Molecular Disordering Research Articles

Analysis of Molecular Disordering Processes in the Phase Transition of Liquid Crystals Observed by Patterned-Illumination Time-Resolved Phase Microscopy.

The initial processes of the phase transition dynamics of liquid crystals (LCs) subject to UV pulse irradiation were clarified using a nanosecond time-resolved imaging technique called pattern-illumination time-resolved phase microscopy (PI-PM). Two types of LCs were studied: a photo-responsive LC and dye-doped LCs. We found two steps of molecular disordering processes in the phase transition, namely local disordering proceeding anisotropically, followed by the spreading of the isotropic phase. These two processes were separated for a photo-responsive LC while being simultaneously observed for the dye-doped LCs. It was found that the photomechanical dyes induced the phase transition process faster than the photothermal dyes.

Reversible colossal barocaloric effects near room temperature in 1-X-adamantane (X=Cl, Br) plastic crystals

Plastic crystals undergo phase transitions with unusually large volume and entropy changes related to strong molecular orientational disordering. These features have led to a resurgent interest in these materials because recently they have shown great potential in solid-state cooling applications driven by pressure. Here we demonstrate that two plastic crystals derived from adamantane -1-Br-adamantane and 1-Cl-adamantane- undergo colossal reversible barocaloric effects under moderate pressure changes in a wide temperature span near room temperature thanks to a relatively small hysteresis and very high sensitivity of the transition temperature to pressure. In particular, 1-Cl-adamantane displays an optimal operational temperature range covering from ~40 K below and up to room temperature, and under a pressure change of 1 kbar this compound outperforms any other barocaloric material known so far. Our work gives strong support to plastic crystals as best candidates for barocaloric cooling. We also provide insight into the physical origin of the entropy changes through the analysis of the disorder on the involved phases.

Transition in the Acid–Base Component of Surface Free Energy of Ice upon the Premelting of Its Second Molecular Bilayer

Molecular disordering of the ice surface occurs below the bulk melting temperature of 273 K, termed surface premelting. The top-most molecular layer begins gradually premelting at 200 K, and has been linked to its low coefficient of friction through an increase in molecular mobility. The second molecular bilayer premelts around 257 K, but no study has linked this transition to a change in any macroscopic phenomena. Here, we show that the thermodynamic work of adhesion between polydimethylsiloxane (PDMS) and ice changes abruptly at 257.0 $\pm$ 0.1 K. Surface-sensitive sum frequency generation spectroscopy shows that there are no molecular level changes at the PDMS surface or the ice-PDMS interface near the transition in adhesion, indicating that the transition arises from changes of the ice surface. Using existing contact angle data in the literature, we show that this transition is due to a decrease in the acid-base component of the surface free energy of ice by 17 $\pm$ 2 mJ/m$^2$ at 257.0 $\pm$ 0.1 K. The change in surface energy provides a possible explanation for a variety of unexplained phenomena seen across the literature including ice adhesion, friction, and the morphology of snowflakes.

Electro-Optic Modulation in Hybrid Metal Halide Perovskites.

Rapid and efficient conversion of electrical signals to optical signals is needed in telecommunications and data network interconnection. The linear electro-optic (EO) effect in noncentrosymmetric materials offers a pathway to such conversion. Conventional inorganic EO materials make on-chip integration challenging, while organic nonlinear molecules suffer from thermodynamic molecular disordering that decreases the EO coefficient of the material. It has been posited that hybrid metal halide perovskites could potentially combine the advantages of inorganic materials (stable crystal orientation) with those of organic materials (solution processing). Here, layered metal halide perovskites are reported and investigated for in-plane birefringence and linear electro-optic response. Phenylmethylammonium lead chloride (PMA2 PbCl4 ) crystals are grown that exhibit a noncentrosymmetric space group. Birefringence measurements and Raman spectroscopy confirm optical and structural anisotropy in the material. By applying an electric field on the crystal surface, the linear EO effect in PMA2 PbCl4 is reported and its EO coefficient is determined to be 1.40 pm V-1 . This is the first demonstration of this effect in hybrid metal halide perovskites, materials that feature both highly ordered crystalline structures and solution processability. The in-plane birefringence and electro-optic response reveal that layered perovskite crystals could be further explored for potential applications in polarizing optics and EO modulation.

All-solution-processed PIN architecture for ultra-sensitive and ultra-flexible organic thin film photodetectors

An ideal organic thin film photodetectors (OTFPs) should adopt a hierarchical, multilayer p-type/blend-type/n-type (PIN) structure, with each layer having a specific purpose which could greatly improve the exciton dissociation while guarantee efficient charge transport. However, for the traditional layer-by-layer solution fabrication procedure, the solvent used can induce organic material mixing and molecular disordering between each layer. Hence, such architecture for OTFPs can now only be formed via thermal evaporation. In this paper, a contact-film-transfer method is demonstrated to all-solution processing organic PIN OTFPs on flexible substrates. The fabricated PIN OTFPs exhibit high photoresponse and high stability under continuous mechanical bending. Hence, the method we described here should represent an important step in the development of OTFPs in the future.

Polarization-dependent NEXAFS study of adsorption of long-chain surfactants on mechanically milled iron powder

In this work we have demonstrated the possibility of using the polarization-dependent NEXAFS spectra to study the structure of organic layers at the surface of powdered materials with plate-like shaped particles. The polarization dependence of the NEXAFS spectra may be easily obtained by just changing the angle between the X-ray beam direction and the substrate onto which the powder particles are set. For the first time, we have carried out a detailed study of the surfactant layers (n-perfluorononanoic and stearic acid), which are formed at the surface of iron plate-like particles under mechanical milling of iron powder with an addition of corresponding surfactants. The surfactant molecules are predominantly oriented perpendicular to the surface of the mechanically milled particles. Such orientation is similar to the arrangement of the molecules in the layers formed under equilibrium conditions, e.g. deposition from solutions. The changes in the chemical environment occurring in the molecule tails (defluorination or dehydrogenation) under mechanochemical treatment, do not result in a significant change in the molecular orientation and disordering of the adsorbed layer.

Ring patterns in liquid crystals aligned by phase-separated two-component monolayers

We investigate the alignment of 4′-pentyl-4-cynobiphenyl (5CB) liquid crystal on phase-separated two-component organosilane monolayers that consist of the protruding islands surrounded by a continuous phase. The molecular disordering at the protruding island edge locally perturbs the homeotropic alignment of the 5CB induced the phase-separated monolayers, leading to the formation of ring patterns. The edge effect on the 5CB alignment decreases with the decrease of the height difference between the protruding islands and the surrounding monolayer and increases with the decrease of the size of the protruding islands.

Enhancement of Surface Potential Buildup and Decay of Tris(8-quinolinato)aluminum Film Deposited on Ultraviolet/Ozone-Treated Gold Electrodes

The enhancement of the surface potential of tris(8-quinolinato)aluminum (Alq3) film deposited on ultraviolet (UV)/ozone-treated gold electrodes was confirmed by surface potential measurement, and the ordering of molecular dipoles was suggested. The enhanced surface potential of the order of 10 V decayed rapidly by photoirradiation, due to molecular dipole disordering, but a surface potential of 100–400 mV remained. Interestingly, this remaining surface potential of Alq3 on the UV/ozone-treated gold electrode was always smaller than that of Alq3 film on an untreated gold electrode. It was concluded that the UV/ozone treatment is effective in decreasing the density of surface states at the Alq3-metal interface as well as for the enhancement of molecular dipole ordering.

A crystal structure of ultra-dispersed form of polytetrafluoroethylene based on X-ray powder diffraction data

An X-Ray powder diffraction study of ultra-dispersed polytetrafluoroethylene was carried out. As well as a regular polytetrafluoroethylene the ultra-dispersed form contents a high proportion of the crystalline phase. The X-ray diffraction pattern could be described with two-dimensional hexagonal unit cell [a=5.685(1) Å, symmetry group p6mm]. Structural modeling with a continuous electron density approach as well as with a discrete disordered atoms distribution was accomplished. The model was refined using the Rietveld method. The structure is characterized by a spiral arrangement of polymers (CF2-)n along the z-axis with complete mutual disordering by rotational displacement around z, as well as a partial molecular translation along the z-axis. Molecular disordering results in a systematic absence of reflections with 1≠0 and as a sequence in two-dimensional unit cell effect. The presence of complete rotational disordering distinguishes the ultra-dispersed form of polytetrafluoroethylene from the standard one (fluoroplast-4), where only partial disordering is observed.

Effects of temperature and glycerol on the de-organisation of hsian-tsao ( Mesona procumbens Hemsl) polysaccharide solution by electron spin resonance spectroscopy

The effect of different glycerol concentration on the molecular transition on heating of hsian-tsao polysaccharide solution was investigated by electron spin resonance (ESR), using nitroxide radicals (4-amino TEMPO) as labels that covalently attached to the carboxyl groups of uronic acid of the hsian-tsao polysaccharide chains. As indicated by the increased rotational correlation time ( τ c) and broadening ESR resonance signals due to anisotropic effects, the mobility of the polymer chains became more restricted with reducing temperature and increasing glycerol concentration (0–80%). A change in anisotropic ratio ( p value) likely reflecting the molecular dissociation and conformational disordering of the polysaccharide was observed during heating at 40–50 °C, and being more remarkable as glycerol concentration increased. The increment in glycerol concentration resulted in a two-fold increase in activation energy of the conformational transition (from 17.93 to 35.15 kJ/mol).

X-RAY CRYSTALLOGRAPHIC INVESTIGATIONS AND CHARACTERIZATIONS OF THREE MIXED ANIONIC HEXACYANO COMPLEXES

The mixed anionic complexes, Cd3 1l[(CrIII/IrIII)(CN)6]2 13H2O, Co3 11[(CoIII/IrIII)(CN)6]2-13H2O, and Pd3 II[(CrIII/IrIII)(CN)6]2 13H2O, crystallize in the cubic centrosymmetric space group Fm3m (No. 225) with αo= 10.884(1), 10.396(1), and 10.455(2)Å, respectively. The disordered structures contain 11/3 molecules in the unit cell. Full-matrix least-squares refinements yielded final reliability (R) index factors of 0.040, 0.050, and 0.069 with ‚goodness-of-fit’ (GOF, ∑2) values of 1.26, 1.25, and 1.11, respectively. All metal atoms in each compound are six-coordinate octahedral and the cationic divalent metal atoms are linked to the (1:1) mixed trivalent metal atoms by cyanide bridging. Networks of hydrogen-bonding are setup within cavities of the crystal lattices (created by molecular disordering). Characterization of each hexacyano complex includes physical property determinations, conoscopic studies, I. R. spectroscopic identifications, and thermal gravimetric analyses (TGA). Selective bond distances and angles are tabularized.

Probing the structure of solids in a liquid environment: a recent in-situ crystallisation experiment using high energy wavelength scanning X-ray diffraction

Structural development associated with the crystallisation of n-eicosane is probed in-situ, using high energy wavelength scanning X-ray diffraction (HEWSXRD) at the synchrotron radiation source at HASYLAB, Germany. Preliminary data reveal confirmatory evidence for an undefined structural phase transformation close to the melting point, which is probably related to increased molecular disordering as a function of temperature. The experiment confirms that information on the structure of crystalline particles can be obtained in-situ as they form.

Molecular disordering in natural cokes approaching dyke and sill contacts

Optical properties of vitrinites often display reversals in trend approaching contacts with igneous intrusions. Reflectance (maximum, average or random) is the most common property affected within thermal aureoles, but parameters of optical anisotropy, for example bireflectance or bireflectance ratio, may also show reversed trends close to intrusions. The suggestion that increasing randomness of the direction of polarization of discrete areas within affected coal material is responsible for a fall in bireflectance ratio, does not seem to explain all cases. The reversal phenomenon may possibly be related to more general molecular disordering, due to the thermal effects, and/or to difficulties in producing a polished surface of quality after the vitrinite has been transformed to carbonaceous material with a turbostratic or paracrystalline structure, or to fully ordered graphite. General molecular disordering as a cause is now a real possibility on the basis of correlated optical and X-ray diffraction data from samples analysed from within a thermal aureole of a Tertiary dyke emplaced in Permian coal-bearing strata. The influences of different factors (principally temperature level, time of exposure to elevated temperature, degree of consolidation of sediments and their porosities and permeabilities, initial maturation level of the vitrinite before intrusion, overburden pressure and stress) on the maturation level of the organic matter are considered. There is as yet no unequivocal single explanation for the reversals of property trends, but the reversals seem to be related to ‘optical graphites’ that have developed at much lower temperatures than those required to produce graphite in the laboratory. The reversals may be due to a combination of molecular disordering and variable surface quality, upon which differing orientations of highly optically anisotropic fragments in the samples may also have some bearing.

Membrane tolerance to ethanol is rapidly lost after withdrawal: a model for studies of membrane adaptation.

The structural properties of liver microsomes and erythrocytes obtained from rats that had been chronically administered ethanol were examined by electron spin resonance (ESR) following ethanol withdrawal for 1-10 days. Membranes obtained from control animals exhibited considerable molecular disordering upon the addition of ethanol in vitro (50-100 mM). Conversely, microsomal and erythrocyte membranes from alcoholic animals were resistant to this disordering by ethanol (membrane tolerance). These membrane properties were also apparent in lipid bilayers comprised of either total lipids or phospholipids isolated from the control and alcoholic animals. While several weeks of ethanol administration were required for both erythrocytes and microsomes to develop membrane tolerance, erythrocytes from alcoholic animals were disordered by ethanol in vitro after the animals had been withdrawn from ethanol for only 1 day. The same rapid loss of tolerance was observed in microsomes after 2 days of withdrawal. The same time course for the loss of tolerance was observed in lipid bilayers prepared from the total lipid and phospholipid extracts. No significant differences in the cholesterol/phospholipid ratio were observed between the microsomal or erythrocyte membranes isolated before and after withdrawal. Thus, alterations in the microsomal and erythrocyte phospholipids, and not cholesterol content, were responsible for conveying membrane tolerance. Membrane structural properties can be rapidly adjusted in a mammalian system in response to the withdrawal of the external membrane perturbant ethanol. The withdrawal model, which begins with established membrane tolerance and leads to rapid and complete loss of tolerance, provides a model to analyze the compositional changes responsible for this tolerance to disordering by ethanol.

Chronic ethanol ingestion increases calcium uptake and resistance to molecular disordering by ethanol in liver microsomes.

Calcium uptake and molecular ordering were studied in hepatic microsomes from rats treated chronically with ethanol and were compared with the effects of ethanol in vitro. Calcium uptake was increased by 30% in microsomes from ethanol-fed animals, the Vmax being increased and the Km remaining unchanged. In Arrhenius plots of calcium uptake, the transition temperature in the 25-30 degrees C range was higher by 1.5 degrees C in microsomes from ethanol-fed rats. Chronic ethanol administration resulted in more rigid microsomal membranes, as evidenced by EPR spectra of the spin probe 5-doxylstearic acid. In a plot of the order parameter, S, as a function of temperature, microsomes from ethanol-fed rats showed a discontinuity at a higher temperature than those from controls (29.9 versus 24.2 degrees C). Ethanol, in vitro, inhibited microsomal calcium uptake in both preparations. However, the degree of inhibition was greater in controls. The rate of calcium uptake by microsomes from ethanol-treated animals in the presence of 100 mM ethanol was the same as those from controls in the absence of ethanol. In vitro, increasing concentrations of ethanol progressively decreased molecular ordering of control microsomes, whereas little change was noted in microsomes after chronic ethanol treatment.

Disorder in the crystal structures of paradichlorobenzene

A theory for certain molecular crystals of the relation of lattice vacancies to molecular orientational disorder is developed and applied to the case of para-dichlorobenzene. For a melt-grown or solution-grown crystal substantial molecular orientational disordering is predicted between 250 K and the melting point at 326 K. Comparison with experimental hydrogen atom Debye-Waller factors supports this conclusion. Such disorder may be common in molecular crystals, the requirements being (a) a high concentration of relatively immobile vacancies (> 10-3), (b) two other crystal structures close in free energy to the stable structure (differing in internal energies by less than 8 kJ mole-1).

Rheological properties of rubbery polymers at low shear stresses

The rheological behavior of the rubbery amorphous polymers SKB-35 (sodium-butadiene rubber), SKN-26M(butadiene-nitrile rubber), and PIB-85 (polyisobutylene) has been investigated in relation to the creep process. The tests were conducted at low shear stresses, in the constant shear stress regime, on the temperature interval from 0 to 100°C using a parallel-plate viscometer. We have shown, for the first time, by a viscometric method that in the high-elastic state rubbery polymers have a particular equilibrium structure corresponding to each temperature. A reduction in temperature leads to molecular ordering processes associated with an increase in the viscosity of the polymer. A temperature rise leads to molecular disordering and a gradual fall in viscosity. At a given temperature the two processes converge to the same value of the equilibrium viscosity. The rate of the process of equilibrium structure formation is determined not only by the temperature but also by the nature of the polymer. The higher the polarity, the more slowly the equilibrium structure is established.

Infrared studies of chain folding in polymers. X. polycaprolactam

Abstract The 1210-, 1233-, and 1288—cm−1 infrared bands in the α form of nylon 6 are shown to result from a unique conformation in a tightly folded chain with adjacent reentry. The assignment of the 1210- and 1288-cm−1 bands to motions of a gauche nitrogen-methylene group in the fold and the 1233-cm−1 band to motions of a gauche carboxyl-methylene group in the fold is supported by the infrared studies of the α form of nylon 6, the γ form of nylon 6, poly-D(-)β-methyl- e -caproamide (PBCA), and the high- and low-temperature forms of the cyclic dimer of nylon 6. The γ- form of nylon 6 and PBCA do not exhibit a unique conformation in the fold. The 1198-cm−1 band can be assigned to the trans conformation of the amide group which exists in the normal planar zigzag conformation in the α-form crystal. A mechanism for the α to γ iodine-induced transition in nylon 6 is proposed showing the role of the fold in maintaining a minimum of molecular disordering during the transformation.