Co-continuous Networks Research Articles

Modulating Thermal Conductivity and Flame Retardancy of Polyolefin Composites via Distributed Structures of Magnesium Hydroxide and Hexagonal Boron Nitride.

Thermally conductive and flame-retardant polyolefin composites are facing great challenges in meeting the increasing demands for fire safety and thermal management. Aiming at simultaneously enhancing thermal conductivity and flame retardancy, hexagonal boron nitride (hBN) and magnesium hydroxide (MH) were adopted in ethylene-vinyl acetate copolymer/polyolefin elastomer (EVA/POE) blends to design composites with selective filler distributions and co-continuous networks via different processing schemes. The thermal conductivity and flame retardancy show strong dependence on the distributed structure of hBN and MH. The composites with hBN-rich centers and MH-rich edges in the filled POE phase show a thermal conductivity of 0.70 W/(m·K) and an LOI of 27.7%, which are very close to the thermal conductivity of EVA/POE/hBN and the LOI of EVA/POE/MH at the same total filler content. The composites with MH-rich centers and hBN-rich edges show pHRR, THR and TSP values of 169 kW/m2, 49.8 MJ/m2 and 1.8 m2, which are decreased by 40%, 33% and 62% in comparison with EVA/POE/MH, respectively. Modulating the filler structure distribution provides a strategy to co-enhance thermal conductivity and flame retardancy.

Reaction-induced phase separation towards in situ network in epoxy resins for simultaneously improving thermal conductivity and fire safety

Simultaneously improving the thermal conductivity and fire safety of the polymeric thermal management materials (PTMMs) still remains a challenge. In this work, through reaction-induced phase separation (RIPS), co-continuous networks were in situ formed between polyetherimide (PEI) and epoxy resin (EP) cured with either commercially available 4,4′-diaminodiphenyl sulfone (DDS) or the synthesized bis(4-aminophenyl) phenylphosphonate (BAPP). Boron nitride nanosheets (BNNSs) were selectively localized in the interfacial region between the EP-rich and PEI-rich phases as the thermally conductive fillers; therefore the thermally conductive networks were constructed. BNNSs and BAPP simultaneously endowed the composites with enhanced thermal conductivity and expected fire safety. In detail, the EPBAPP/PEI/BNNS composite with barely 1 wt% BNNSs showed a thermal conductivity increment over 67%; and both the heat and smoke release were greatly suppressed in cone calorimetry: the peak heat release rate (PHRR) reduced by 58%, total heat release (THR) by 56% and total smoke production (TSP) by 46%, compared with the reference. The successful application of the RIPS method in constructing in situ thermally conductive networks provided promising perspectives for designing versatile PTMMs.

Advanced adjustable sensor for multi-signal analysis via construction of co-continuous dual-sensing networks

Multifunctional sensors (MFS) have broad prospects in fields such as healthcare and environmental monitoring, which should integrate multi-signal detection. But the signal crosstalk problem limits the functional expansion of sensors, which is an urgent problem that few people have completely solved. We firstly propose to combine two matrices with different sensing function in a co-continuous network as a composite of ‘‘water channel’’ and elastic skeleton, to achieve the non-interfering integration of different sensing capabilities in multifunctional sensor. Then, through the study of multi-response law, a mathematical model is innovatively proposed to analyze the synthetic response signals and solve for monitoring signals. From the results, the MFS can maintain excellent mechanical properties while producing superior responses to the triple factors of temperature, humidity and mechanical stimuli. The mathematical model can accurately describe the synergistic effects of temperature and humidity, and the theoretical results are in good agreement with the actual results, which demonstrates the effectiveness of this strategy for discriminating multi-signal responses. Thus, the simple and effective fabricating concept and method for sensors are proposed with the construction of “water channel”, which can effectively distinguish temperature and humidity responses through two-phase co-continuous networks with different independent function and a mathematical model.

Template-Enabled Biofabrication of Thick 3D Tissues with Patterned Perfusable Macrochannels.

Interconnected pathways in 3D bioartificial organs are essential to retaining cell activity in thick functional 3D tissues. 3D bioprinting methods have been widely explored in biofabrication of functionally patterned tissues; however, these methods are costly and confined to thin tissue layers due to poor control of low-viscosity bioinks. Here, cell-laden hydrogels that could be precisely patterned via water-soluble gelatin templates are constructed by economical extrusion 3D printed plastic templates. Tortuous co-continuous plastic networks, designed based on triply periodic minimal surfaces (TPMS), serve as a sacrificial pattern to shape the secondary sacrificial gelatin templates. These templates are eventually used to form cell-encapsulated gelatin methacryloyl (GelMA) hydrogel scaffolds patterned with the complex interconnected pathways. The proposed fabrication process is compatible with photo-crosslinkable hydrogels wherein prepolymer casting enables incorporation of high cell populations with high viability. The cell-laden hydrogel constructs are characterized by robust mechanical behavior. In vivo studies demonstrate a superior cell ingrowth into the highly permeable constructs compared to the bulk hydrogels. Perfusable complex interconnected networks within cell-encapsulated hydrogels may assist in engineering thick and functional tissue constructs through the permeable internal channels for efficient cellular activities in vivo.

Power of Aerogel Platforms to Explore Mesoscale Transport in Catalysis.

We describe the opportunity to deploy aerogels-an ultraporous nanoarchitecture with co-continuous networks of meso/macropores and covalently bonded nanoparticulates-as a platform to address the nature of the electronic, ionic, and mass transport that underlies catalytic activity. As a test case, we fabricated Au||TiO2 junctions in composite guest-host aerogels in which ∼5 nm Au nanoparticles are incorporated either directly into the anatase TiO2 network (Au "in" TiO2, AuIN-TiO2 aerogel) or deposited onto preformed TiO2 aerogel (Au "on" TiO2, AuON/TiO2 aerogel). The metal-meets-oxide nanoscale interphase as visualized by electron tomography feature extended three-dimensional (3D) interfaces, but AuIN-TiO2 aerogels impose a greater degree of Au contact with TiO2 particles than does the AuON/TiO2 form. Both aerogel variants enable transport of electrons over micrometer-scale distances across the TiO2 network to Au||TiO2 junctions, as evidenced by electron paramagnetic resonance (EPR) and ultrafast visible pump-IR probe time-resolved absorption spectroscopy. The siting of gold nanoparticles in the TiO2 network more effectively disperses trapped electrons. Density functional theory (DFT) calculations find that increased physical contact between Au and TiO2, induced by oxygen vacancies, produces increased hybridization of midgap states and quenches unpaired trapped electrons. We assign the apparent differences in electron-transport capabilities to a combination of the relatively better-wired Au||TiO2 junctions in AuIN-TiO2 aerogels, which have a greater capacity to dilute accumulated charge over a larger interfacial surface area, with an enhanced ability to discharge the accumulated electrons via catalytic reduction of adsorbed O2 to O2- at the interface. Solid-state 1H nuclear magnetic resonance experiments show that proton spin-lattice relaxation times and possibly proton diffusion are strongly coupled to Au||TiO2 interfacial design, likely through spin coupling of protons to unpaired electrons trapped at the TiO2 network. Taken together, our results show that Au||TiO2 interfacial design strongly impacts charge carrier (electron and proton) transport over mesoscale distances in catalytic aerogel architectures.

Rechargeable Zn–Air Batteries Enabled By Zn Sponge Anodes and Bi(tri?)Functional Cathodes

Zinc–air batteries have the potential to overcome many of the disadvantages of current Li-ion batteries: safety concerns associated with toxic and flammable electrolytes, high costs, and specific energy <200 Wh kg–1. Zinc–air provides higher specific energy (up to 450 Wh kg–1), while using safe, low-cost components and aqueous electrolytes. Although commercially successful as a primary battery, broader implementation of Zn–air batteries is hindered by limited rechargeability and modest zinc utilization (typically <60% of theoretical discharge capacity). This long-standing performance roadblock is attributed to the manner in which Zn electrodes are commonly fabricated—by mixing Zn powder with binders and gelling agents to form composite electrodes. During discharge of these ad hoc structures, Zn is oxidized and forms soluble zincate, which diffuses from its point of electrogeneration until it supersaturates and then dehydrates to high-impedance products (e.g., ZnO). Upon electrochemical recharge, ZnO, in equilibrium with soluble zincate, is reduced back to Zn0, often accompanied by a shape change. Discharge–charge cycling leads to pronounced morphological changes, especially at points of high local current density, where short-inducing dendrites can form. We developed a Zn “sponge” form factor comprising interpenetrating, co-continuous networks of solid and void which imposes more uniform current distribution throughout the volume of the electrode structure, minimizes uneven reaction loci, and suppresses formation of dendrites. We have demonstrated that the Zn sponge electrode can be cycled in prototype Ag–Zn and Ni–Zn cells without the formation of dendrites, but translating that to the Zn–air case requires the use of air-cathodes that also support electrochemical discharge and recharge. For that more ambitious cell chemistry, we turn to powder-composite air cathodes that include mesoporous oxides of manganese and/or a solid-solution or mixed-phase oxides of transition metals such as nickel and/or iron prepared via sol–gel synthetic methods. Zinc–air cells were constructed and cycled at high depths-of-discharge relative to the Zn, an achievement that not only validates the ability for the air cathode to be bifunctionally catalyzed, but to operate at capacities and rates that turn all of the Zn in the anode over enough times to generate a technologically relevant specific energy.

Molecular dynamics simulation of thermotropic bolaamphiphiles with a swallow-tail lateral chain: formation of cubic network phases.

T-shaped bolaamphiphiles (TBA) with a swallow-tail lateral chain have been found to provide a fertile platform to produce complex liquid crystalline phases that are accessible through changes of temperature and lateral chain length and design. In this work, we use molecular simulations of a simple coarse-grained model to map out the phase behavior of this type of molecules. This model is based on the premise that the crucial details of the fluid structure stem from close range repulsions and the strong directional forces typical of hydrogen bonds. Our simulations confirm that TBAs exhibit a rich phase behavior upon increasing the length of their lateral chain. The simulations detect a double gyroid phase and an axial-bundle columnar phase which bear some structural resemblance to those found in the experiment. In addition, simulations predict two cocontinuous phases with 3D-periodicity: the "single" diamond and the "single" plumber's nightmare phase. Our analysis of energetic and entropic contributions to the free energy of phases formed by TBA with either swallow-tail or linear side-chains suggest that the 3D-periodic network phases formed by the former are stabilized by the large conformation entropy of the side-chains.

Morphology and Mechanical Properties of Star Block Copolymer Modified Epoxy Resin Blends

In this work, we investigate the morphological and mechanical properties of blends of diglycidyl ether of bisphenol-A (DGEBA) and epoxidized polystyrene–polybutadiene star block copolymer (epST3) using diamino-diphenyl sulphone (DDS) as a hardener. The epoxidation of the star block copolymer was performed using meta-chloroperoxybenzoic acid (m-CPBA). Chemical modification of the copolymer was characterized by Fourier transform infrared (FTIR) spectroscopy while the morphology of the blends was analyzed by microscopic techniques. The mechanical behavior of the blends was analyzed by recording microindentation as well as impact tests. It has been demonstrated that the completely macrophase-separated morphology of the blends could be dispersed into the structures comprising co-continuous networks of the block copolymer phase and the epoxy resin leading to an increment in impact strength. At the same time, the hardness and stiffness of the blends, as shown by the indentation microhardness and indentation modulus, was maintained to a pretty high level.

Synergistic Hybrid Organic–Inorganic Aerogels

A class of inorganic-organic hybrid mesoporous aerogel structure was synthesized by growing gel in a gel. In Type 1, silica gels were grown inside the macropores of thermoreversible syndiotactic polystyrene (sPS) gel, while Type 2 hybrid aerogels were obtained by thermoreversible gelation of sPS chains in the mesopores of preformed silica gel. The hybrid gels were converted into aerogels by exchanging the solvent with liquid carbon dioxide followed by supercritical drying. The hybrid aerogels presented cocontinuous networks of pearl-necklace silica particles and crystalline strands of sPS and exhibited the "petal effect" due to the presence of superhydrophobic sPS and hygroscopic silica. The compressive modulus and compressive strain show large enhancements over sPS and silica aerogels indicating synergy, although Type 1 hybrid aerogels were found to be more robust. The hybrid aerogels showed fast absorption and high absorption capacity for a representative hydrocarbon liquid.

Three-dimensional nanoscale organization of polymer solar cells

The performance of polymer solar cells (PSCs) strongly depends on the three-dimensional morphological organization of the compounds within the bulk heterojunction active layer. Donor and acceptor materials should form co-continuous networks with nanoscale phase separation to sustain effective dissociation of excitons into free electrons and holes at the donor/acceptor interface and to guarantee fast charge carrier transport from any place in the photoactive layer to the corresponding electrodes. Here, we describe applications of the technique of electron tomography to directly visualize with nanometre resolution and study in detail the 3D organization in the photoactive layers of PSCs, with the aim of identifying the critical morphology parameters contributing to high efficiency of bulk heterojunction systems.

Erosion‐dependant continuity development in high viscosity ratio blends of very low interfacial tension

AbstractThis work studies continuity development and cocontinuity in high viscosity ratio EPDM/PP blends. A very low interfacial tension (0.3 mN/m) between the blend components together with high viscosity ratios (11 and 17) result in a variety of unusual morphological features, including isolated nanometer diameter fibers, very large particles, partially coalesced particles, and numerous particles interconnected by fibers. This unique combination of morphologies leads the blend to a novel and stable cocontinuous structure of partially coalesced particles and particles interconnected by fibers. Compared with low to medium viscosity ratio EPDM/PP blends, these cocontinuous networks demonstrate early percolation thresholds, rapid continuity development, and attain cocontinuity at lower compositions of minor phase. The slow surface erosion of the high viscosity EPDM phase during melt blending is shown to be responsible for the generation of these unusual morphological structures. Typically the timescale for erosion phenomena are so small that they have defied study in the mixing environment itself and typical blend morphology studies almost always examine the final steady‐state morphology obtained after several minutes of mixing. The combination of very low interfacial tension and very high viscosity ratios of these EPDM/PP systems provide a unique opportunity to examine erosion phenomena persisting over longer time scales during melt mixing. We propose a new concentration‐dependant erosion mechanism that is based on particle collision–coalescence–separation dynamics. The proposed conceptual mechanism is shown to dramatically accelerate the erosion process and maintain cocontinuity over prolonged periods of mixing. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1919–1929, 2006

Macroporous poly( l-lactide) of controlled pore size derived from the annealing of co-continuous polystyrene/poly( l-lactide) blends

A detailed study on the static annealing of co-continuous polystyrene/poly( l-lactide) (PLLA) blends is presented. The effects of temperature, time at temperature, viscosity of the phases and interfacial modification on the coarsening of the blend are discussed. In this paper, polystyrene and PLLA are blended at compositions of 50/50 and 60/40 to form co-continuous morphologies. These co-continuous morphologies are coarsened under quiescent annealing conditions, and the subsequent removal of the polystyrene phase leaves a macroporous PLLA structure. The microstructure is analyzed using three different techniques: the BET nitrogen adsorption technique, mercury intrusion porosimetry and SEM combined with image analysis. It is shown that static annealing can be used to generate a series of co-continuous networks with controlled pore sizes ranging from 1 to hundreds of microns. A non-linear pore size growth rate is observed for these systems due to the degradation of PLLA and this study indicates that controlled degradation can be used as an additional tool for morphology control. Compatibilized polystyrene/PLLA blends demonstrate significantly reduced coarsening effects due to the reduction of interfacial tension. The coarsening rate of the co-continuous structure was examined in terms of the pore size, R and this growth rate is discussed in terms of a previously proposed coarsening mechanism. This approach is a route towards the preparation of a macroporous PLLA structure with pore sizes in the range required for scaffolds for tissue regeneration.

Co-extrusion of biocompatible polymers for scaffolds with co-continuous morphology.

A methodology for the preparation of porous scaffolds for tissue engineering using co-extrusion is presented. Poly(epsilon-caprolactone) is blended with poly(ethylene oxide) in a twinscrew extruder to form a two-phase material with micron-sized domains. Selective dissolution of the poly(ethylene oxide) with water results in a porous material. A range of blend volume fractions results in co-continuous networks of polymer and void spaces. Annealing studies demonstrate that the characteristic pore size may be increased to larger than 100 microm. The mechanical properties of the scaffolds are characterized by a compressive modulus on the order of 1 MPa at low strains but displaying a marked strain-dependence. The results of osteoblast seeding suggest it is possible to use co-extrusion to prepare polymer scaffolds without the introduction of toxic contaminants. Polymer co-extrusion is amenable to both laboratory- and industrial-scale production of scaffolds for tissue engineering and only requires rheological characterization of the blend components. This method leads to scaffolds that have continuous void space and controlled characteristic length scales without the use of potentially toxic organic solvents.

Development of Biodegradable Polymer Scaffolds Using Co-Extrusion

AbstractA methodology for the preparation of porous scaffolds for tissue engineering using co-extrusion is presented. Poly(ε-caprolactone) is blended with poly(ethylene oxide) in a twin-screw extruder to form a two-phase material with micrometer-sized domains. Selective dissolution of the poly(ethylene oxide) with water results in a porous material. This method of polymer extrusion permits the preparation of scaffolds having continuous void space and controlled characteristic length scales without the use of potentially toxic organic solvents. A range of blend volume fractions results in co-continuous networks of polymer and void spaces. Annealing studies demonstrate that the characteristic pore size may be increased to larger than 100 μm. The mechanical properties of the scaffolds are characterized by a compressive modulus on the order of 1 MPa at low strains and approximately 10 MPa at higher strains. The results of osteoblast seeding suggest it is possible to use co-extrusion to prepare polymer scaffolds without the introduction of toxic contaminants.

SPINODAL DECOMPOSITION WITH SURFACE TENSION DRIVEN FLOWS

Abstract The Cahn-Hilliard theory of phase separation by spinodal decomposition has been extended to include hydrodynamics caused by surface tension. Numerical solutions for phase separation in binary polymer mixtures in two spatial dimension are presented. The surface tension influences the growth rate of the average domain size and the morphology for near critical quenches that would otherwise form cocontinuous networks. The growth rate exponent depends on the value of dimensionless surface tension, γ = [(ρ0 m 3/2)/(Dμ)][(κυ s )/(RgT)]½, and ranges from 0.30 ±0.03, for γ = 0 (no hydrodynamics) to 0.69 ±0.04 for γ = 1 for a critical quench. For an off-critical quench in which a dispersed phase would be formed by diffusion alone, the scaling exponent shows little effect of surface tension and ranges from 0.28 ±0.02 for γ = 0 to 0.32 ±0.02 for γ = l. While not conclusive, the results suggest that it is infeasible to prepare bulk samples of cocontinuous polymer phases by the process known as compositional q...

A novel approach to polymeric microdispersions

Novel microdispersions of one polymer in another have been created by solvent devolatilization. Two incompatible polymers are first dissolved in a common solvent. The solvent is then rapidly evaporated, causing the residual polymeric solution to enter the two-phase region. Depending on the relative amounts of the two polymers and the rate of evaporation, polymer blends of widely differing morphologies are formed. Unlike previous methods employed for their production, polymer dispersions created by devolatilization have remarkably uniform particles, with the average size controllable in the range of 0.5 μm to 20 μm. If the solvent free volume fractions of the two polymers are more ore less equal, co-continuous networks are formed.