Orientational Transition Research Articles

Структурный переход и температурные зависимости коэффициентов теплового расширения NaNO-=SUB=-3-=/SUB=-, внедренного в нанопористое стекло

The temperature evolution of the crystal structure of a nanocomposite material obtained by introducing sodium nitrate NaNO3 from a melt under pressure into a nanoporous alkali borosilicate glass with an average pore diameter of 7 nm has been studied by the method of diffraction of synchrotron radiation in a wide temperature range upon heating. Analysis of the experimental diffraction patterns revealed a significant decrease in the temperature of the structural (orientational) transition by more than 50 K (up to 496 K) compared to bulk sodium nitrate. From the temperature dependence of the intensity of the superstructure peak (113), the dependence of the critical exponent β (T) for this transition was obtained and a significant difference from the critical exponent for a bulk material was found in the temperature range from 455 K to the transition temperature. Analysis of the broadening of Bragg reflections made it possible to estimate the average size (~ 40 nm) of sodium nitrate nanoparticles into the pores. An increase in the linear coefficient of thermal expansion in the [001] direction was found in NaNO3 nanoparticles in comparison with bulk material at temperatures above 450 K.

Studying the effects of carbon nanotube contents on stretch-induced crystallization behavior of polyethylene/carbon nanotube nanocomposites using molecular dynamics simulations.

In the present work, we used molecular dynamics simulations to study the effects of carbon nanotube (CNT) contents on stretch-induced crystallization behavior in CNT filled polyethylene systems. During high-temperature stretching, the stretching is responsible for the orientation of CNTs, which then facilitates segment orientation and conformational transition from the gauche-conformation into the trans-conformation in interfacial regions. The systems with higher CNT contents have a higher degree of orientation and higher contents of trans-conformation during stretching, resulting in the formation of more precursors. During subsequent crystallization, the initial crystallization rate increases with the increase of the CNT content due to the increase in precursor contents in interfacial regions. However, after the CNT content exceeds a certain value, a filler network would be formed by CNTs, which can restrict chain movements and then lead to a decrease in the overall crystallization rate in the systems with high CNT contents.

Development of the molten pool and solidification characterization in single bead multilayer direct energy deposition

Although single-layer directed energy deposition has been extensively studied, an in-depth understanding of the cladding melting and solidification mechanism of single bead multilayer directed energy deposition is equally promising for industry, considering practical application scenarios. Based on the single-layer directed energy deposition, a numerical model of multilayer directed energy deposition was developed and validated to reveal the cladding forming process and solidification structure of multilayer directed energy deposition. The simulation results show the complex Marangoni convection pattern generated by the surface tension gradient in the melt pool. Moreover, the multilayer directed energy deposition enhances the penetration effect of the melt pool under the influence of the heat accumulation effect, resulting in a cross-layer mass transfer different from that of the single-layer directed energy deposition. The evolution of the multilayer cladding from columnar crystals at the bottom to equiaxed crystals at the top and the shift of its growth orientation were also analyzed based on the obtained solid-liquid interface temperature gradients and solidification velocities during the solidification of the melt pool. Finally, the relationship between cladding grain size and solidification properties was theoretically predicted by counting the grain size of different layers in the cladding, and the effect of grain growth on the mechanical properties was superficially discussed. • Penetration of multilayer deposition melt pools enhanced by heat accumulation effects. • Cross-layer mass transfer exists for multilayer deposition. • "C" or "U" type of grain growth orientation transition. • The recrystallization after solidification promotes the mechanical properties.

Multistage Structural Ordering and Crystallization of Poly(trimethylene terephthalate) during Sub-Tg Stretching: Synergetic Effects of Chain Orientation and Conformational Transition

Crystallizable glassy polymers can undergo multistage structural evolutions and form multiple types of ordered phases (e.g., mesomorphic and crystalline phases) during stretching below their glass transition temperatures (Tg). However, the structural evolution of polymers during sub-Tg stretching and the associated molecular mechanisms have not been well understood. In this work, we investigated the multistage structural ordering of glassy polymers and the related mechanisms during stretching below Tg by studying poly(trimethylene terephthalate) (PTT) as a model system. Roles of chain orientation and conformational transition on the multistage structural evolution of melt-quenched PTT were elucidated through a loading–unloading experiment. When stretched below Tg (e.g., 25 °C), the orientation of polymer chains and the stacking of phenylene rings lead to an intermolecular ordering, which induces the generation of a smectic C-like mesomorphic phase. The orientation degree of chains in such a mesomorphic phase increases with further stretching. This highly oriented mesomorphic phase transforms into a triclinic crystalline phase when the applied stress is unloaded. Stretching at a higher temperature (e.g., 40 °C) results in the formation of a more ordered mesomorphic phase and facilitates the induced crystallization due to the enhanced chain mobility. Fourier transform infrared results demonstrated that the unloading-induced crystallization of stretched PTT is originated from the trans-gauche conformational transition.

Is pressure the key to hydrogen ordering ice IV?

Hydrochloric-acid-doped ice IV prepared at increasing pressures leads to growing endotherms observed with ambient pressure calorimetry. The endotherms are irreversible suggesting three possible scenarios for their origins: (1) a weakly hydrogen-ordered counterpart of ice IV is formed, but ambient pressure favours hydrogen-disordered ice IV, (2) increased pressure creates increased strain within the crystal structure of the ice, which is released upon heating yielding the endotherms or (3) the endotherms are kinetic overshoot effects related to the underlying orientational glass transition. X-ray diffraction cannot distinguish between these scenarios. Recent controversies regarding the preparation of ice IV are also discussed.

Prediction Power of Logistic Regression (LR) and Multi-Layer Perceptron (MLP) Models in Exploring Driving Forces of Urban Expansion to Be Sustainable in Estonia

Estonia mainly experienced urban expansion after regaining independence in 1991. Employing the CORINE Land Cover dataset to analyze the dynamic changes in land use/land cover (LULC) in Estonia over 28 years revealed that urban land increased by 33.96% in Harju County and by 19.50% in Tartu County. Therefore, after three decades of LULC changes, the large number of shifts from agricultural and forest land to urban ones in an unplanned manner have become of great concern. To this end, understanding how LULC change contributes to urban expansion will provide helpful information for policy-making in LULC and help make better decisions for future transitions in urban expansion orientation and plan for more sustainable cities. Many different factors govern urban expansion; however, physical and proximity factors play a significant role in explaining the spatial complexity of this phenomenon in Estonia. In this research, it was claimed that urban expansion was affected by the 12 proximity driving forces. In this regard, we applied LR and MLP neural network models to investigate the prediction power of these models and find the influential factors driving urban expansion in two Estonian counties. Using LR determined that the independent variables “distance from main roads (X7)”, “distance from the core of main cities of Tallinn and Tartu land (X2)”, and “distance from water land (X11)” had a higher negative correlation with urban expansion in both counties. Indeed, this investigation requires thinking towards constructing a balance between urban expansion and its driving forces in the long term in the way of sustainability. Using the MLP model determined that the “distance from existing residential areas (X10)” in Harju County and the “distance from the core of Tartu (X2)” in Tartu County were the most influential driving forces. The LR model showed the prediction power of these variables to be 37% for Harju County and 45% for Tartu County. In comparison, the MLP model predicted nearly 80% of variability by independent variables for Harju County and approximately 50% for Tartu County, expressing the greater power of independent variables. Therefore, applying these two models helped us better understand the causative nature of urban expansion in Harju County and Tartu County in Estonia, which requires more spatial planning regulation to ensure sustainability.

Impact of graphene-molecular interaction on collective orientation barrier for organic film growth

The temperature-dependent molecular orientation variation of pentacene (PEN) on a graphene-covered substrate (PEN/Gr) was investigated via p-polarized multiple-angle incidence resolution spectrometry (pMAIRS). The temperature regime of the orientation transition of PEN/Gr was different from that of PEN/SiO2. The collective orientation barrier (COB), an energy barrier that molecules need to overcome to form a standing orientation, was estimated via pMAIRS. Consequently, the COB of PEN/Gr was found to be 10 times larger than that of PEN/SiO2. This indicated that the COB is valuable for understanding the effect of substrate interaction on molecular orientation.

Highly sensitive label-free liquid crystal-based aptasensor to detect alpha-fetoprotein

ABSTRACT In this study, a novel label-free optical aptasensor based on liquid crystals (LCs) for sensitive and specific alpha-fetoprotein (AFP) detection was developed. The modification parameters were optimised to limit the effect of the background signal and increase the sensitivity of the sensor. The AFP-specific aptamer was immobilised on the substrates using an APTES + GA bridge. The binding of AFP to its aptamer causes changes in the surface topography and disrupts the LC alignment. The orientational transition of LC could be observed, from dark to bright, using POM. The changes in the topography of the surfaces were confirmed by AFM, ellipsometry, and water contact angle measurements. Under optimal conditions, the proposed LC-based aptasensor exhibited optimal performance in AFP detection. The detection limit for AFP reached 12.62 pg mL−1 in HEPES buffer and 18.65 pg mL−1 in human serum, with the detection range from 20–800 mL−1 for both the conditions. Owing to advantages such as simplicity, speed, and high sensitivity and specificity, the proposed label-free LC-based aptasensor has potential applications in the clinical diagnosis of various tumour markers.

Entropy-driven phases at high coverage adsorption of straight rigid rods on two-dimensional square lattices.

Polymers are frequently deposited on different surfaces, which has attracted the attention of scientists from different viewpoints. In the present approach polymers are represented by rigid rods of length k (k-mers), and the substrate takes the form of an L×L square lattice whose lattice constant matches exactly the interspacing between consecutive elements of the k-mer chain. We briefly review the classical description of the nematic transition presented by this system for k≥7 observing that the high-coverage (θ) transition deserves a more careful analysis from the entropy point of view. We present a possible viewpoint for this analysis that justifies the phase transitions. Moreover, we perform Monte Carlo (MC) simulations in the grand canonical ensemble, supplemented by thermodynamic integration, to first calculate the configurational entropy of the adsorbed phase as a function of the coverage, and then to explore the different phases (and orientational transitions) that appear on the surface with increasing the density of adsorbed k-mers. In the limit of θ→1 (full coverage) the configurational entropy is obtained for values of k ranging between 2 and 10. MC data are discussed in comparison with recent analytical results [D. Dhar and R. Rajesh, Phys. Rev. E 103, 042130 (2021)2470-004510.1103/PhysRevE.103.042130]. The comparative study allows us to establish the applicability range of the theoretical predictions. Finally, the structure of the high-coverage phase is characterized in terms of the statistics of k×l domains (domains of l parallel k-mers adsorbed on the surface). A distribution of finite values of l (l≪L) is found with a predominance of k×1 (single k-mers) and k×k domains. The distribution is the same in each lattice direction, confirming that at high density the adsorbed phase goes to a state with mixed orientations and no orientational preference. An order parameter measuring the number of k×k domains in the adsorbed layer is introduced.

Delicately Controlled Polymer Orientation for High-Performance Non-Fullerene Solar Cells with Halogen-Free Solvent Processing.

Molecular orientation in polymer solar cells (PSCs) is a critical subject of investigation that promotes the quality of bulk heterojunction morphology and power conversion efficiency (PCE). Herein, the intrinsic polymer orientation transition can be found upon delicate control over the branching point position of the irregular alkoxy side chain in difluoroquinoxaline-thiophene-based conjugated polymers. Three polymers with branching points at the third, fourth, and fifth positions away from the backbone were synthesized and abbreviated as PHT3, PHT4, and PHT5, respectively. Temperature-dependent absorption behavior manifests the polymer aggregation ability in the order of PHT3 < PHT4 < PHT5. Surprisingly, the polymer orientation transition from typical face-on to edge-on emerged between PHT4 and PHT5, as evidenced by X-ray-scattering analysis. The enhanced face-on crystallinity of PHT4 endowed the o-xylene-processed PHT4:IT-4Cl-based devices with the highest PCE of 13.40%. For PHT5 with stronger aggregation, the related o-xylene-processed PSCs still showed a good PCE of 12.66%. Our results demonstrate that a delicate polymer orientation transition could be realized through a precisely controlled strategy of the side chain, yielding green-solvent-processed high-performance PSCs.

A novel biosensing of histamine based on liquid crystal through dielectric and electro-optical approaches

The effects of histamine on blood vessels play a key role in immune response at the time of physical damage or some allergic reaction. The prevalent measurement methods for the detection of histamine are amperometric and capacitive biosensor techniques. Here, we present an easy fabrication and optimization studies of a novel liquid crystal (LC) based biosensor capable of detecting histamine level. The proposed biosensor is based on the orientational transition of nematic LC (NLC), 4′-octyl-4-biphenylcarbonitrile by enzymatic reaction of biomolecules. The sensing mechanism is based on homeotropic-to-planar configurational change of long molecular axes of LC when diamine oxidase/histamine was immobilized on the pretreated glass substrate. The developed LC-based histamine biosensor provides a good optical and dielectric response at a detection limit of 20 mg/L–500 mg/L.

Two-stage strike-slip faulting of the Altyn Tagh Fault revealed by magnetic fabrics in the Qaidam Basin

The timing and mechanism of the changes in the stress/strain field direction on the northeastern Tibetan Plateau are controversial, which limits our understanding of the tectonic evolution of the northeastern Tibetan Plateau and the Altyn Tagh Fault (ATF). Here, we conducted a detailed anisotropy of low-field magnetic susceptibility analysis of successive fluvial-lacustrine sediments deposited since the early Oligocene at Eboliang in the middle-western Qaidam Basin. Magnetic fabrics of the upper part (the Qigequan Formation) are predominantly considered to be primary sedimentary magnetic fabrics, while those of the lower-middle parts (the Shangganchaigou to Shizigou Formations) are mainly pre-folding, layer-parallel shortening induced embryonic tectonic fabrics. The magnetic lineation revealed ~50° successive clockwise rotations of the compressional strain orientations from nearly N-S to NE-SW in the middle-western Qaidam Basin during ~15–7 Ma. The nearly N-S compressional event is consistent with the generally N-S-directed India-Asia collision and thus was attributed to its far-field effects, whereas the post-mid-Miocene transition of ~50° clockwise rotations of the compressional strain orientations is consistent with the tectonic deformation in the basin interior. This transition lagged far behind that in the northern Qaidam marginal thrust belt (NQMTB), which began in the early Oligocene. The laterally diachronous transition in compressional strain orientation and deformations in the middle-western and northern Qaidam Basin were most likely due to the two-stage, sinistral strike-slip faulting of the ATF, with confined sinistral shear occurring along the ATF and partially transferred into the NQMTB during the early Oligocene to middle Miocene, while distributed sinistral shear occurred on the northeastern Tibetan Plateau beginning in the middle Miocene. A bidirectional propagation of compressional strain variation and tectonic deformation from the northern Qaidam Basin since the early Oligocene is proposed to reflect the tectonic evolution of the northeastern Tibetan Plateau.

Controlling Spin Orientation and Metamagnetic Transitions in Anisotropic van der Waals Antiferromagnet CrPS4 by Hydrostatic Pressure

AbstractControlling the phases of matter is a central task in condensed matter physics and materials science. In 2D magnets, manipulating spin orientation is of great significance in the context of the Mermin–Wagner theorem. Herein, a systematic study of temperature‐ and pressure‐dependent magnetic properties up to 1 GPa in van der Waals CrPS4 is reported. Owing to the temperature‐dependent change of the magnetic anisotropy energy, the material undergoes a first‐order spin reorientation transition with magnetic moments realigning from being almost parallel with the c axis in the ac plane to the quasi‐1D chains of CrS6 octahedra along the b axis upon heating. The spin reorientation temperature is suppressed after applying pressure, shifting the high‐temperature phase to lower temperatures with the emergence of spin‐flop transitions under magnetic fields applied along the b axis. The saturation field increases with pressure, indicating the enhancement of interlayer antiferromagnetic coupling. However, the Néel temperature is slightly reduced, which is ascribed to the suppression of intralayer ferromagnetic coupling. The work demonstrates the control of spin orientation and metamagnetic transitions in layered antiferromagnets, which may provide new perspectives for exploring 2D magnetism and related spintronic devices.

Orientational order and phase transitions in deuterated methane: a neutron total scattering and reverse Monte Carlo study

We report a study of the orientational order and phase transitions in crystalline deuterated methane, carried out using neutron total scattering and the reverse Monte Carlo method. The resultant atomic configurations are consistent with the average structures obtained from Rietveld refinement of the powder diffraction data, but additionally enable us to determine the C–D bond orientational distribution functions (ODF) for the disordered molecules in the high-temperature phase, and for both ordered and disordered molecules in the intermediate-temperature phase. We show that this approach gives more accurate information than can been obtained from fitting a bond ODF to diffraction data. Given the resurgence of interest in orientationally-disordered crystals, we argue that the approach of total scattering with the RMC method provides a unique quantification of orientational order and disorder.

Enabling three-dimensional porous architectures via carbonyl functionalization and molecular-specific organic-SERS platforms

Molecular engineering via functionalization has been a great tool to tune noncovalent intermolecular interactions. Herein, we demonstrate three-dimensional highly crystalline nanostructured D(C7CO)-BTBT films via carbonyl-functionalization of a fused thienoacene π-system, and strong Raman signal enhancements in Surface-Enhanced Raman Spectroscopy (SERS) are realized. The small molecule could be prepared on the gram scale with a facile synthesis-purification. In the engineered films, polar functionalization induces favorable out-of-plane crystal growth via zigzag motif of dipolar C = O···C = O interactions and hydrogen bonds, and strengthens π-interactions. A unique two-stage film growth behavior is identified with an edge-on-to-face-on molecular orientation transition driven by hydrophobicity. The analysis of the electronic structures and the ratio of the anti-Stokes/Stokes SERS signals suggests that the π-extended/stabilized LUMOs with varied crystalline face-on orientations provide the key properties in the chemical enhancement mechanism. A molecule-specific Raman signal enhancement is also demonstrated on a high-LUMO organic platform. Our results demonstrate a promising guidance towards realizing low-cost SERS-active semiconducting materials, increasing structural versatility of organic-SERS platforms, and advancing molecule-specific sensing via molecular engineering.

The Orientation Transition and Control of REBaCu3Oy Films Grown by Liquid Phase Epitaxy

The Orientation Transition and Control of REBaCu₃O_<i>y</i> Films Grown by Liquid Phase Epitaxy

Modeling of the Effects of Metal Complexation on the Morphology and Rheology of Xanthan Gum Polysaccharide Solutions

Polysaccharide solutions commonly contain metal ions, which form cross-linking complexes with polymer ligands and significantly affect the polymer morphology and solution rheology. Here, we study by multiscale computational modeling complemented with atomic force microscopy (AFM) measurements the molecular mechanisms of metal complexation with examples of xanthan gum (XG) solutions containing ZnCl2. We develop original atomistic molecular dynamics (MD) and coarse-grained dissipative particle dynamics (DPD) models for XG chains of different compositions and Zn complexation. MD simulations reveal that Zn dramatically affects conformations of individual XG chains, causing a reduction in the radius of gyration by the formation of various types of intrachain cross-linking that are enhanced by the increased pyruvate content in the chains. DPD simulations at the static conditions and under the shear flow show that salt addition leads to gelation in XG systems via a sol–gel transition due to enhanced interchain cross-linking. Two morphological transitions occur in the shear flow: an orientational transition to a nematic phase with XG chains aligned in the direction of flow and a bundling transition accompanied by a reverse gel–sol transition at high Zn concentrations. The presence of the increased pyruvate content in the chains leads to stronger aggregation and stiffer aggregates, in qualitative agreement with AFM results. The proposed computational methodology of modeling the effects of metal complexation and chain composition on morphological and rheological properties may be extended and applied to various polysaccharide systems.

Preparation of poly(lactic acid) with excellent comprehensive properties via simple deformation or microfibrillation of spherulites

AbstractOur work overcomes the long‐standing challenge to achieve comprehensive improvements in thermo/mechanical properties of poly(lactic acid) (PLA), especially for impact toughness, strength, ductility, stiffness, and heat resistance. Simple deformation or microfibrillation of spherulites can create excellent thermo/mechanical properties making PLA compete against conventional petrochemical‐based polymers. It was performed by pressure‐induced‐flow (PIF) processing, and produced typical deformed spherulite structure and brick‐mud structure. It was found that the critical deformation degree of spherulites was around 2.7, beyond which spherulites would fragment. Through three transition stages of spherulite structures, deformed spherulite structures, and brick‐mud structure, PLA exhibited an enhancement of orientation, storage modulus and glass transition temperature. And a substantial increase was proved in impact strength (105.6 kJ/m2), tensile strength (148.4 MPa) and elongation at break (107.7%), which can be comparable to most engineering plastics. Both the deformed spherulite structures and the brick‐mud structures can enhance the impact toughness of PLA by increasing the fracture path of cracks. Besides, the evolution of the microstructure for a sandwich structure during PIF‐processing was proposed.

Liquid crystal-amplified optofluidic biosensor for ultra-highly sensitive and stable protein assay

Protein assays show great importance in medical research and disease diagnoses. Liquid crystals (LCs), as a branch of sensitive materials, offer promising applicability in the field of biosensing. Herein, we developed an ultrasensitive biosensor for the detection of low-concentration protein molecules, employing LC-amplified optofluidic resonators. In this design, the orientation of LCs was disturbed by immobilized protein molecules through the reduction of the vertical anchoring force from the alignment layer. A biosensing platform based on the whispering-gallery mode (WGM) from the LC-amplified optofluidic resonator was developed and explored, in which the spectral wavelength shift was monitored as the sensing parameter. The microbubble structure provided a stable and reliable WGM resonator with a high Q factor for LCs. It is demonstrated that the wall thickness of the microbubble played a key role in enhancing the sensitivity of the LC-amplified WGM microcavity. It is also found that protein molecules coated on the internal surface of microbubble led to their interactions with laser beams and the orientation transition of LCs. Both effects amplified the target information and triggered a sensitive wavelength shift in WGM spectra. A detection limit of 1 fM for bovine serum albumin (BSA) was achieved to demonstrate the high-sensitivity of our sensing platform in protein assays. Compared to the detection using a conventional polarized optical microscope (POM), the sensitivity was improved by seven orders of magnitude. Furthermore, multiple types of proteins and specific biosensing were also investigated to verify the potential of LC-amplified optofluidic resonators in the biomolecular detection. Our studies indicate that LC-amplified optofluidic resonators offer a new solution for the ultrasensitive real-time biosensing and the characterization of biomolecular interactions.

A stable liquid crystals sensing platform decorated with cationic surfactant for detecting thrombin

A liquid crystals (LCs) sensing platform to detect thrombin was demonstrated based on high specificity of aptamer recognition. Initially, LCs doped with a cationic surfactant, octadecyl-trimethylammonium bromide (OTAB), adopted homeotropic alignment corresponding to dark optical image. Upon introduction of thrombin aptamer, the orientational transition of LCs from homeotropic to tilted state occurred due to the electrostatic interaction between it and OTAB. And accordingly a visual dark-to-bright alteration was observed for the polarized light microscope (POM) image of LCs. In the presence of thrombin with which the aptamer preferred to bind, OTAB released could induce the vertical arrangement of LCs and a dark optical image was regained. Therefore, by observing the changes of bright and dark optical appearances for LCs, thrombin could be analyzed with a detection limit of ~ 136 nM. The LCs sensing system, assisted with the specific combination of target molecule and its corresponding aptamer, reveals a simple and effective way to achieve label-free and low-cost detection of biomolecules in related clinic diagnosis.