elastomers via In Situ Small-Angle X-ray Scattering

The tunable properties of thermoplastic elastomers (TPEs), through polymer chemistry manipulations, enable these technologically critical materials to be employed in a broad range of applications. The need to "dial-in" the mechanical properties and responses of TPEs generally requires the design and synthesis of new macromolecules. In these designs, TPEs with nonlinear macromolecular architectures outperform the mechanical properties of their linear copolymer counterparts, but the differences in the deformation mechanism providing enhanced performance are unknown. Here, in situ small-angle X-ray scattering (SAXS) measurements during uniaxial extension reveal distinct deformation mechanisms between a commercially available linear poly(styrene)-poly(butadiene)-poly(styrene) (SBS) triblock copolymer and the grafted SBS version containing grafted poly(styrene) (PS) chains from the poly(butadiene) (PBD) midblock. The neat SBS (φSBS = 100%) sample deforms congruently with the macroscopic dimensions, with the domain spacing between spheres increasing and decreasing along and transverse to the stretch direction, respectively. At high extensions, end segment pullout from the PS-rich domains is detected, which is indicated by a disordering of SBS. Conversely, the PS-grafted SBS that is 30 vol % SBS and 70% styrene (φSBS = 30%) exhibits a lamellar morphology, and in situ SAXS measurements reveal an unexpected deformation mechanism. During deformation, there are two simultaneous processes: significant lamellar domain rearrangement to preferentially orient the lamellae planes parallel to the stretch direction and crazing. The samples whiten at high strains as expected for crazing, which corresponds with the emergence of features in the 2D SAXS pattern during stretching consistent with fibril-like structures that bridge the voids in crazes. The significant domain rearrangement in the grafted copolymers is attributed to the new junctions formed across multiple PS domains by the grafting of a single chain. The in situ SAXS measurements provide insights into the enhanced mechanical properties of grafted copolymers that arise through improved physical cross-linking that leads to nanostructure domain reorientation for self-reinforcement and craze formation where fibrils help to strengthen the polymer.

Correction: A generalized method for alignment of block copolymer films: solvent vapor annealing with soft shear

Correction for 'A generalized method for alignment of block copolymer films: solvent vapor annealing with soft shear' by Zhe Qiang et al., Soft Matter, 2014, 10, 6068-6076, https://doi.org/10.1039/C4SM00875H.

Influence of the Nature of Aliphatic Hydrophobic Physical Crosslinks on Water Crystallization in Copolymer Hydrogels

The local environment within a hydrogel influences the properties of water, including the propensity for ice crystallization. Water-swollen amphiphilic copolymers produce tunable nanoscale environments, which are defined by hydrophobic associations, for the water molecules. Here, the antifreeze properties for equilibrium-swollen amphiphilic copolymers with a common hydrophilic component, hydroxyethyl acrylate (HEA), but associated through crystalline (octadecyl acrylate, ODA) or rubbery (ethylhexyl acrylate, EHA) hydrophobic segments, are examined. Differences in the efficacy of the associations can be clearly enunciated from compositional solubility limits for the copolymers in water (<2.6 mol % ODA vs ≤14 mol % EHA), and these differences can be attributed to the strength of the association. The equilibrium-swollen HEA-ODA copolymers are viscoelastic solids, while the swollen HEA-EHA copolymers are viscoelastic liquids. Cooling these swollen copolymers to nearly 200 K induces some crystallization of the water, where the fraction of water frozen depends on the details of the nanostructure. Decreasing the mean free path of water by increasing the ODA composition from 10 to 25 mol % leads to fractionally more unfrozen water (66-87%). The swollen HEA-EHA copolymers only marginally inhibit ice (<13%) except with 45 mol % EHA, where nearly 60% of the water remains amorphous on cooling to 200 K. In general, the addition of the EHA leads to less effective ice inhibition than analogous covalently crosslinked HEA hydrogels (19.9 ± 1.8%). These results illustrate that fluidity of confining surfaces can provide pathways for critical nuclei to form and crystal growth to proceed.

Size and print path effects on mechanical properties of material extrusion 3D printed plastics

Print conditions for thermoplastics by filament-based material extrusion (MatEx) are commonly optimized to maximize the elastic modulus. However, these optimizations tend to ignore the impact of thermal history that depends on the specimen size and print path selection. Here, we investigate the effect of size print path (raster angle and build orientation) and print sequence on the mechanical properties of polycarbonate (PC) and polypropylene (PP). Examination of parallel and series printing of flat (XY) and stand-on (YZ) orientation of Type V specimens demonstrated that to observe statistical differences in the mechanical response that the interlayer time between printed roads should be approximately 5 s or less. The print time for a single layer in XY orientation is much longer than that for a single layer in YZ orientation, so print sequence only impacts the mechanical response in the YZ orientation. However, the specimen size and raster angle did influence the mechanical properties in XY orientation due to the differences in thermal history associated with intralayer time between adjacent roads. Moreover, all of these effects are significantly larger when printing PC than PP. These differences between PP and PC are mostly attributed to the mechanism of interface consolidation (crystallization vs. glass formation), which changes the requirements for a strong interface between roads (crystals vs. entanglements). These results illustrate how the print times dictated by the print path layout impact observed mechanical properties. This work also demonstrated that the options available in some standards developed for traditional manufacturing will change the quantitative results when applied to 3D printed parts.

Supplementary Information

The online version contains supplementary material available at 10.1007/s40964-022-00275-w.

Sodium dodecyl sulfate modulates the structure and rheological properties of Pluronic F108-poly(acrylic acid) coacervates)

Micelles formed within coacervate phases can impart functional properties, but it is unclear if this micellization provides mechanical reinforcement of the coacervate whereby the micelles act as high functionality crosslinkers. Here, we examine how sodium dodecyl sulfate (SDS) influences the structure and properties of Pluronic F108-polyacrylic acid (PAA) coacervates as SDS is known to decrease the aggregation number of Pluronic micelles. Increasing the SDS concentration leads to larger water content in the coacervate and an increase in the relative concentration of PAA to the other solids. Rheological characterization with small angle oscillatory shear (SAOS) demonstrates that these coacervates are viscoelastic liquids with the moduli decreasing with the addition of the SDS. The loss factor (tan δ) initially increases linearly with the addition of SDS, but a step function increase in the loss factor occurs near the reported CMC of SDS. However, this change in rheological properties does not appear to be correlated with any large scale structural differences in the coacervate as determined by small angle X-ray scattering (SAXS) with no signature of Pluronic micelles in the coacervate when SDS concentration is >4 mM during formation of the coacervate, which is less than that observed (6 mM SDS) in initial Pluronic F108 solution despite the higher polymer concentration in the coacervate. These results suggest that the mechanical properties of polyelectrolyte-non-ionic surfactant coacervates are driven by the efficicacy of binding between the complexing species driving the coacervate, which can be disrupted by competitive binding of the SDS to the Pluronic.

Non-destructive determination of functionalized polyelectrolyte placement in layer-by-layer films by IR ellipsometry

Layer-by-layer (LbL) assembly facilitates controlled coatings on a variety of surfaces with the ability to manipulate the composition through the thickness by selection of the complementary pairs. However, the characterization of these composition profiles tends to be destructive and requires significant compositional differences that can limit their utility. Here, we demonstrate the ability to non-destructively quantify the depth dependence of the allyl content associated with the selective incorporation of poly(sodium acrylate-co-allylacrylamide) (84 : 16 mol : mol) (allyl-PAA) in LbL films based on the assembly of poly(diallyldimethylammonium chloride) (PDAC)/poly(acrylic acid) (PAA) and PDAC/allyl-PAA. Although the atomic composition of the film is not dramatically influenced by the change between PAA and allyl-PAA, the absorption in the IR near 1645 cm^-1 by the allyl group provides sufficient optical contrast to distinguish the LbL components with spectroscopic ellipsometry. The use of IR spectroscopic ellipsometry can determine the thickness of layers that contain allyl-PAA and also gradients that develop due to re-arrangements during the LbL process. With multiple films fabricated simultaneously, the location of the gradient between the 1st and 2nd series of multilayers (e.g., first PDAC/PAA bilayers and then PDAC/allyl-PAA bilayers) can be readily assessed. The results from a variety of different multilayer architectures indicate that the gradient is located within the thickness expected for the 1st deposited bilayer stack (PDAC/PAA or PDAC/allyl-PAA). These results are indicative of a dynamic dissolution-deposition process (in- and out- diffusion) during the fabrication of these LbL films. These results provide additional evidence into the mechanisms for exponential growth in LbL assemblies. The ability to quantify a gradient with the low contrast system examined indicates that spectroscopic IR ellipsometry should be able to non-destructively determine compositional gradients for most polymer films where such gradients exist.

Microwave-Enabled Size Control of Iron Oxide Nanoparticles on Reduced Graphene Oxide

Nanoparticle-functionalized 2D material networks are promising for a wide range of applications, but in situ formation of nanoparticles is commonly challenged by rapid growth. Here, we demonstrate controlled synthesis of small and dispersed iron oxide nanoparticles on reduced graphene oxide (rGO) networks through rapid localized heating with microwaves with low-cost iron nitrate as the precursor. The strong coupling of the microwave radiation with the rGO network rapidly heats the network locally to decompose the iron nitrate and generate iron oxide nanoparticles, while cessation of microwaves leads to rapid cooling to minimize crystal growth. Small changes in the microwave reaction time (<1 min) led to very large changes in the iron oxide morphology. The solid-state microwave syntheses produced narrower nanoparticle size distribution than conventional heating. These results illustrate the potential of solid-state microwave syntheses to control the nanoparticle size on 2D materials through rapid localized heating under the microwave process conditions, which should be extendable to a variety of transition metal oxide-rGO systems.

Water dynamics within nanostructured amphiphilic statistical copolymers from quasielastic neutron scattering

Understanding the properties of water under either soft or hard confinement has been an area of great interest, but nanostructured amphiphilic polymers that provide a secondary confinement have garnered significantly less attention. Here, a series of statistical copolymers of 2-hydroxyethyl acrylate (HEA) and 2-(N-ethylperfluorooctane sulfonamido)ethyl methacrylate (FOSM) are swollen to equilibrium in water to form nanostructured physically cross-linked hydrogels to probe the effect of soft confinement on the dynamics of water. Changing the composition of the copolymer from 10 to 21 mol. % FOSM decreases the average size of the assembled FOSM cross-link, but also the spacing between the cross-links in the hydrogels with the mean distance between the FOSM aggregates decreasing from 3.9 to 2.7 nm. The dynamics of water within the hydrogels were assessed with quasielastic neutron scattering. These hydrogels exhibit superior performance for inhibition of water crystallization on supercooling in comparison to analogous hydrogels with different hydrophilic copolymer chemistries. Despite the lower water crystallinity, the self-diffusion coefficient for these hydrogels from the copolymers of HEA and FOSM decreases precipitously below 260 K, which is a counter to the nearly temperature invariant water dynamics reported previously with an analogous hydrogel [Wiener et al., J. Phys. Chem. B 120, 5543 (2016)] that exhibits nearly temperature invariant dynamics to 220 K. These results point to chemistry dependent dynamics of water that is confined within amphiphilic hydrogels, where the interactions of water with the hydrophilic segments can qualitatively alter the temperature dependent dynamics of water in the supercooled state.

Controlling nanostructure and mechanical properties in triblock copolymer/monomer blends via reaction-induced phase transitions

Thermoplastic elastomers based on ABA triblock copolymers are typically limited in modulus and strength due to crack propagation within the brittle regions when the hard end-block composition favors morphologies that exhibit connected domains. Increasing the threshold end-block composition to achieve enhanced mechanical performance is possible by increasing the number of junctions or bridging points per chain, but these copolymer characteristics also tend to increase the complexity of the synthesis. Here, we report an in situ polymerization method to successfully increase the number of effective junctions per chain through grafting of poly(styrene) (PS) to a commercial thermoplastic elastomer, poly(styrene)-poly(butadiene)-poly(styrene) (SBS). The strategy described here transforms a linear SBS triblock copolymer-styrene mixture into a linear-comb-linear architecture in which poly(styrene) (PS) grafts from the mid-poly(butadiene) (PBD) block during the polymerization of styrene. Through systematic variation in the initial SBS/styrene content, nanostructural transitions from disordered spheres to lamellar through reaction-induced phase transitions (RIPT) were identified as the styrene content increased. Surprisingly, maximum mechanical performance (Young's modulus, tensile strength, and elongation at break) was obtained with samples exhibiting lamellar nanostructures, corresponding to overall PS contents of 61-77 wt% PS (including the original PS in SBS). The PS grafting from the PBD block increases the modulus and the strength of the thermoplastic elastomer while preventing brittle fracture due to the greater number of junctions afforded by the PS grafts. The work presented here demonstrates the use of RIPT to transform standard SBS materials into polymer systems with enhanced mechanical properties.

Mechanisms of Directional Polymer Crystallization

Zone annealing, a directional crystallization technique originally used for the purification of semiconductors, is applied here to crystalline polymers. Tight control over the final lamellar orientation and thickness of semicrystalline polymers can be obtained by directionally solidifying the material under optimal conditions. It has previously been postulated by Lovinger and Gryte that, at steady state, the crystal growth rate of a polymer undergoing zone annealing is equal to the velocity at which the sample is drawn through the temperature gradient. These researchers further implied that directional crystallization only occurs below a critical velocity, when crystal growth rate dominates over nucleation. Here, we perform an analysis of small-angle X-ray scattering, differential scanning calorimetry, and cross-polarized optical microscopy of zone-annealed poly(ethylene oxide) to examine these conjectures. Our long period data validate the steady-state ansatz, while an analysis of Herman’s orientation function confirms the existence of a transitional region around a critical velocity, v_crit, where there is a coexistence of oriented and isotropic domains. Below v_crit, directional crystallization is achieved, while above v_crit, the mechanism more closely resembles that of conventional isotropic isothermal crystallization.

Aqueous polypropylene glycol induces swelling and severe plasticization of high Tg amphiphilic copolymers containing hexafluoroisopropanol groups

Poly(ethylene glycol) (PEG) tends to be considered low fouling, which has led to its use in a wide variety of applications. Amphiphilic polyols, such as Antifoam 204, are commonly used as surfactants in fermentation processes due to their limited toxicity and low cost, but these polyols in aqueous solutions can unexpectedly swell membranes. Here we examine the interactions of PEG or poly(propylene glycol) (PPG) with amphiphilic substituted norbornene copolymers through swelling in dilute aqueous solution. The effect of molecular mass (M_n) of the polyol (PEG and PPG) in aqueous (1 wt% butanol) solution on the swelling and mechanical properties of a series of poly(alkyl norbornene-co-hexafluoroisopropanol norbornene) is systematically investigated using a quartz crystal microbalance with dissipation. At 10 ppm of PEG, the swelling is less than 20% for all of the copolymers examined and the swelling is independent of PEG M_n. Although PPG at the lowest M_n examined leads to similar swelling to PEG, the swelling induced by PPG increases with M_n to reach a maximum at M_n = 3.1 kg mol^-1. Pluronic L121 is similar compositionally to Antifoam 204, but the equilibrium swelling is decreased by nearly a factor of 2, which is attributed to the higher M_n of Pluronic L121. The limited dependence on the alkyl chain in the copolymer suggest that the interactions between the polyol and hexafluoroisopropanol moiety in the copolymer drive the uptake by the membrane through bound water with the unassociated ether in the PPG that increases swelling with increasing M_n, but a finite size effect limits the swelling for sufficiently large polymer additives.

Microwave Processing Controls the Morphology of Block Copolymer-Templated Mesoporous Cobalt Oxide Films

Microwave heating provides an efficient method to rapidly heat materials through interaction of microwaves with the media. Here, we demonstrate the rapid synthesis of mesoporous cobalt oxide films through the heating of the silicon substrate by microwaves. A non-sol-gel approach based on cobalt nitrate-citric acid complex cooperative assembly with a poly[methoxy poly(ethylene glycol)methacrylate]-block-poly(butyl acrylate) (PMPEGMA-b-PBA) block copolymer was used to fabricate the cobalt oxide through a cobalt carbonate intermediate. The time required to convert cobalt carbonate to cobalt oxide with the full removal of the PMPEGMA-b-PBA template can be decreased by two orders of magnitude with microwaves in comparison to standard heating in a furnace at 350 °C. At the highest microwave power examined (1500 W), this can be accomplished within 2 s, while >5 min is required at 350 °C in a furnace. At a microwave power of <400 W, there is insufficient energy to induce the transition from carbonate to oxide, but even at only 420 W, the oxide can be formed within 26 s. The rapid heating by the microwaves tends to increase the crystallinity and mean crystal size of the cobalt oxide within the mesoporous films. Despite the growth of larger average crystals, the pore size and porosity tend to be larger when the film is processed using microwaves. Higher microwave power leads to larger average crystals and average pore size. These results suggest that rapid processing to crystallize frameworks in mesoporous materials may allow for highly crystalline frameworks without loss of the templated mesostructure.

Influence of Sodium Salts on the Swelling and Rheology of Hydrophobically Cross-linked Hydrogels Determined by QCM-D

Hydrophobically modified copolymers provide a versatile platform of hydrogel materials for diverse applications, but the influence of salts on the swelling and material properties of this class of hydrogels has not been extensively studied. Here, we investigate model hydrogels with three different sodium salts with anions chosen from the classic Hofmeister series to determine how these counterions influence the swelling and mechanical properties of neutral hydrogels. The gel chosen was based on a statistical copolymer of dimethylacrylamide and 2-(N-ethylperfluorooctane sulfonamido) ethyl acrylate (FOSA). Our measurements utilize a quartz crystal microbalance with dissipation (QCM-D) to quantify both swelling and rheological properties of these gels. We find that a 1 mol/L solution of Na₂SO₄, corresponding to a kosmotropic anion, leads to nearly a 2.6-fold gel deswelling and correspondingly, the complex modulus increases by an order of magnitude under these solution conditions. In contrast, an initial increase in swelling and then a swelling maximum is observed for a 0.02 mol/L concentration in the case of a chaotropic anion, NaClO₄, but the changes in the degree of gel swelling in this system are not directly correlated with changes in the gel shear modulus. The addition of NaBr, an anion salt closer to the middle of the chaotropic to kosmotropic range, leads to hydrogel deswelling where the degree of deswelling and the shear modulus are both nearly independent of salt concentration. Overall, the observed trends are broadly consistent with more kosmotropic ions causing diminished solubility ("salting out") and strongly chaotropic ions causing improved solubility ("salting in"), a trend characteristic of the Hoffmeister series governing the solubility of many proteins and synthetic water-soluble polymers, but trends in the shear stiffness with gel swelling are clearly different from those normally observed in chemically cross-linked gels and are correspondingly difficult to interpret. The salt specificity of swelling and mechanical properties of nonionic hydrogels is important for any potential application in which a wide range of salt concentrations and types are encountered.

Control of Pore Size in Ordered Mesoporous Carbon-Silica by Hansen Solubility Parameters of Swelling Agent

The cooperative assembly of functional precursors with block copolymers (BCPs) is a powerful, general route to fabricate ordered mesoporous materials, but the precise tuning of the mesopore size generally requires trial and error to obtain the correct BCP template or appropriate swelling agent. Here, we demonstrate the ability to effectively modulate both expansion and contraction of the ordered cylindrical mesopores relative to those obtained from cooperatively assembled Pluronic F127, resol, and tetraethylorthosilicate. The two key physical parameters for the swelling agents are their hydrophobicity, as quantified by the octanol-water partition coefficient (K_ow), and Hansen solubility parameters that describe the interactions of the solvent with the different components of the BCP template. Four low volatility solvents are examined that span a wide K_ow with up to 90 wt % solvent relative to the Pluronic F127. Glycerol triacetate (K_ow < 1) can decrease the average mesopore size from 5.9 to 4.8 nm due to segmental screening of the interactions in the Pluronic F127 to decrease chain stretching at intermediate loadings. A modest increase in mesopore size to 8.1 nm can be achieved with trimethylbenzene (TMB, K_ow = 3.42). Dioctyl phthalate (DOP), which is slightly more hydrophobic (K_ow = 8.1), is more effective than TMB at expanding the pore size (maximum: 13.5 nm) without loss of ordered structure. A more hydrophobic solvent, tris (2-ethylhexyl) trimellitate (K_ow = 12.5), is less effective at increasing the pore size (maximum: 8.2 nm). The Hansen solubility parameters for DOP most closely match those of the hydrophobic segment in the Pluronic F217 template. We attribute this similarity, which is related to the solvent quality, to the improved efficacy of DOP in increasing the pore size. These results illustrate that both the Hansen solubility parameters (relative to the hydrophobic segment of the template) and relative hydrophobicity of the swelling agent determine the obtainable pore sizes in cooperatively assembled ordered mesoporous materials.

Li-Ion Capacitor Integrated with Nano-network-Structured Ni/NiO/C Anode and Nitrogen-Doped Carbonized Metal-Organic Framework Cathode with High Power and Long Cyclability

Lithium-ion capacitors (LICs) represent a new type of energy-storage devices, which have combined merits of high energy density Li-ion battery and high power density supercapacitor. Nevertheless, one significant challenge for LICs is the imbalanced kinetics between the fast capacitive cathode and relatively slow intercalation anode that limit the energy-storage performance. Here, the asymmetric LIC devices were developed based on a nitrogen-doped, carbonized zeolitic imidazolate framework (ZIF-8) cathode and a three-dimensional, nano-network-structured, conversion reaction-based Ni/NiO/C anode. These nanostructures associated with both the cathode and anode enable rapid electron and ions transport in the LIC devices, which allows the asymmetric LICs to be operated on either high energy mode (energy density of 114.7 Wh/kg at power density of 98.0 W/kg) or high power mode (power density of 60.1 kW/kg at energy density of 17.6 Wh/kg). The device also exhibited long-term cycle stability with 87% capacitance retention after 12 000 cycles. These results demonstrate that the rational design of nanoporous electrode structures can deliver a balanced, high-performance-activated cZIF-8|Ni/NiO/C-based lithium-ion capacitor.

Tuning Cooperative Assembly with Bottlebrush Block Co-polymers for Porous Metal Oxide Films Using Solvent Mixtures

Block copolymer templating enables the generation of well-defined pore sizes and geometries in a wide variety of frameworks, typically through evaporation-induced self-assembly (EISA). Here, we systematically modulate the solvent quality with mixtures of tetrahydrofuran-ethanol (THF-EtOH) to manipulate the unimer/micelle ratio in the precursor solution to explore how the associated solution structure influences the final pore morphology. A bottlebrush block copolymer (BBCP) with poly(ethylene oxide) and poly(t-butyl acrylate) side chains was used as the template for pore formation. Irrespective of the solvent composition, a bimodal pore size distribution was obtained with mesopores templated by small aggregates of the BBCP unimers (potentially low aggregation number micelles) and macropores templated by large self-assembled BBCP micelles. The morphology and pore characteristics of the metal oxide films were dependent on the THF-EtOH composition. Interestingly, an intermediate solvent composition where the volume of micelles is approximately half the volume of unimers (in the precursor solution) leads to the best ordering of micelle-templated pores and also the maximum porosity in the films. The micelle/unimer ratios in the precursor solutions do not correspond directly to the bimodal pore distribution in the metal oxide films, which we attribute to kinetically trapped assembly of the BBCP at a low THF content. The increased critical micelle concentration at high THF composition leads to changes in the unimer/micelle ratio during solvent evaporation. These results appear to be universal for a number of metal oxides (cobalt, magnesium, and zinc) with the porosity maximized at a THF/EtOH ratio of 3:1. These results suggest the potential for enhancements in the porosity of block copolymer-templated films by EISA methods through judicious solvent selection.

Tunable Piezoresistivity from Magnetically Aligned Ni(Core)@Ag(Shell) Particles in an Elastomer Matrix

Core-shell (Ni@Ag) particles are aligned through the thickness of a poly(dimethylsiloxane) (PDMS) film using a magnetic field in a continuous roll-to-roll process. The alignment of the particles dramatically decreases the percolation threshold for electrical conductivity through the thickness of the film by nearly an order of magnitude from 28 vol % without the field to ≈1 vol % with a 52 mT magnetic field during curing. However, the magnetic forces lead to rough surface topography for intermediate Ni@Ag loadings, but confining the Ni@Ag/PDMS composite by a glass constraint provides a smooth surface. This difference in geometry changes the morphology of the vertically aligned "chains" of Ni@Ag particles where the chains are more aggregated when the film is unconstrained. As the Ni@Ag concentration is decreased below 3.6% for the constrained film, breaks in the aligned particles evident from X-ray tomography lead to pressure sensitive resistance across that film with a large decrease in resistance above a threshold pressure. The threshold pressure is demonstrated to be controllable from ≈15 to ≈290 kPa through the loading of aligned Ni@Ag in the PDMS, but this threshold pressure decreases on cyclic loading. These magnetically aligned composites represent a facile route to mechanically responsive films that could be used in a variety of applications where cyclic loading above and below the threshold pressure is not required, such as disposable pressure sensors for ensuring reliability of products through transportation and embedded structural health monitoring for identifying critical displacements.

Polyelectrolyte-micelle coacervates: intrapolymer-dominant vs. interpolymer-dominant association, solute uptake and rheological properties

The effects of polyelectrolyte charge density, polyelectrolyte-to-surfactant ratio, and micelle species on coacervation were studied by turbidity, dynamic light scattering, and zeta potential measurements to examine the coacervation of the weak polyelectrolyte branched polyethylenimine (BPEI) and oppositely charged sodium dodecyl sulfate (SDS) micelles as well as BPEI and mixed micelles composed of SDS and poly(ethylene glycol) 4-nonylphenyl 3-sulfopropyl ether potassium salt (PENS). The results of dynamic light scattering and zeta potential measurements are discussed in terms of pH and BPEI-to-surfactant ratio. An intrapolymer-dominant to interpolymer-dominant association model for the BPEI-micelle coacervates was proposed based on the variation of size and zeta potential of coacervate particles by their BPEI-to-surfactant ratio. The partition coefficient of solutes into BPEI-micelle coacervates was determined using UV-vis measurements as a function of pH, BPEI-to-surfactant ratio, and mixed micelle composition. Both the hydrophobicity of solutes and micelles, as well as the association mode of coacervates, impact the solute uptake efficiency. Dynamic rheological measurements on the coacervates suggest that the rheological properties of the complex coacervates are impacted by the association mode of the coacervates as well as the charge density on BPEI chains during coacervation.

Mechanically tunable, human mesenchymal stem cell viable poly(ethylene glycol)-oxime hydrogels with invariant precursor composition, concentration, and stoichiometry

Hydrogels are used widely for exploratory tissue engineering studies. However, currently no hydrogel systems have been reported that exhibit a wide range of elastic modulus without changing precursor concentration, identity, or stoichiometry. Herein, ester and amide-based PEG-oxime hydrogels with tunable moduli (~5-30 kPa) were synthesized with identical precursor mass fraction, stoichiometry, and concentration by varying the pH and buffer concentration of the gelation solution, exploiting the kinetics of oxime bond formation. The observed modulus range can be attributed to increasing amounts of network defects in slower forming gels, as confirmed by equilibrium swelling and small angle neutron scattering (SANS) experiments. Finally, hMSC viability was confirmed in these materials in a 24 h assay. While only an initial demonstration of the potential utility, the controlled variation in defect density and modulus is an important step forward in isolating system variables for hypothesis-driven biological investigations.

Strain rate dependent nanostructure of hydrogels with reversible hydrophobic associations during uniaxial extension

An energy dissipation mechanism during deformation is required to impart toughness to hydrogels. Here we describe how in situ small angle X-ray scattering (SAXS) provides insight into possible energy dissipation mechanisms for a tough hydrogel based on an amphiphilic copolymer where nanoscale associations of the hydrophobic moieties act as effective crosslinks. The mechanical properties of the hydrogels are intimately coupled with the nanostructure that provides reversible crosslinks and evolves during deformation. As the extension rate increases, more mechanical energy is dissipated from rearrangements of the crosslinks. The scattering is consistent with hopping of hydrophobes between the nanoscale aggregates as the primary rearrangement mechanism. This rearrangement changes the network conformation that leads to non-affine deformation, where the change in the nanostructure dimension from SAXS is less than 15% of the total macroscopic strain. These nanostructure changes are rate dependent and correlated with the relaxation time of the hydrogel. At low strain rate (0.15% s-1), no significant change of the nanostructure was observed, whereas at higher strain rates (1.5% s-1 and 8.4% s-1) significant nanostructure anisotropy occurred during extension. These differences are attributed to the ability for the network chains to rearrange on the time scale of the deformation; when the characteristic time for extension is longer than the average segmental relaxation time, no significant change in nanostructure occurs on uniaxial extension. These results illustrate the importance of strain rate in the mechanical characterization and consideration of relaxation time in the design of tough hydrogels with reversible crosslinks.

Effect of Adjacent Hydrophilic Polymer Thin Films on Physical Aging and Residual Stress in Thin Films of Poly(butylnorbornene-ran-hydroxyhexafluoroisopropyl norbornene)

The properties of thin supported polymer films can be dramatically impacted by the substrate upon which it resides. A simple way to alter the properties of the substrate (chemistry, rigidity, dynamics) is by coating it with an immiscible polymer. Here we describe how ultrathin (ca. 2 nm) hydrophilic polymer layers of poly(acrylic acid), PAA, and poly(styrenesulfonate), PSS, impact the aging behavior and the residual stress in thin films of poly(butylnorbornene-ran-hydroxyhexafluoroisopropyl norbornene), BuNB-r-HFANB. The aging rate decreases as the film thickness (h) is decreased, but the extent of this change depends on the adjacent layer. Even for the thickest films (h>500 nm), there is a decrease in the aging rate at 100 °C when BuNB-r-HFANB is in contact with PSS. In an effort to understand the origins of these differences in the aging behavior, the elastic modulus and residual stress (σR) in the films were determined by wrinkling as a function of aging time. The change in the elastic modulus during aging does not appear to be directly correlated with the densification or expansion of the films, but the aging rates appear to roughly scale as hσR 1/3. These results illustrate that the physical aging of thin polymer films can be altered by adjacent polymers.

Impact of surface wettability on dynamics of supercooled water confined in nitrogen-doped ordered mesoporous carbon

Confinement of water to nanoscale dimensions enables substantial supercooling through disruption of the hydrogen bonding network. However, there remain questions associated with the importance of the nature of the water-surface interactions relative to physical confinement defined by the pore geometry on the dynamics of supercooled water. Here, a simple route to tune the surface wetting properties through nitrogen doping of carbon is reported. This method leads to nearly indistinguishable mesopore sizes to enable separation of surface wettability and pore size effects. Quasielastic neutron scattering (QENS) is used to probe the proton dynamics of water confined within the mesopores with an average diameter of 4.85 ± 0.05 nm as a function of temperature from 267 K to 189 K. The motions of water in the mesopores follow jump-diffusion. For the temperatures examined, the diffusivity of water in the mesopores decreases with increasing nitrogen doping of the carbon framework. The activation energy associated with proton dynamics increases by approximately 30% with N-doping when compared to the undoped carbon surface, which is attributed to the enhanced surface wettability (favorable interactions between water and pore surface). This acts to provide an energy barrier for the water motions. This work suggests that the influence of surface chemistry on the dynamics of supercooled water confined in mesopores is less than the influence of nanopore size.

Sulfur Diffusion within Nitrogen-Doped Ordered Mesoporous Carbons Determined by in Situ X-ray Scattering

The low intrinsic conductivity of sulfur necessitates conductive additives, such as mesoporous carbons, to the cathode to enable high-performance metal-sulfur batteries. Simultaneous efforts to address polysulfide shuttling have introduced nitrogen-doped carbons to provide both conductivity and suppressed shuttling because of their strong interaction with sulfur. The strength of this interaction will impact the ability to fill the mesopores with sulfur via melt infusion. Here, we systematically investigate how nitrogen doping influences the rate that molten sulfur can infiltrate the mesopores and the overall extent of pore filling of highly ordered mesoporous doped carbons using in situ small angle X-ray scattering (SAXS). The similarity in electron density between molten sulfur and the soft carbon framework of the mesoporous material leads to a precipitous decrease in the scattered intensity associated with the ordered structure as voids are filled with sulfur. As the nitrogen doping increases from 1 to 20 at. %, the effective diffusivity of sulfur in the mesopores decreases by an order of magnitude (2.7 × 10^-8 to 2.3 × 10^-9 cm/s). The scattering becomes nearly invariant within 20 min of melt infiltration at 155 °C for all but the most doped carbon, which indicates that submicron-sized mesoporous carbon particles can be filled rapidly. Additionally, the nitrogen doping decreases the sulfur content that can be accommodated within the mesopores from 95% of the mesopores filled without doping to only 64% filled with 20 at. % N as determined by the residual scattering intensity. Sulfur does not crystallize within the mesopores of the nitrogen-doped carbons, which is further indicative of the strong interactions between the nitrogen species and sulfur that can inhibit polysulfide shuttling. In situ SAXS provides insights into the diffusion of sulfur in mesopores and how the surface chemistry of nitrogen-doped carbon appears to significantly hinder the infiltration by sulfur.

Anomalous Confinement Slows Surface Fluctuations of Star Polymer Melt Films

The unusually large film thickness at which confinement effects manifest themselves in surface fluctuations of unentangled four-arm star polymers has been defined using film thicknesses from 10R_g to 107R_g. For 15k four-arm star polystyrene (SPS), confinement appears at a thickness between 112 nm (40R_g) and 72 nm (26R_g), which is remarkably larger than the thicknesses at which confinement appears for unentangled 6k linear (<15 nm, <7R_g) and 6k and 14k cyclic (24 and 22 nm, respectively) polystyrenes. Data for 15k star films can be rationalized using a two-layer model with a 17 nm (6R_g) thick highly viscous layer at the substrate, which is significantly thicker than the 1R_g thick "irreversibly adsorbed" layer. For a 29 nm (10R_g) thick film, more striking confinement occurs due to the combined influence of both interfaces. These results underscore the extraordinary role long-chain branching plays in dictating surface fluctuations of thin films.

Mechanical properties of bulk graphene oxide/poly(acrylic acid)/poly(ethylenimine) ternary polyelectrolyte complex

Ternary complexes formed in a single pot process through the mixing of cationic (branched polyethylenimine, BPEI) and anionic (graphene oxide, GO, and poly(acrylic acid), PAA) aqueous solutions exhibit superior mechanical performance in comparison to their binary analogs. The composition of the ternary complex can be simply tuned through the composition of the anionic solution, which influences the water content and mechanical properties of the complex. Increasing the PAA content in the complex decreases the overall water content due to improved charge compensation with the BPEI, but this change also significantly improves the toughness of the complex. Ternary complexes containing ≤32 wt% PAA were too brittle to generate samples for tensile measurements, while extension in excess of 250% could be reached with 57 wt% PAA. From this work, the influence of GO and PAA on the mechanical properties of GO/PAA/BPEI complexes were elucidated with GO sheets acting to restrain the viscous flow and improve the mechanical strength at low loading (<12.6 wt%) and PAA more efficiently complexes with BPEI than GO to generate a less swollen and stronger network. This combination overcomes the brittle nature of GO-BPEI complexes and viscous creep of PAA-BPEI complexes. Ternary nanocomposite complexes appear to provide an effective route to toughen and strengthen bulk polyelectrolyte complexes.

Enhanced Impact Resistance of Three-Dimensional-Printed Parts with Structured Filaments

Net-shape manufacture of customizable objects through three-dimensional (3D) printing offers tremendous promise for personalization to improve the fit, performance, and comfort associated with devices and tools used in our daily lives. However, the application of 3D printing in structural objects has been limited by their poor mechanical performance that manifests from the layer-by-layer process by which the part is produced. Here, this interfacial weakness is overcome using a structured, core-shell polymer filament where a polycarbonate (PC) core solidifies quickly to define the shape, whereas an olefin ionomer shell contains functionality (crystallinity and ionic) that strengthen the interface between the printed layers. This structured filament leads to improved dimensional accuracy and impact resistance in comparison to the individual components. The impact resistance from structured filaments containing 45 vol % shell can exceed 800 J/m. The origins of this improved impact resistance are probed using X-ray microcomputed tomography. Energy is dissipated by delamination of the shell from PC near the crack tip, whereas PC remains intact to provide stability to the part after impact. This structured filament provides tremendous improvements in the critical properties for manufacture and represents a major leap forward in the impact properties obtainable for 3D-printed parts.

Quantitative Rheometry of Thin Soft Materials Using the Quartz Crystal Microbalance with Dissipation

In the inertial limit, the resonance frequency of the quartz crystal microbalance (QCM) is related to the coupled mass on the quartz sensor through the Sauerbrey expression that relates the mass to the change in resonance frequency. However, when the thickness of the film is sufficiently large, the relationship becomes more complicated and both the frequency and damping of the crystal resonance must be considered. In this regime, a rheological model of the material must be used to accurately extract the adhered film's thickness, shear modulus, and viscoelastic phase angle from the data. In the present work we examine the suitability of two viscoelastic models, a simple Voigt model ( Physica Scripta 1999, 59, 391-396) and a more realistic power-law model ( Langmuir 2015, 31, 4008-4017), to extract the rheological properties of a thermoresponsive hydrogel film. By changing temperature and initial dry film thickness of the gel, the operation of QCM was traversed from the Sauerbrey limit, where viscous losses do not impact the frequency, through the regime where the QCM response is sensitive to viscoelastic properties. The density-shear modulus and the viscoelastic phase angle from the two models are in good agreement when the shear wavelength ratio, d/λ _n, is in the range of 0.05-0.20, where d is the film thickness and λ _n is the wavelength of the mechanical shear wave at the n^th harmonic. We further provide a framework for estimating the physical properties of soft materials in the megahertz regime by using the physical behavior of polyelectrolyte complexes. This provides the user with an approximate range of allowable film thicknesses for accurate viscoelastic analysis with either model, thus enabling better use of the QCM-D in soft materials research.

Enhanced stability of smoothly electrodeposited amorphous Fe2O3@electrospun carbon nanofibers as self-standing anodes for lithium ion batteries

Fe₂O₃ and carbon nanofiber (Fe₂O₃@CNFs) composite anodes for lithium ion batteries (LIBs) were fabricated by electrospinning and electrodeposition to provide a simple route for modulating the morphology of the anodes and their performance.

Rapid assessment of crystal orientation in semi-crystalline polymer films using rotational zone annealing and impact of orientation on mechanical properties

Crystal orientation in semi-crystalline polymers tends to enhance their performance, such as increased yield strength and modulus, along the orientation direction. Zone annealing (ZA) orients the crystal lamellae through a sharp temperature gradient that effectively directs the crystal growth, but the sweep rate (V_ZA) of this gradient significantly impacts the extent of crystal orientation. Here, we demonstrate rotational zone annealing (RZA) as an efficient method to elucidate the influence of V_ZA on the crystal morphology of thin films in a single experiment using isotactic poly(1-butene), PB-1, as a model semi-crystalline polymer. These RZA results are confirmed using standard, serial linear ZA to tune the structure from an almost unidirectional oriented morphology to weakly oriented spherulites. The overall crystallinity is only modestly changed in comparison to isothermal crystallization (maximum of 55% from ZA vs. 48% for isothermal crystallization). However, the average grain size increases and the spherulites become anisotropic from ZA. Due to these structural changes, the Young's modulus of the oriented films, both parallel and perpendicular to the spherulite orientation direction, is significantly increased by ZA. The modulus does become anisotropic after ZA due to the directionality in the crystal structure, with more than a threefold increase in the modulus parallel to the orientation direction for the highest oriented film in comparison to the modulus from isothermal crystallization. RZA enables rapid identification of conditions to maximize orientation of crystals in thin polymer films, which could find utility in determining conditions to improve crystallinity and performance in organic electronics.

Three-Dimensional Printed Shape Memory Objects Based on an Olefin Ionomer of Zinc-Neutralized Poly(ethylene-co-methacrylic acid)

Three-dimensional printing enables the net shape manufacturing of objects with minimal material waste and low tooling costs, but the functionality is generally limited by available materials, especially for extrusion-based printing, such as fused deposition modeling (FDM). Here, we demonstrate shape memory behavior of 3D printed objects with FDM using a commercially available olefin ionomer, Surlyn 9520, which is zinc-neutralized poly(ethylene-co-methacrylic acid). The initial fixity for 3D printed and compression-molded samples was similar, but the initial recovery was much lower for the 3D printed sample (R = 58%) than that for the compression-molded sample (R = 83%). The poor recovery in the first cycle is attributed to polyethylene crystals formed during programming that act to resist the permanent network recovery. This effect is magnified in the 3D printed part due to the higher strain (lower modulus in the 3D printed part) at a fixed programming stress. The fixity and recovery in subsequent shape memory cycles are greater for the 3D printed part than for the compression-molded part. Moreover, the programmed strain can be systematically modulated by inclusion of porosity in the printed part without adversely impacting the fixity or recovery. These characteristics enable the direct formation of complex shapes of thermoplastic shape memory polymers that can be recovered in three dimensions with the appropriate trigger, such as heat, through the use of FDM as a 3D printing technology.

Transport-Limited Adsorption of Plasma Proteins on Bimodal Amphiphilic Polymer Co-Networks: Real-Time Studies by Spectroscopic Ellipsometry

Traditional hydrogels are commonly limited by poor mechanical properties and low oxygen permeability. Bimodal amphiphilic co-networks (β-APCNs) are a new class of materials that can overcome these limitations by combining hydrophilic and hydrophobic polymer chains within a network of co-continuous morphology. Applications that can benefit from these improved properties include therapeutic contact lenses, enzymatic catalysis supports, and immunoisolation membranes. The continuous hydrophobic phase could potentially increase the adsorption of plasma proteins in blood-contacting medical applications and compromise in vivo material performance, so it is critical to understand the surface characteristics of β-APCNs and adsorption of plasma proteins on β-APCNs. From real-time spectroscopic visible (Vis) ellipsometry measurements, plasma protein adsorption on β-APCNs is shown to be transport-limited. The adsorption of proteins on the β-APCNs is a multistep process with adsorption to the hydrophilic surface initially, followed by diffusion into the material to the internal hydrophilic/hydrophobic interfaces. Increasing the cross-linking of the PDMS phase reduced the protein intake by limiting the transport of large proteins. Moreover, the internalization of the proteins is confirmed by the difference between the surface-adsorbed protein layer determined from XPS and bulk thickness change from Vis ellipsometry, which can differ up to 20-fold. Desorption kinetics depend on the adsorption history with rapid desorption for slow adsorption rates (i.e., slow-diffusing proteins within the network), whereas proteins with fast adsorption kinetics do not readily desorb. This behavior can be directly related to the ability of the protein to spread or reorient, which affects the binding energy required to bind to the internal hydrophobic interfaces.

Operando Grazing Incidence Small-Angle X-ray Scattering/X-ray Diffraction of Model Ordered Mesoporous Lithium-Ion Battery Anodes

Emergent lithium-ion (Li⁺) batteries commonly rely on nanostructuring of the active electrode materials to decrease the Li⁺ ion diffusion path length and to accommodate the strains associated with the insertion and de-insertion of Li⁺, but in many cases these nanostructures evolve during electrochemical charging-discharging. This change in the nanostructure can adversely impact performance, and challenges remain regarding how to control these changes from the perspective of morphological design. In order to address these questions, operando grazing-incidence small-angle X-ray scattering and X-ray diffraction (GISAXS/GIXD) were used to assess the structural evolution of a family of model ordered mesoporous NiCo₂O₄ anode films during battery operation. The pore dimensions were systematically varied and appear to impact the stability of the ordered nanostructure during the cycling. For the anodes with small mesopores (≈9 nm), the ordered nanostructure collapses during the first two charge-discharge cycles, as determined from GISAXS. This collapse is accompanied by irreversible Li-ion insertion within the oxide framework, determined from GIXD and irreversible capacity loss. Conversely, anodes with larger ordered mesopores (17-28 nm) mostly maintained their nanostructure through the first two cycles with reversible Li-ion insertion. During the second cycle, there was a small additional deformation of the mesostructure. This preservation of the ordered structure lead to significant improvement in capacity retention during these first two cycles; however, a gradual loss in the ordered nanostructure from continuing deformation of the ordered structure during additional charge-discharge cycles leads to capacity decay in battery performance. These multiscale operando measurements provide insight into how changes at the atomic scale (lithium insertion and de-insertion) are translated to the nanostructure during battery operation. Moreover, small changes in the nanostructure can build up to significant morphological transformations that adversely impact battery performance through multiple charge-discharge cycles.

A family of mechanically adaptive supramolecular graphene oxide/poly(ethylenimine) hydrogels from aqueous assembly

Composite hydrogels containing graphene oxide (GO) offer advantageous mechanical properties, but tuning these properties generally requires the synthesis of new hydrogels or if the hydrogel is thermally responsive, utilization of a chemistry determined temperature window. Here, we demonstrate a simple route to generate a family of GO-based hydrogels from aqueous solution based assembly of GO with polycationic poly(ethylenimine), PEI, without any secondary chemical crosslinking. Tuning the ratio of GO : PEI during the assembly produces a family of hydrogels that responds to mechanical compression by irreversibly altering their equilibrium water content and mechanical properties in a controllable manner. Despite the lack of chemical crosslinks, the hydrogels are stable when stored in an excess of water or NaCl solutions (up to 1 M) and exhibit a tunable swelling ratio (mass hydrogel : mass solid) between 44 and 162 based on both composition and compression history. Consequently, the storage modulus from shear rheology can be increased by more than 3 orders of magnitude from this irreversible mechanical compression of the hydrogel. This stiffening of the hydrogels in response to mechanical stimuli enables the prior compression loading of the hydrogel to be determined. We demonstrate that this strategy is generalizable to other anionic 2D materials such as clay (cloisite). This family of mechanically adaptive hydrogels enables facile fabrication and tuning of physical properties that could be advantageous for sensing, energy dissipation, and other applications.

Solid state microwave synthesis of highly crystalline ordered mesoporous hausmannite Mn3O4films

Microwave calcination of ordered micelle templated manganese carbonate films leads to highly crystalline, ordered mesoporous manganese oxide, while similar temperatures in a furnace lead to disordered, amorphous manganese oxide.

Accelerated Amidization of Branched Poly(ethylenimine)/Poly(acrylic acid) Multilayer Films by Microwave Heating

Chemical cross-linking of layer-by-layer assembled films promotes mechanical stability and robustness in a wide variety of environments, which can be a challenge for polyelectrolyte multilayers in saline environments or for multilayers made from weak polyelectrolytes in environments with extreme pHs. Heating branched poly(ethylenimine)/poly(acrylic acid) (BPEI/PAA) multilayers at sufficiently high temperatures drives amidization and dehydration to covalently cross-link the film, but this reaction is rather slow, typically requiring heating for hours for appreciable cross-linking to occur. Here, a more than one order of magnitude increase in the amidization kinetics is realized through microwave heating of BPEI/PAA multilayers on indium tin oxide (ITO)/glass substrates. The cross-linking reaction is tracked using infrared spectroscopic ellipsometry to monitor the development of the cross-linking products. For thick films (∼1500 nm), gradients in cross-link density can be readily identified by infrared ellipsometry. Such gradients in cross-link density are driven by the temperature gradient developed by the localized heating of ITO by microwaves. This significant acceleration of reactions using microwaves to generate a well-defined cross-link network as well as being a simple method for developing graded materials should open new applications for these polymer films and coatings.

Tough Stretchable Physically-Cross-linked Electrospun Hydrogel Fiber Mats

Nature uses supramolecular interactions and hierarchical structures to produce water-rich materials with combinations of properties that are challenging to obtain in synthetic systems. Here, we demonstrate hierarchical supramolecular hydrogels from electrospun, self-associated copolymers with unprecedented elongation and toughness for high porosity hydrogels. Hydrophobic association of perfluoronated comonomers provides the physical cross-links for these hydrogels based on copolymers of dimethyl acrylamide and 2-(N-ethylperfluorooctane sulfonamido)ethyl methacrylate (FOSM). Intriguingly, the hydrogel fiber mats show an enhancement in toughness in comparison to compression molded bulk hydrogels. This difference is attributed to the size distribution of the hydrophobic aggregates where narrowing the distribution in the electrospun material enhances the toughness of the hydrogel. These hydrogel fiber mats exhibit extensibility more than double that of the bulk hydrogel and a comparable modulus despite the porosity of the fiber mat leading to >25 wt % increase in water content.

Hierarchical Electrospun and Cooperatively Assembled Nanoporous Ni/NiO/MnOx/Carbon Nanofiber Composites for Lithium Ion Battery Anodes

A facile method to fabricate hierarchically structured fiber composites is described based on the electrospinning of a dope containing nickel and manganese nitrate salts, citric acid, phenolic resin, and an amphiphilic block copolymer. Carbonization of these fiber mats at 800 °C generates metallic Ni-encapsulated NiO/MnOx/carbon composite fibers with average BET surface area (150 m(2)/g) almost 3 times higher than those reported for nonporous metal oxide nanofibers. The average diameter (∼900 nm) of these fiber composites is nearly invariant of chemical composition and can be easily tuned by the dope concentration and electrospinning conditions. The metallic Ni nanoparticle encapsulation of NiO/MnOx/C fibers leads to enhanced electrical conductivity of the fibers, while the block copolymers template an internal nanoporous morphology and the carbon in these composite fibers helps to accommodate volumetric changes during charging. These attributes can lead to lithium ion battery anodes with decent rate performance and long-term cycle stability, but performance strongly depends on the composition of the composite fibers. The composite fibers produced from a dope where the metal nitrate is 66% Ni generates the anode that exhibits the highest reversible specific capacity at high rate for any composition, even when including the mass of the nonactive carbon and Ni(0) in the calculation of the capacity. On the basis of the active oxides alone, near-theoretical capacity and excellent cycling stability are achieved for this composition. These cooperatively assembled hierarchical composites provide a platform for fundamentally assessing compositional dependencies for electrochemical performance. Moreover, this electrospinning strategy is readily scalable for the fabrication of a wide variety of nanoporous transition metal oxide fibers.

Salt-responsive polyzwitterionic materials for surface regeneration between switchable fouling and antifouling properties

Development of smart regenerative surface is a highly challenging but important task for many scientific and industrial applications. Specifically, very limited research efforts were made for surface regeneration between bio-adhesion and antifouling properties, because bioadhesion and antifouling are the two highly desirable but completely opposite properties of materials. Herein, we developed salt-responsive polymer brushes of poly(3-(1-(4-vinylbenzyl)-1H-imidazol-3-ium-3-yl) propane-1-sulfonate) (polyVBIPS), which can be switched reversibly and repeatedly between protein capture/release and surface wettability in a controllable manner. PolyVBIPS brush has demonstrated its switching ability to resist both protein adsorption from 100% blood plasma/serum and bacterial attachment in multiple cycles. PolyVBIPS brush also exhibits reversible surface wettability from ∼40° to 25° between in PBS and in 1M NaCl solutions in multiple cycles. Overall, the salt-responsive behaviors of polyVBIPS brushes can be interpreted by the "anti-polyelectrolyte effect", i.e. polyVBIPS brushes adopt a collapsed chain conformation at low ionic strengths to achieve surface adhesive, but an extended chain conformation at high ionic strength to realize antifouling properties. We expect that polyVBIPS will provide a simple, robust, and promising system for the fabrication of smart surfaces with biocompatible, reliable, and regenerative properties.

STATEMENT OF SIGNIFICANCE

Unlike many materials with "one-time switching" capability for surface regeneration, we developed a new regenerative surface of zwitterionic polymer brush, which exhibits a reversible salt-induced switching property between a biomolecule-adhesive state and a biomolecule repellent state in complex media for multiple cycles. PolyVBIPS is easily synthesized and can be straightforward coated on the surface, which provides a simple, robust, and promising system for the fabrication of smart surfaces with biocompatible, reliable, regenerative properties.