Temperature-dependent Structural Behavior Research Articles

Thermal Polymorphism in CsCB11H12.

Thermal polymorphism in the alkali-metal salts incorporating the icosohedral monocarba-hydridoborate anion, CB11H12-, results in intriguing dynamical properties leading to superionic conductivity for the lightest alkali-metal analogues, LiCB11H12 and NaCB11H12. As such, these two have been the focus of most recent CB11H12- related studies, with less attention paid to the heavier alkali-metal salts, such as CsCB11H12. Nonetheless, it is of fundamental importance to compare the nature of the structural arrangements and interactions across the entire alkali-metal series. Thermal polymorphism in CsCB11H12 was investigated using a combination of techniques: X-ray powder diffraction; differential scanning calorimetry; Raman, infrared, and neutron spectroscopies; and ab initio calculations. The unexpected temperature-dependent structural behavior of anhydrous CsCB11H12 can be potentially justified assuming the existence of two polymorphs with similar free energies at room temperature: (i) a previously reported, ordered R3 polymorph stabilized upon drying and transforming first to R3c symmetry near 313 K and then to a similarly packed but disordered I43d polymorph near 353 K and (ii) a disordered Fm3 polymorph that initially appears from the disordered I43d polymorph near 513 K along with another disordered high-temperature P63mc polymorph. Quasielastic neutron scattering results indicate that the CB11H12- anions in the disordered phase at 560 K are undergoing isotropic rotational diffusion, with a jump correlation frequency [1.19(9) × 1011 s-1] in line with those for the lighter-metal analogues.

Temperature influence on impact protection performance of steel-plastic structures–Manifested by head impact against pillars of passenger car

Structures for impact protection often consist of multiple thin-walled sections with metallic and polymer-based materials for lightweighting. In the range of vehicle service temperature from -40 °C to 80 °C, mechanical properties of steels are not temperature sensitive while those of polymer-based materials may change as much as three times, causing different impact responses of steel-plastic structures in summer and winter. The temperature dependent structural behavior is further compounded by limited space for intrusion. Vehicle B-pillar has a thin-walled steel section for meeting its stiffness and strength requirements and a thin-walled plastic trim interior for occupant head impact protection. The structure has limited depth along the impact direction and its materials have different temperature properties, and therefore, it is a good example for analyzing temperature influence on impact response of steel-plastic structures. In this study, we have revealed that the impact response is highly temperature dependent and the mechanisms of causing downgrade protection performance are different in winter and summer. For head impact against vehicle B-pillar interior, at winter temperature, the plastic trim is harder, and thus the peak acceleration of the head is greater than that at room temperature, which is not good for protection. At summer temperature, the plastic trim is softer, and thus the head impact tends to bottom out the plastic trim and the residual kinetic energy would further deform the steel section, leading to higher peak acceleration and larger intrusion than that at room temperature.

Toward a Designable Extracellular Matrix: Molecular Dynamics Simulations of an Engineered Laminin-Mimetic, Elastin-Like Fusion Protein.

Native extracellular matrices (ECMs) exhibit networks of molecular interactions between specific matrix proteins and other tissue components. Guided by these naturally self-assembling supramolecular systems, we have designed a matrix-derived protein chimera that contains a laminin globular-like (LG) domain fused to an elastin-like polypeptide (ELP). This bipartite design offers a flexible protein engineering platform: (i) laminin is a key multifunctional component of the ECM in human brains and other neural tissues, making it an ideal bioactive component of our fusion, and (ii) ELPs, known to be well-tolerated in vivo, provide a self-assembly scaffold with tunable physicochemical (viscoelastic, thermoresponsive) properties. Experimental characterization of novel proteins is resource-intensive, and examining many conceivable designs would be a formidable challenge in the laboratory. Computational approaches offer a way forward: molecular dynamics (MD) simulations can be used to analyze the structural/physical behavior of candidate LG-ELP fusion proteins, particularly in terms of conformational properties salient to our design goals, such as assembly propensity in a temperature range spanning the inverse temperature transition. As a first step in examining the physical characteristics of a model LG-ELP fusion protein, including its temperature-dependent structural behavior, we simulated the protein over a range of physiologically relevant temperatures (290-320 K). We find that the ELP region, built upon the archetypal (VPGXG)5 scaffold, is quite flexible and has a propensity for β-rich secondary structures near physiological (310-315 K) temperatures. Our trajectories indicate that the temperature-dependent burial of hydrophobic patches in the ELP region, coupled to the local water structure dynamics and mediated by intramolecular contacts between aliphatic side chains, correlates with the temperature-dependent structural transitions in known ELP polymers. Because of the link between compaction of ELP segments into β-rich structures and differential solvation properties of this region, we posit that future variation of ELP sequence and composition can be used to systematically alter the phase transition profiles and, thus, the general functionality of our LG-ELP fusion protein system.

Temperature-dependent structural behavior of mixed-metal hydroxides in air

A co-precipitation process using transition metal hydrates was used to prepare a mixture of M(OH)2 and MO(OH) (M=Ni0.5Co0.2Mn0.3). This powder consisted of dense, quasi-spherical particles each composed of plate-like particles of ca. 250nm across. By increasing the temperature, M(OH)2 was made to transform into MO(OH) at ca. 220°C, then into M3O4 at ca. 330°C. Moreover, the M3O4 phase persisted without further reaction until 700°C. During these phase transitions, the mixed-metal (M) in M(OH)2, MO(OH) and M3O4 retained disordered cation distributions at the same crystallographic sites of their host lattices without appearance of any phases related to Ni, Co and Mn. The MO(OH) phase appeared owing to exposure to air. M(OH)2 at room temerature had the lattice parameters (a=b=2.9991(2) Å and c=8.5892(2) Å) with space group (P63/mmc).

Self-Assembled Nanostructures of Fully Hydrated Monoelaidin–Elaidic Acid and Monoelaidin–Oleic Acid Systems

In recent years, there has been a surge of interest in exploring the effect of trans-fatty acids (TFAs) on biological membrane properties. The research studies are motivated by an increasing body of evidence suggesting that the consumption of TFAs increases the risk of developing negative health effects such as coronary heart disease and cancer. The ultimate goal of studying the lipid-fatty acid interactions at the molecular level is to predict the biological role of fatty acids in cells. In this regard, it is interesting to elucidate the effect of loading TFAs and their counterpart cis-fatty acids (CFAs) on the physical properties of lipid model membranes. Here, the present study focuses on discussing the following: (1) the effect of mixing monoelaidin (ME, TFA-containing lipid) with its counterpart monoolein (MO, CFA-containing lipid) on modulating the fully hydrated self-assembled structure, and (2) the influence of solubilizing oleic acid (OA) and its trans counterpart elaidic acid (EA) on the fully hydrated ME system. The ME model membrane was selected due to its sensitivity to variations in lipid composition and temperature. Synchrotron small-angle X-ray scattering (SAXS) was applied for studying the temperature-dependent structural behavior of the fully hydrated ME/MO-based system prepared with an equal ME/MO weight ratio and also for characterizing the fully hydrated OA- and EA-loaded ME systems. Wide-angle X-ray (WAXS) experiments were also performed for characterizing the formed crystalline lamellar phases at ambient temperatures. The results demonstrate the significant influence of the partial replacement of ME by MO on the phase behavior. The addition of MO induces the lamellar-nonlamellar phase transitions at ambient temperatures and promotes the formation of the inverted type hexagonal (H(2)) phase above 72 °C. The fully hydrated ME/EA and ME/OA systems with their rich polymorphism exhibit an interesting temperature-dependent complex behavior. The experimental findings show that the temperature-induced phase transitions are dictated by the solubilized fatty acid concentration and its configuration. Both EA and OA have a significant impact on the fully hydrated ME system. Similar to previous published studies, OA induces a significantly stronger mean negative membrane curvature as compared to EA. The two phase diagrams are discussed in terms of water-lipid and lipid-fatty acid interactions, membrane bending, and lipid packing concepts. A newly observed interesting epitaxial relationship for the lamellar-hexagonal phase transition in the EA-loaded ME system is illustrated and discussed in detail.

Erratum: Temperature-dependent structural behavior of self-avoiding walks on three-dimensional Sierpinski sponges [Phys. Rev. E81, 051119 (2010)

Erratum: Temperature-dependent structural behavior of self-avoiding walks on three-dimensional Sierpinski sponges [Phys. Rev. E<b>81</b>, 051119 (2010)

Temperature-dependent structural behavior of self-avoiding walks on three-dimensional Sierpinski sponges

The statistics of self-avoiding random walks (SAWs), consisting of up to N=1280 steps, on deterministic fractal structures with infinite ramification, modeled by Sierpinski cubic lattices, in the presence of a finite temperature is investigated as a model for polymers absorbed on a disordered medium. Thereby, the three-dimensional Sierpinski sponge is defined by two types of sites with energy 0 and ϵ>0 , respectively, yielding a deterministic fractal energy landscape. The probability distribution function of the end-to-end distance of SAWs is obtained and its scaling behavior studied. In the limiting case of temperature T → ∞, the known behavior of SAWs on regular cubic lattices is recovered, while for T → 0 the resulting scaling exponents are confronted with previous calculations for much shorter linear chains based on the exact enumeration technique. For finite temperatures, the structural behavior of SAWs in three dimensions is compared to its two-dimensional counterpart and found to be intermediate between the two limiting cases (T → 0 and T → ∞, respectively), where the characteristic exponents, however, display a nontrivial dependence on temperature.

A Luminescent Ionic Liquid Crystal: [C12mim]4[EuBr6]Br

Abstract[C12mim]4[EuBr6]Br·CH3CN (2) was obtained at –15 °C by reacting europium bromide with [C12mim]Br (1‐dodecyl‐3‐methylimidazolium bromide) in acetonitrile. 2 crystallizes in the triclinic space group P$\bar {1}$ (No. 2) with two formula units in the unit cell. The structure is characterized by double layers of [C12mim] cations with interdigitated alkyl chains which are separated by anion layers formed by [EuBr6] octahedra and Br···CH3CN units. Upon warming to room temperature 2 looses acetonitrile to form the room temperature ionic liquid crystal [C12mim]4[EuBr6]Br (1). The temperature dependent structural behavior has been characterized by differential scanning calorimentry and hot‐stage polarizing optical microscopy. In the temperature range of –3 to 98 °C 1 adopts a smectic liquid crystal phase. At 77 K 1 shows a strong red emission with a lifetime of 2.6 ms. Analysis of the emission spectrum yields a quantum efficiency of 0.45.

Temperature-dependent structural behavior of self-avoiding walks on Sierpinski carpets

We study the temperature-dependent structural behavior of self-avoiding walks (SAWs) on two-dimensional Sierpinski carpets as a simple model of polymers adsorbed on a disordered surface. Thereby, the Sierpinski carpet defines two types of sites with energy 0 and >0 , respectively, yielding a deterministic fractal energy landscape. In the limiting cases of temperature T-->0 and T-->infinity , the known behaviors of SAWs on Sierpinski carpets and on regular square lattices, respectively, are recovered. For finite temperatures, the structural behavior is found to be intermediate between the two limiting cases; the characteristic exponents, however, display a nontrivial dependence on temperature.

Structural behavior, crystal chemistry, and phase transitions in substituted leucite; high-resolution neutron powder diffraction studies

High-resolution neutron powder diffraction was used to study phase transitions in the leucite phases of KAlSi206, RbAlSi20, CsAlSi206, and KFeSi206. The temperature-dependent structural behavior involves two mechanisms: relaxation of the tetrahedral framework about channel cations, and slowly changing T-O bond lengths. The high-temperature cubic phase is characterized by a fully-extended tetrahedral framework; thermal expansion occurs by an increase in mean T-0 bond lengths. On decreasing temperature, a displacive phase transition to tetragonal symmetry is manifested by an optic instability; twisting of tetragonal prisms of comer-linked (AI,Si)04 tetrahedra about [001] leads to collapse of the (111) structural channels and concomitant volume reduction.