Phase-separated Structure Research Articles

The Interplay between Phase Separation and Gelation Controlling the Morphologies of the Reactive Covalent Network/Polymer Blends

The solution processability of a gelating polyurea network has enabled its blending with polymers, allowing facile preparation of bicontinuous nanoporous materials. To exploit the method for synthesizing various functional porous structures, a deeper understanding of the structure-forming mechanism and the relationship between the experimental parameters and the morphology is crucial. Herein, we experimentally show that the thermodynamic parameters of the mixture of the covalent network and polymer govern the formation of initial blend morphology, and control over the covalent network’s gelation kinetics under solvent evaporation is critical to obtain the nanoscale structures otherwise difficult to achieve. We first investigated the phase behaviors of various covalent network/polymer mixtures in solutions and the morphologies of their resultant blends and porous structures. The different morphologies were mapped in a single diagram based on the polymer composition and the empirical descriptor devised to include the components’ mutual interactions and relative sizes. To capture the phase-separated structures at the nanoscale, we found that the two coexisting phenomena should be controlled: the gelation via interconnection of the covalent network nanogel particles and the growth of phase-separated domains with solvent evaporation. We proposed a general mechanism involving the interplay of gelation and phase separation with an evaporative cue using a ternary phase diagram of the polymer/covalent network/solvent. This work lays the groundwork for designing and optimizing functional porous materials derived from gelating covalent network/polymer mixtures, in addition to establishing an empirical approach in a reactive colloidal polymer/polymer mixture system.

Nanoindentation of polyurethane with phase-separated fibrillar structure

Polyurethane elastomer is investigated by atomic force microscopy (AFM) in a regime of fast nanoindentation (simultaneous getting relief and mechanical properties of the surface). The hard phase of the polymer has a fibrillar structure and inhomogeneously distributed in the softer matrix. The surface is covered by a nanolayer of the soft phase and the obtained relief depends on the depth of indentation: it is necessary to pierce this outer layer to reach the internal fibrillar structure of the polymer. The rate of indentation is not constant and affected by the spring properties of AFM-probe, local phase composition and the depth of indentation. Analysis of the kinetics of the probe-surface interactions allowed to determine the thickness of the outer layer and to establish relationships between the indentation rate and elastic and dissipative properties of the soft and hard regions of the polyurethane surface. • Relief depends on a depth of indentation. • Layer of the soft phase covers the bulk structure of the polyurethane. • The rate of indentation depend on local surface properties. • Measured elastic and inelastic properties depend on the rate of indentation.

Deep learning model for predicting phase diagrams of block copolymers

Block copolymers show various microphase-separated structures depending on their chain architecture and the interaction parameters between the different chemical structures of monomeric unit χ. Self-consistent field theory is a powerful tool to predict such phase-separated structures, and phase diagrams of block copolymers have been reported using self-consistent field theory. However, obtaining stable morphology of each polymer structure and the χ parameter requires intensive computational study. We applied deep learning to predict phase diagrams from metastable structures obtained by crude, cost-effective self-consistent field calculation. We used a 3D convolutional neural network for classification of the metastable structures, and a limited number of sets of block copolymer structures and χ parameters with stable phase labels were used for training. After the model was trained using the training set, it successfully assigned the metastable structures of a wide variety of diblock and triblock copolymers to the correct stable phases. This approach is capable of predicting the phase diagrams of block copolymers effectively without intensive self-consistent field calculation.

Effect of interfacial structure based on grafting density of poly(2-methoxyethyl acrylate) on blood compatibility

An excellent blood-compatible polymer, poly(2-methoxyethyl acrylate) (PMEA), exhibits nanometer-scale phase-separated structures at the interface with water or phosphate-buffered saline (PBS), and fibrinogen adsorption is suppressed, especially on the water-rich region. To understand the correlation between the interfacial structure based on the grafting density of PMEA and blood compatibility, grafted PMEA (gPMEA) surfaces with controlled density were prepared by immobilizing thiol-terminated PMEA on a gold substrate. The amount of adsorbed fibrinogen and the number of adhered platelets on gPMEAs decreased first with the increasing grafting density (σ), but increased after showed minimum at σ of approximately 0.11 chains/nm2. The interfacial structures of the gPMEA/PBS interface changed with grafting density, and the maximum area of water-rich region was obtained at σ = 0.11. The water contact angle at σ = 0.11 is smaller than that at the other grafting density. These results revealed that hydration to the polymer is very effective to suppress the platelet adhesion and water-rich region shows excellent blood compatibility on gPMEA surfaces. This work clearly indicated that the density of PMEA affects the interfacial structure and plays an important role in the blood compatibility of the material.

Preparation and study of spirocyclic cationic side chain functionalized polybiphenyl piperidine anion exchange membrane

Research on the ion conductivity and mechanical stability of anion exchange membranes (AEMs) has achieved great progress, it is more urgent to prepare AEMs with high alkali stability. Azaspirocyclic cations are among the most alkali-stable cations. In this study, a synthesized long-chain 3-(3-(1-(8-bromooctyl) piperidin-4-yl) propyl)-6-azaspiro[5.5] undecan-6-ium bromide(BOP-ASU) cation was introduced into a portion of a piperidine ring on a PBP backbone to prepare PBP-BOP-ASU, and AEMs based on PBP-ASU and PBP-BOP-ASU were prepared. The structure of each product was characterized (1H NMR, MS), and the prepared anion exchange membrane was also characterized using micromorphology (SEM, TEM, AFM) and performance tests (TGA, WU, SR, ion conductivity, alkali stability). The PBP-BOP-ASU (8% membrane) showed the highest ion conductivity (117.43 mS/cm) at 80 °C. In addition, it showed excellent alkali stability in a test environment of 2 M NaOH solution at 80 °C for 1400 h. Moreover, the introduction of side chain spiro cations could improve the microscopic phase separation structure of the AEMs, and it also increased their ionic conductivity, thus ensuring the potential for their application in anion exchange membrane fuel cells.

(Invited) Engineering Polypeptides for Controlled Assembly of Ionomer Thin Films

Increasing evidence suggests that control of ionomer thin film structures on metal surfaces is pivotal for efficient performance in electrochemical devices such as fuel cells and electrolyzers. Unfortunately, the assembly of these thin film structures is difficult to control using conventional methods and ideal arrangements remain unknown. Engineered polypeptides have emerged as powerful biomolecular tools in electrode assembly because binding sites and polypeptide structures can be easily modulated by changing the amino acid sequence. However, no studies have been conducted showing polypeptides can be engineered to interact with ionomers, attach them to metal surfaces, and control their arrangement. Our lab has recently developed this technology, using an elastin-like polypeptide to bind to metals, and bind to acidic and basic ionomer via ionic interactions. We use a quartz crystal microbalance with dissipation to provide detailed information about the loading, thickness, and binding behavior of the polypeptide and ionomer layers. We also use atomic force microscopy and grazing-incidence small-angle X-ray scattering to understand the impact of the biomolecules on ionomer phase separation. Finally, we have begun to analyze the performance (ionic conductivity) of the assembled films using interdigitated electrodes and electrochemical impedance spectroscopy. Through these techniques, we show that 1) our biomolecular system is highly flexible, easily adapting to different materials, 2) the polypeptide sequence can dictate ionomer phase separated structures, and 3) this system can be used to improve the performance of ionomer thin films and gain structure-function understanding. Generally, our results demonstrate that engineered polypeptides are promising tools for ionomer control in electrode engineering.

Alloy Design, Thermodynamics, and Electron Microscopy of Ternary Ti-Ag-Nb Alloy with Liquid Phase Separation.

The Ti–Ag alloy system is an important constituent of dental casting materials and metallic biomaterials with antibacterial functions. The binary Ti–Ag alloy system is characterized by flat liquidus lines with metastable liquid miscibility gaps in the phase diagram. The ternary Ti–Ag-based alloys with liquid phase separation (LPS) were designed based on the mixing enthalpy parameters, thermodynamic calculations using FactSage and Scientific Group Thermodata Europe (SGTE) database, and the predicted ground state diagrams constructed by the Materials Project. The LPS behavior in the ternary Ti–Ag–Nb alloy was investigated using the solidification microstructure analysis in arc-melted ingots and rapidly solidified melt-spun ribbons via trans-scale observations, combined with optical microscopy (OM), scanning electron microscopy (SEM) including electron probe micro analysis (EPMA), transmission electron microscopy (TEM), and scanning transmission electron microscopy (STEM). The solidification microstructures depended on the solidification processing in ternary Ti–Ag–Nb alloys; macroscopic phase-separated structures were observed in the arc-melted ingots, whereas fine Ag globules embedded in the Ti-based matrix were observed in the melt-spun ribbons.

Interaction, structure and tensile property of swollen Nafion® membranes

Commercial Nafion® membranes are fabricated from an ionomer with strongly hydrophobic polytetrafluoroethylene (PTFE) backbone and strongly hydrophilic pendant sulfonic group. Understanding the difference in the phase separated structures for them in dry and solvent-swollen states is crucial for their optimal application and recycling. Structure changes in solvent swollen membranes were studied by Small angle X-ray scattering (SAXS), their thermodynamic origins from polymer-solvent interactions, solvent uptake and tensile properties were explored. Dry membranes have tightly packed backbone and crystallite, acting as the matrix that afford strong mechanic strength in tensile tests. Solvents that can swell both hydrophobic matrix and ionic clusters lead to high level of solvent uptake, evidenced from larger correlation lengths for both polymer matrix and ionic clusters, and the swollen membranes become soft and flexible. Semi-quantitative relationships to correlate polymer-solvent interactions, the length and distribution of structural domains, solvent uptake and tensile properties were presented. Dimethylacetamide as the primary solvent and 40% isopropanol-water as the binary solvent, both have intermediate difference in Hansen solubility with the PTFE backbone, can permeate into hydrophobic matrix and ionic clusters, resulting in the most swollen Nafion membranes. Based on correlation matrix analysis, we also found that solvents with larger molecule volume will benefit the elongation of swollen membranes.

Morphology of Isotactic Polypropylene–Polyethylene Block Copolymers Driven by Controlled Crystallization

A study of the morphology of diblock copolymers composed of two crystalline blocks of isotactic polypropylene (iPP) and polyethylene (PE) is shown. The samples form phase-separated structures in th...

Mixed substituent ether‐containing polyphosphazene/poly(bis‐phenoxyphosphazene) blends as membranes for CO2 separation from N2

AbstractPolyphosphazenes gain much of their physical and chemical properties from the substituents attached to the phosphorus atoms in the backbone. Poly(phosphazenes) that have short‐chain polyether containing substituents, such as, 2‐(2‐methoxyethoxy)ethanol, have high CO2 permeability and selectivity over N2 as well as resistance to degradation under humidification at 60°C. However, the principal shortcomings of this polymer are the poor mechanical and surface characteristics. Blending of an example of this polyphosphazene (MEEP‐80) with poly(bis‐phenoxyphosphazene) (PPOP) has yielded more durable materials that have a non‐adhesive surface. In this work, the blended polymer membranes were found to have increased CO2 permeability with higher selectivity over N2. Thermal analysis and the application of transport models support a structure of the blends that is not either an intimate blend or phase separated bulk structure, but one where PPOP domains retain their crystallinity and are dispersed within an amorphous MEEP‐80 phase.

Visualization of the electronic phase separation in superconducting KxFe2\u2212ySe2

Type-II iron-based superconductors (Fe-SCs), the alkali-metal-intercalated iron selenide AxFe2−ySe2 (A = K, Tl, Rb, etc.) with a superconducting transition temperature of 32 K, exhibit unique properties such as high Néel temperature, Fe-vacancies ordering, antiferromagnetically ordered insulating state in the phase diagram, and mesoscopic phase separation in the superconducting materials. In particular, the electronic and structural phase separation in these systems has attracted intensive attention since it provides a platform to unveil the insulating parent phase of type-II Fe-SCs that mimics the Mott parent phase in cuprates. In this work, we use spatial- and angle-resolved photoemission spectroscopy to study the electronic structure of superconducting KxFe2−ySe2. We observe clear electronic phase separation of KxFe2−ySe2 into metallic islands and insulating matrix, showing different K and Fe concentrations. While the metallic islands show strongly dispersive bands near the Fermi level, the insulating phase shows an energy gap up to 700 meV and a nearly flat band around 700 meV below the Fermi energy, consistent with previous experimental and theoretical results on the superconducting K1−xFe2Se2 (122 phase) and Fe-vacancy ordered K0.8Fe1.6Se2 (245 phase), respectively. Our results not only provide important insights into the mysterious composition of phase-separated superconducting and insulating phases of KxFe2−ySe2, but also present their intrinsic electronic structures, which will shed light on the comprehension of the unique physics in type-II Fe-SCs.

A unique route of colloidal phase separation yields stress-free gels.

Phase separation often leads to gelation in soft and biomatter. For colloidal suspensions, we have a consensus that gels form by the dynamical arrest of phase separation. In this gelation, percolation of the phase-separated structure occurs before the dynamical arrest, leading to the generation of mechanical stress in the gel network. Here, we find a previously unrecognized type of gelation in dilute colloidal suspensions, in which percolation occurs after the local dynamical arrest, i.e., the formation of mechanically stable, rigid clusters. Thus, topological percolation generates little mechanical stress, and the resulting gel is almost stress-free when formed. We also show that the selection of these two types of gelation (stressed and stress-free) is determined solely by the volume fraction as long as the interaction is short-ranged. This universal classification of gelation of particulate systems may have a substantial impact on material and biological science.

Cluster-Like Structure of Fe-Based Alloys with Enhanced Magnetostriction

The microstructure of several Fe–xGa alloys with phase-separated structure have been studied by neutron diffraction with high Δ d/d resolution. Analysis of diffraction data shows that the microstructure of these alloys is organized as nano-sized clusters with a better-ordered atomic structure coherently embedded in a disordered or less-ordered matrix. The characteristic size of the clusters depends on the Ga content and ranges from 100–2000 A.

In situ observation of the dynamics in the middle stage of spinodal decomposition of a silicate glass via scanning transmission electron microscopy

Real-space observations in the middle stage of spinodal decomposition have not been achieved because of the small sizes and unclear interfaces of the phases. Here, we performed in situ real-space experiments with heating to quantitatively reveal the local dynamics and structures of a calcium aluminosilicate glass in the middle stage of spinodal decomposition by scanning transmission electron microscopy. This glass separate into CaO–Al2O3–SiO2 phase and SiO2 phases. The observations revealed that the CaO–Al2O3–SiO2 phases with low Ca concentrations behaved as if they were in the middle stage, whereas the CaO–Al2O3–SiO2 phases with high Ca concentrations coarsened as if they were in the final stage. This coexistence of stages suggested that the behaviors of the phases depend not only on time, but also on their local relative concentrations. Differing from the well-known final stage of spinodal decomposition, the phases with relatively high Ca concentrations in the middle stage showed peculiar directional migration guided by the phases with relatively low Ca concentrations. These findings have not been observed before and thus have the potential to provide a new way to control the phase-separated structure.

Polar Metal Phase Induced by Oxygen Octahedral Network Relaxation in Oxide Thin Films

ABO3 perovskite materials and their derivatives have inherent structural flexibility due to the corner sharing network of the BO6 octahedron, and the large variety of possible structural distortions and strong coupling between lattice and charge/spin degrees of freedom have led to the emergence of intriguing properties, such as high-temperature superconductivity, colossal magnetoresistance, and improper ferroelectricity. Here, an unprecedented polar ferromagnetic metal phase in SrRuO3 (SRO) thin films is presented, arising from the strain-controlled oxygen octahedral rotation (OOR) pattern. For compressively strained SRO films grown on SrTiO3 substrate, oxygen octahedral network relaxation is accompanied by structural phase separation into strained tetragonal and bulk-like orthorhombic phases, and the asymmetric OOR evolution across the phase boundary allows formation of the polar phase, while bulk metallic and ferromagnetic properties are maintained. From the results, it is expected that other oxide perovskite thin films will also yield similar structural environments with variation of OOR patterns, and thereby provide promising opportunities for atomic scale control of material properties through strain engineering.

Synthesis of Silicon Hybrid Phenolic Resins with High Si-Content and Nanoscale Phase Separation Structure

In this paper, a set of silicon hybrid phenolic resins (SPF) with high Si-content were prepared by mixing phenolic resins with self-synthesized silicon resins. In order to obtain the nanoscale phase structure, condensation degree and the amount of Si-OH groups in silicon resins were controlled by the amount of inhibitor ethanol in the hydrolytic condensation polymerization of siloxane. Increasing the amount of ethanol resulted in more silanol groups and a lower degree of condensation for silicon resins, which then led to more formation of Si-O-Ph bonds in hybrid resin and improved compatibility between silicon resin and phenolic resin. When 400% ethanol by weight of siloxane was used in the sample SPF-4, nanoscale phase separation resulted. The residual weight of the cured SPF-4 at 1000 °C (R1000) significantly increased compared to pure phenolic resins. The result of the oxyacetylene flame ablation and the Cone Calorimeter test confirmed the improved ablative property and flammability after the modification. The performance improvement of the cured SPF-4 was attributed to the nanoscale phase structure and high silicon content, which promoted the formation of dense silica protective layers during pyrolysis.

The prion-like domain of Fused in Sarcoma is phosphorylated by multiple kinases affecting liquid- and solid-phase transitions.

Fused in Sarcoma (FUS) is a ubiquitously expressed protein that can phase-separate from nucleoplasm and cytoplasm into distinct liquid-droplet structures. It is predominantly nuclear and most of its functions are related to RNA and DNA metabolism. Excessive persistence of FUS within cytoplasmic phase-separated assemblies is implicated in the diseases amyotrophic lateral sclerosis and frontotemporal dementia. Phosphorylation of FUS’s prion-like domain (PrLD) by nuclear phosphatidylinositol 3-kinase-related kinase (PIKK)-family kinases following DNA damage was previously shown to alter FUS’s liquid-phase and solid-phase transitions in cell models and in vitro. However, proteomic data suggest that FUS’s PrLD is phosphorylated at numerous additional sites, and it is unknown if other non-PIKK and nonnuclear kinases might be influencing FUS’s phase transitions. Here we evaluate disease mutations and stress conditions that increase FUS accumulation into cytoplasmic phase-separated structures. We observed that cytoplasmic liquid-phase structures contain FUS phosphorylated at novel sites, which occurred independent of PIKK-family kinases. We engineered phosphomimetic substitutions within FUS’s PrLD and observed that mimicking a few phosphorylation sites strongly inhibited FUS solid-phase aggregation, while minimally altering liquid-phase condensation. These effects occurred independent of the exact location of the phosphomimetic substitutions, suggesting that modulation of PrLD phosphorylation may offer therapeutic strategies that are specific for solid-phase aggregation observed in disease.

Efficient Exciton Diffusion in Micrometer-Sized Domains of Nanographene-Based Nonfullerene Acceptors with Long Exciton Lifetimes in Blend Films with Conjugated Polymer.

Phase-separated structures in photoactive layers composed of electron donors and acceptors in organic photovoltaics (OPVs) generally exert a profound impact on the device performance. In this study, nonfullerene acceptors (NFAs) where a heteronanographene central core was furnished with branched alkoxy chains of different lengths, TACIC-EH, TACIC-BO, and TACIC-HD, were prepared to adjust the aggregation tendency and systematically probe the relationships of film structures with photophysical and photovoltaic properties. The side-chain length showed negligible effects on the absorption properties and energy levels of TACICs. In addition, regardless of the chain length, all TACIC films exhibited characteristically long singlet exciton lifetimes (1330-2330 ps) compared to those in solution (≤220 ps). Using a conjugated polymer donor, PBDB-T, the best OPV performance was achieved with TACIC-BO that contained medium-length chains, exhibiting a power conversion efficiency (PCE) of 9.92%. TACIC-HD with the longest chains showed deteriorated electron mobility due to the long insulating alkoxy groups. Therefore, the PBDB-T:TACIC-HD-based device revealed a low charge collection efficiency and PCE (8.21%) relative to the PBDB-T:TACIC-BO-based device, but their film morphologies were analogous. Meanwhile, TACIC-EH with the shortest chains showed low solubility and formed micrometer-sized large aggregates in the blend film with PBDB-T. Although the charge collection efficiency of PBDB-T:TACIC-EH was lower than that of PBDB-T:TACIC-BO, the efficiencies of exciton diffusion to the donor-acceptor interface were sufficiently high (>98%) owing to the elongated singlet exciton lifetime of TACIC-EH. The PCE of the PBDB-T:TACIC-EH-based device remained moderate (7.10%). Therefore, TACICs with the long singlet exciton lifetimes in the films provide a clear guideline for NFAs with low sensitivity of OPV device performance to the blend film structures, which is advantageous for large-scale OPV production with high reproducibility.

Tuning the effects of N1 substituents on the 2-methylimidazolium functionalized polynorbornene alkaline anion exchange membranes

The N1-substituted (methyl, isopropyl, butyl, benzyl or dodecyl) 2-methylimidazolium functionalized polynorbornene (rPNB-RMIm-30) AEMs are prepared and the effects of substituents are studied. The N1-butyl or isopropyl-substituted 2-methylimidzolium (rPNB-BuMIm-30 or rPNB-iPrMIm-30) anion exchange membrane (AEM) reveals much better alkaline durability than that of methyl, benzyl and dodecyl-substituted 2-methylimidazolium-based AEMs, reflected by the almost unaltered FT-IR spectra and TGA curves after being subjected to 1 M NaOH for 720 h at 60 °C, with a conductivity degradation of 17% and 14% respectively. Furthermore, a highest hydroxide conductivity (51.5 mS cm−1, 80 °C) is achieved of the rPNB-BuMIm-30 AEM, attributing to its well-defined phase-separated structure, as evidenced by results of atomic force microscope (AFM) and X-ray scattering (SAXS). Membrane electrode assembly (MEA) with rPNB-BuMIm-30 AEM exhibits a peak power density of 205 mWcm−2 at 60 °C. This study paves the way of N1-Substituented 2-methylimidazolium-based AEM for fuel cells practical application.

Facile synthesis of poly (arylene ether ketone)s containing flexible sulfoalkyl groups with enhanced oxidative stability for DMFCs

After tethering sodium 2-mercaptoethanesulfonate (MTS) to the bromomethylated poly(arylene ether ketone) precursor, a novel clustered sulfonated poly(arylene ether ketone) containing flexible sulfoalkyl groups (MTSPAEK) was prepared and used as polymer electrolyte membrane for application in DMFCs. The chemical structure and the degree of grafting of MTSPAEK copolymers were identified by 1H NMR spectra. The resulted MTSPAEK copolymers exhibited excellent thermal stability (Td5% > 259 °C) and good mechanical properties (tensile strength at break > 52 MPa). Compared to conventional sulfonated aromatic hydrocarbon polymers, MTSPAEK membranes displayed enhanced oxidative stability in Fenton's reagent owing to the elimination of free radicals by the sulfide groups located on the polymer side chains. Especially, MTSPAEK-2.10 with the highest content of flexible sulfoalkyl groups exhibited a highest proton conductivity of 0.181 S cm−1 at 80 °C. It could be attributed to the obvious hydrophilic/hydrophobic phase-separated structure within the membrane, which was confirmed by AFM images. Moreover, MTSPAEK-2.10 membrane performed a peak power density of 70 mW cm−2 in DMFC when feeding with 2 M methanol at 80 °C, which was comparable to the performance of recast Nafion as reported. Therefore, the combination of good thermal stability and mechanical properties, good oxidative stability, and good methanol barrier performance of MTSPAEK membranes indicated that they have potential to be alternative materials for PEMs in DMFCs.