Lamellar Stacks Research Articles

Cross hatched structure of polyethylene spherulites

We apply electron microscopy to evaluate the structure of spherulites in films of linear low density polyethylene (LLDPE). Improvements in this imaging technique allow us to show that the spherulites consist of two crossing patterns of lamellar stacks; one type of stack is composed of the usual radial lamellae while the second type of lamellae stacks is tangential. Radial lamellae are thicker and have a helical twist while tangential lamellae do not twist but rather create parallel stacks. The observed structure is akin to the structure frequently encountered in spherulites of polypropylene and shows the same dependence on crystallization temperature. At the same time the formation mechanism must be different from widely discussed epitaxial overgrowth in polypropylene because the polyethylene orthorhombic crystal lattice does not provide the same matching of lattice parameters as alpha phase of polypropylene.

Combustion and gasification characteristics of low-temperature pyrolytic semi-coke prepared through atmosphere rich in CH4 and H2

Abstract Thermoanalysis was used in this research to produce a comparative study on the combustion and gasification characteristics of semi-coke prepared under pyrolytic atmospheres rich in CH4 and H2 at different proportions. Distinctions of different semi-coke in terms of carbon chemical structure, functional groups, and micropore structure were examined. The results indicated that adding some reducing gases during pyrolysis could inhibit semi-coke reactivity, the inhibitory effect of the composite gas of H2 and CH4 was the most observable, and the effect of H2 was higher than that of CH4; moreover, increasing the proportion of reducing gas increased its inhibitory effect. X-ray diffractometer and Fourier-transform infrared spectrometer results indicated that adding reducing gases in the atmosphere elevated the disordering degree of carbon microcrystalline structures, boosted the removal of hydroxyl- and oxygen-containing functional groups, decreased the unsaturated side chains, and improved condensation degree of macromolecular networks. The nitrogen adsorption experiment revealed that the types of pore structure of semi-coke are mainly micropore and mesopore, and the influence of pyrolytic atmosphere on micropores was not of strong regularity but could inhibit mesopore development. Aromatic lamellar stack height of semi-coke, specific surface area of mesopore, and pore volume had a favorable linear correlation with semi-coke reactivity indexes.

Simultaneous measurement of electrical characteristics and microstructure of crystallised PEDOT:PSS based OECTs under strain

Abstract Here is reported the evolution of the transconductance and resistance, along with microstructure, of organic electrochemical transistors (OECTs) under progressively greater strain. A decrease in transconductance and increase in resistance over several orders of magnitude are reported for strains up to 75%. These changes in electrical characteristics are correlated with changes in the microstructure of the PEDOT:PSS characterised by a positive shift, and reduction in amplitude, of the characteristic (100) peak of the grazing incidence wide-angle X-ray scattering (GIWAXS) profile, recorded in tandem with the electrical characteristics during stretching. The (100) peak is associated with the lamella stacking of the PEDOT and PSS chains, and it is shown that this lamella spacing is reduced by up to around 3.5 A on application of strain up to 75%.

Simulation of SAXS patterns from oriented lamellar structures and their elliptical trajectories

Small-angle X-ray Scattering (SAXS) patterns are useful for studying the organization of crystalline and amorphous regions that make up a semi-crystalline polymer and influence its properties. To understand how these patterns originate and evolve during deformation, three distinct SAXS patterns (2-point, 4-point ‘eyebrow’, and 4-point ‘butterfly’) were generated using simple microstructural models and compared in detail with experimental data from poly(ethylene-co-octene). As the patterns typically contain a single unique reflection, it is not possible to fully determine the structure from the patterns alone. Therefore, structural models with lamellar stacks were used to simulate SAXS patterns. When stack orientation (interlamellar shear) and chain slip act to rotate lamellae in the same sense, the result is an ‘eyebrow’ pattern. Opposite senses give a ‘butterfly’ pattern. Experimental patterns often have reflections where the peak position traces out an ellipse. Adding moderate amounts of rotational disorder to the models produces this effect without any elliptical correlations or deformed lattice in the material.

Alignment of linear polymeric grains for highly stable N-type thin-film transistors

Summary Temperature-insensitive properties are attractive for most electronics, including polymeric semiconducting devices. Especially, polymeric field-effect transistors (FETs) with high mobility have been important research targets due to their broad applications. However, polymeric FETs with stable charge transport operating at extremely cold or hot zones are faced with enormous challenges. In this study, the polyacrylonitrile was found to significantly tune the sizes of pre-aggregates of polymers in solutions and the crystallinity of the polymeric films. The orientation of 5–25 μm linear grains in the films were prepared through the bar-coating process with polyacrylonitrile as an additive, which stabilized the electron mobility over a wide range of temperatures. The linear-grain morphology of the film contributed to reducing the holes and grain boundaries in the transport paths of carriers. Typically, the top-gate FETs based on P(NDI2OD-T2) displayed a stable electron transporting behavior from 200 to 460 K, with mobility greater than 3.5 cm2V−1s−1.

Synthesis and thermotropic liquid-crystalline properties of a hexyloxy-substituted pyridyl-ethynylene-azobenzene and its halogen-bonded complex with tetrafluoroiodophenyl decanoate

This work deals with the synthesis and thermotropic behavior of a hexyloxy-substituted pyridyl-ethynylene-azobenzene and its halogen-bonded complex (XB complex) with tetrafluoroiodophenyl decanoate. The XB complex here reported is a highly anisotropic photoactive mesogen. It displays only smectic-type order where two different lamellar stacks seem to co-exist. The smectic period (determined by XRD) shorter than the theoretical length of the XB-complex suggests a single layer stacking of molecules, which may be arranged head-to-tail to satisfy the electrostatic interactions between fluorinated and non-fluorinated rings. Finally, photo-isomerization and NMR experiments seem indicate that the halogen-bonded complex is stable in solution.

Efficient isotropic water desalination in anisotropic lamellar nano-channels formed by layered black phosphorus membrane

Recent developments in nanomaterials have greatly advanced the design and fabrication of water filtration membranes and hopefully alleviate the deteriorating freshwater scarcity. In this work, we demonstrated that lamellar stacking of layered black phosphorus (BP) can serve as an efficient membrane for seawater desalination. Using molecular dynamics simulations, we show that the nano-channels inside BP membranes can effectively block ionic passage while maintaining high water flux (~ 30 L/cm2/day/MPa) at a large range of interlayer spacing (9 Å to 12 Å). Despite the in-plane anisotropy of the BP membrane, the filtration performance of the nano-channels is barely affected by the orientation of the adjacent BP layers. Quantitative potential of mean force analysis indicates that the excellent water filtration performance is attributed to the high energy barriers (>15 kJ/mol) for ions versus the low energy barrier (~ 2 kJ/mol) for water to enter the nano-channels. Moreover, the effect of defects presented in the BP membrane to water filtration has been addressed which is found to hardly affect the filtration performance, indicating the robustness of the BP-based nano-channels. Therefore, the results presented in this study offer a competitive membrane for seawater desalination which is also expected to guide the future design of membrane materials.

Time-resolving small angle X-Ray scattering analysis of melt crystallization of mixtures of regular and irregular isotactic polypropylene samples

The melting/crystallization properties of blends obtained by mixing two isotactic polypropylene (iPP) samples synthesized using single-site metallocene catalyst systems and containing a high and low concentration of rr triads as stereo-defects, are studied. The changes occurring at lamellar length scale during a heating/cooling cycle at constant scanning rate are followed in situ by performing time-resolved small angle X-ray scattering (SAXS) measurements. Data analysis demonstrates that the evolution of the SAXS intensity with increase/decrease of the temperature is controlled by the separate melting/crystallization of the two components, the differences in the thermal expansion (contraction) coefficient of the amorphous and crystalline phases and the role of thermal fluctuations in electron density. The two components give rise to different populations of intermixed lamellar stacks in the blends which originate from the good miscibility of the low and high stereoregular samples in the melt.

Direct Access to Bowl-Like Nanostructures with Block Copolymer Anisotropic Truncated Microspheres.

Bowl-like nanostructures have attracted significant scientific and technological interest due to their favorable characteristics, such as high specific surface area, interconnected porous channels, and conductivity. However, tailored synthesis of bowl-like nanostructures with well-defined and uniform morphology is still a challenge. Herein, we report a versatile microemulsion assembly approach to prepare bowl-like nanostructures of three different materials: polymer, carbon, and platinum. To this end, polystyrene-block-poly(4vinylpyridine), PS-b-P4VP, block copolymer (BCP) microparticles with truncated-sphere shape and composed of stacks of parallel lamellae were used because those anisotropic microparticles play an important role in the design of bowl-like nanostructures. To form nanolamellae-within-microparticle morphology, a designed PS-b-P4VP/chloroform/CTAB microemulsion can be facilely obtained in the aqueous medium, where the morphology can be tailored by the interplay between macro-phase separations, BCP self-assembly, and interfacial energies of three phases in the presence of cetyltrimethylammonium bromide (CTAB). Finally, protonation or combination of cross-linking and pyrolysis of those truncated microparticles enables formation of polymer or carbon bowl-like nanostructures, respectively. Upon selective adsorption of Pt precursor salt ions with the pyridyl moieties followed by chemical reduction, subsequent calcination permits the synthesis of Pt bowl-like nanostructures. The microemulsion assembly approach opens up new ways to direct and template bowl-like nanostructures.

Effect of Adding Lecithin and Nonionic Surfactant on α-Gels Based on a Cationic Surfactant-Fatty Alcohol Mixture.

α-Gels are often used as base materials for cosmetics and hair conditioners. α-Gel-based commercial products typically contain many types of additives, such as polymers, electrolytes, oily components, and other surfactants, in addition to the three basic components. However, few systematic studies have been conducted on the effect of such additives on α-gels. In this study, we chose surfactant as an example to initiate the effect of such additives on the structure and rheological properties of α-gel samples formulated using cetyl alcohol (C16OH) and cetyltrimethylammonium chloride (CTAC). Optical microscopy analysis demonstrated that the size of the vesicles in the α-gel samples in this study was decreased via the addition of hydrogenated soybean lecithin (HSL) and penta(oxyethylene) cetyl ether (C16EO5), a nonionic surfactant, to them. Rheological measurements revealed that at high C16OH/CTAC ratios, the viscosity and yield stress of the α-gel samples decreased owing to the addition of surfactants to them. Conversely, at low C16OH/CTAC ratios, the opposite tendency was observed. Small-angle X-ray scattering analysis indicated that for the α-gel samples with high C16OH/CTAC ratios, the addition of HSL or C16EO5 to them decreased the interlayer spacing of their lamellar bilayer stack, which led to the changes in the rheological properties of the α-gel samples.

Effects of the addition of coal tar pitch-based mesocarbon microbeads on the preparation of mesophase pitch

Mesophase pitch was prepared by co-carbonization of the hydrocracking tail oil (HCTO) and coal tar pitch-based mesocarbon microbeads (MCMB). The effects of the addition amount of coal tar-based MCMBs on the C/H atomic ratio, softening point, carbon residue, optical texture, aromatic lamellae stacking order degree, and chemical reaction kinetics of mesophase pitch were investigated. The results show that MCMB can be used as a nucleating agent to accelerate the nucleation of mesophase spheres and reduce the energy barriers for the formation of the anisotropic phase. However, excessive MCMB greatly increases the viscosity of the pitch matrix and weakens the mobility of condensed aromatic macromolecules, leading to the formation of mesophase pitch with a mosaic structure and poor lamellar orientation. When the addition amount of MCMB is 5 wt%, the mesophase pitch with a large domain texture, high aromatic lamellar stacking order degree, and low softening point can be formed. This work provides a pathway to efficiently prepare mesophase pitch without catalyst removal.

Improving the Hot Corrosion Resistance of Nanostructured ZrO2-7wt.%Y2O3 Thermal Barrier Coatings Fabricated by Plasma Spraying

Conventional and nanostructured ZrO2-7wt.%Y2O3 thermal barrier coatings (TBCs) were prepared by plasma spraying technology. Microstructure and hot corrosion resistance of two TBCs in 75%Na2SO4+25%NaCl (mass fraction) mixed molten salt under 850 °C were compared. Meanwhile, hot corrosion failure mechanisms of TBCs were investigated. Results demonstrated that both conventional and nanostructured TBCs were typical lamellar stacking characteristic. However, compared with conventional TBC, the nanostructured TBC presented the base phase which is gained from recrystalization of fully melted nano-particles as well as the embedding phase of partially melted nano-particles. The nanostructured TBC has lower porosity and higher density. For both conventional and nanostructured TBCs, hot corrosion failure is caused by partial transformation of tetragonal phase (t-ZrO2) into monoclinic-phase (m-ZrO2) martensite as a response to the consumption of the stabilizer Y2O3. However, the nanostructured TBC generally has lower degree of hot corrosion and better protection to the metal bonding layer and matrix compared with conventional TBC.

Fabrication of Amphiphilic Graphene Oxide—Polyoxometalate with Temperature Response for Epoxidation of Olefin in Water

Here, amphiphilic graphene oxide–polyoxometalate (GO-POM) was fabricated using a new strategy involving control of the stacking of GO lamellae through phosphoric acid and exfoliation by H2O2. The additions of H3PO4 and H2O2 were essential for the formation of the catalytic center of peroxo-POMs. The GO-POM hybrid had one side with hydrophilic properties and another side with hydrophobic properties, which conferred temperature-responding properties. GO-POM could catalyze the epoxidation of cyclooctene with complete conversion and 98% selectivity for epoxide at 50 °C for 12 h in water. Meanwhile, the catalyst could be easily recycled because of its thermosensitive property.

Molecular dynamics simulation of the adsorption of per- and polyfluoroalkyl substances (PFASs) on smectite clay

Molecular dynamics (MD) simulations are used to predict the partitioning of per- and polyfluoroalkyl substances (PFASs) to smectite clay, a high surface area adsorbent ubiquitous in temperate soils. Simulated systems model a stack of flexible smectite lamellae in contact with a bulk aqueous reservoir containing PFAS molecules. Perfluorobutanesulfonic acid (PFBS), perfluorohexanesulfonic acid (PFHxS), and perfluorooctanesulfonic acid (PFOS) are simulated at various aqueous chemistry conditions to examine the effect of PFAS size, salinity, and coordinating cation type (K+, Na+, and Ca2+) on adsorption. The metadynamics technique is employed to facilitate the exploration of the simulation cell and to reconstruct the underlying free energy landscape. Adsorption is favorable on the hydrophobic domains of the external basal surfaces with the fluorinated chain adopting a flat orientation on the surface. Analysis of the adsorption energetics reveals large favorable entropic contributions to adsorption. The enthalpy of adsorption is unfavorable, though much less so in the presence of Ca2+ due to stabilizing ‘lateral cation bridging’ interactions between divalent cations and PFAS sulfonate head groups. Overall, this research advances the mechanistic understanding of PFAS-smectite interactions and provides new insights that could help inform fate and transport models and the development of adsorbents and remediation techniques.

Stretch-induced structural evolution of pre-oriented isotactic polypropylene films: An in-situ synchrotron radiation SAXS/WAXS study

In-situ small- and wide-angle X-ray scattering (SAXS/WAXS) is employed to study the stretch-induced structural evolutions of four different pre-oriented isotactic polypropylene (iPP) cast films during uniaxial stretching at room temperature. The mechanical properties and structural evolutions show distinct features with the improvement in orientation of initial structure. For low-oriented films, the tensile stress induces the amorphization of lamellar crystals parallel to stretching direction, while the shear stress triggers the shearing of lamellar stacks at other angles through crystal slipping. For high-oriented films, the elastic deformation mode dominates as the normals of all lamellae are along stretching direction. Under tensile stress, micro-buckling or bending replaces the lamellar shearing to trigger the initial deformation instability of crystals. And stress-induced amorphization of lamellar crystals might also occur at large strains in the strain hardening zone. This study uncovers the different structural evolution roads of low- and high-oriented films, which will aid to deepen the understanding of the orientation effects on the deformation mechanisms of other semi-crystalline polymers.

Molecular Reorganization during the Formation of the Human Skin Barrier Studied In Situ

In vertebrates, skin upholds homeostasis by preventing body water loss. The skin's permeability barrier is located intercellularly in the stratum corneum and consists of stacked lipid lamellae composed of ceramides, cholesterol, and free fatty acids. We have combined cryo-electron microscopy with molecular dynamics modeling and electron microscopy simulation in our analysis of the lamellae's formation, a maturation process beginning in stratum granulosum and ending in stratum corneum. Previously, we have revealed the lipid lamellae's initial- and end-stage molecular organizations. In this study, we reveal two cryo-electron microscopy patterns representing intermediate stages in the lamellae's maturation process: a single-band pattern with 2.0‒2.5 nm periodicity and a two-band pattern with 5.5‒6.0 nm periodicity, which may be derived from lamellar lipid structures with 4.0‒5.0 nm and 5.5‒6.0 nm periodicity, respectively. On the basis of the analysis of the data now available on the four maturation stages identified, we can present a tentative molecular model for the complete skin barrier formation process.

Symmetric Diblock Copolymers in Cylindrical Confinement: A Way to Chiral Morphologies?

We investigate the confinement-induced formation and stability of helix morphologies in lamella-forming AB diblock copolymers via large-scale, particle-based, single-chain-in-mean-field simulations. Such helix structures are rarely observed in bulk or thin films. Structure formation is induced by quenching incompatibility, χN, from a disordered morphology. If the surfaces of the cylindrical confinement do not prefer one component over the other, we observe that stacked lamellae, with their normals along the cylinder axis, are the preferred morphology. Kinetically, this morphology initially forms close to the cylinder surface, whereas the spontaneous, spinodal microphase separation in the cylinder's interior gives rise to a microemulsion-like morphology, riddled with defects and no directional order. Subsequently, the ordered morphology on the cylinder surface progresses inward, pervading the entire volume. In case that the cylindrical pore is only partially filled, the additional confinement along the cylinder axis generally gives rise to incommensurability between the equilibrium spacing of stacked lamellae and the cylinder height. To accommodate this mismatch, the lamella normals will tilt away from the cylinder axis and generate helices of lamellae on the surface of the cylinder. Again, this order progresses from the cylinder surface inward, generating a chiral morphology. Because the spacing between the internal AB interfaces decreases upon approaching the helix center, the concomitant stress results in a decrease in the number of lamellae and the formation of unique dislocation defects. This type of chiral defect morphology is reproducibly formed by the kinetics of structure formation in partly filled cylindrical pores with nonpreferential surfaces and may find applications in photonic applications.

Toughening alpha-Ti by dislocation-induced phase transformation at crack tips

In this paper, molecular dynamics simulation was used to investigate the crack propagation along the {101‾1} twin boundary in titanium bi-crystals during uniaxial tensile deformation. The whole deformation process can be divided into two periods according to the crack propagation speed: (1) the quick crack propagation period in the early deformation stage and (2) the slow crack propagation period in the later deformation stage. The results show although the pyramidal and basal stacking faults induced from crack tips can reduce the stress concentration in the early deformation stage, they are nonetheless inadequate to inhibit the quick crack propagation with a velocity over 100 m/s. Comparatively, the formation of new face-centered cubic (FCC) grains at crack tips in the later deformation stage can blunt the crack, retard crack propagation and thus promote the toughness of the material. The nucleation and growth of the newly formed FCC grains are controlled by different mechanisms. During the quick crack propagation period, complex interactions between <c+a> partial dislocations on pyramidal planes and <a> dislocations on prismatic planes induce retained stacking faults in FCC lamella and new pyramidal stacking faults in hexagonal close-packed (HCP) matrix. The retained stacking faults develop into FCC nuclei as the tensile strain increases. During the slow crack propagation period, the FCC nuclei grow towards HCP phase and develop into a coarser grain that pins the further propagation of the adjacent crack tip. The growth of FCC nuclei is governed by a prismatic-type phase transformation mechanism, which involves atomic shear, shuffle and extra lattice distortion.

Small angle X-ray scattering from finite sequence of lamellar stacks of crystalline polymers

An analysis method using small-angle X-ray scattering (SAXS) of the lamellar stacks of crystalline polymers was examined. The scattering profile was expressed by a fitting function on the basis of a paracrystalline stacking model of finite sequence of parallel lamellae in a stack with border zones continuously connecting to the mean density in spherulites. The SAXS pattern of poly(butylene terephthalate) was able to be fitted to both the conventional model with infinite number of parallel lamellae in a stack and the present model with finite number. The SAXS pattern of polyethylene could not be fitted by the conventional model supposing an infinite sequence, and it was necessary to suppose a lower finite number; the best fit result was obtained with two crystalline layers. A lower number of parallel lamellae in a stack is consistent with the fact that polyethylene lamellar crystals are twisted in three-dimensional space, and cannot form an infinite sequence of simple stacked structure of lamellae oriented parallel to each other. For the finite sequence with lower number, the determination of crystalline lamellar thickness requires the consideration of border zones. It was also confirmed that an alternative method using triangular shape of a one-dimensional auto-correlation function could be acceptable with better level of agreement for narrower distributions of thickness.

Modifying the electrochemical process of Li–O2 battery by integrated NiMn-layered double hydroxides substrate

As typical two-dimension material, layered double hydroxides (LDHs) attract great interest and are believed to be promising catalysts in Li–O2 battery due to their facile tunability of composition, structures and properties. However, the low electric conductivity and limited catalytic active site exposure caused by the large particle size and severe lamella stacking are the main challenges. Herein, an interconnected, highly active and ultra-thin NiMn-LDHs is deposited on 3D Ni foam and used as integrated porous catalyst substrate (NiMn-LDHs substrate), the NiMn-LDHs substrate is functional as both current collector and catalyst layer, and the NiMn-LDHs substrate exhibits high catalytic activity and robust stability, the substrate successfully plays the function of tunning the electrochemical process of Li–O2 battery. The Li–O2 battery uses this kind of substrate exhibits much improved discharge capacity and long cycle performance compared with pristine Ni substrate. The benefit of the porous NiMn-LDHs in improving the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) catalytic activities is also supported by rotating disc electrode techniques. This work highlights the strategy of fabricating high efficient LDHs catalyst and provides a new direction for achieving highly efficient integrated catalytic cathode for practical use.