Epitaxial etching of organic single crystals

Organic single crystals(OSCs) have received increasing interest in the last decades for their potential applications in flexible electronics. Although there are various subtractive manufacturing methods of organic electronics, the subtractive manufacturing of OSCs is still a challenge, since OSCs are assembled via weak van-der-Waals interactions which are vulnerable and cannot afford damages and suffer the degradation of performances after the process. Here, we develop an epitaxial etching strat...     

Determination of calcium,strontium and barium in single crystals of alkali halides by neutron activation

The determination by neutron activation of trace quantities of calcium, strontium and barium in single crystals of alkali halides has been studied. The matrix is eliminated before the irradiation by an ion-exchange procedure. Gamma spectroscopy after radiochemical separation is used to determine strontium and barium. Calcium is determined by measuring the β-activity of ⁴⁹Sc formed by the decay of ⁴⁹Ca. Long-lived radioactive tracers are used to determine the chemical yields.     

Epitaxial crystallization of polyoxymethylene on NaCl single crystal in dilute solution

The epitaxial crystallization of polyoxymethylene (POM) is observed from 0.01% cyclohexanol solution on the (001) cleavage face of NaCl. High-resolution electron microscopy reveals that the crystals are definitely triangular, rather than rectangular prisms, although the crystals have previously been interpreted as “edge-on” arrangements of lamellar crystals. An electron diffraction pattern obtained from films, which were piled up with a maximum of nine sheets per grid, shows that plane spacing of the epitaxial crystals is 3.87 Å and almost the same as the \[ \left\{ {10\bar 10} \right\} \] spacing (3.86 Å) of hexagonal single crystal of POM: the axes of POM fold chains in the crystals are parallel to the (001) plane. A possible mechanism for the epitaxial crystallization of POM is proposed.     

Crystallizability of ethylene homopolymers by crystallization analysis fractionation

The effect of molecular weight and long‐chain branching on the crystallization analysis fractionation (CRYSTAF) of ethylene homopolymers was investigated. Several ethylene homopolymers were prepared with different molecular weights and levels of long‐chain branching to isolate these effects from the dominant effect of comonomer content on crystallizability measured by CRYSTAF. Molecular weight effects might be significant for samples with number‐average molecular weights below 5000, but this effect can be corrected if terminal methyl groups are taken into account. Long‐chain branching has only a very small effect on the CRYSTAF profile of the samples investigated in this study. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 1616–1628, 2001     

Defect interactions in alkali halides

Pairwise defect interactions between divalent cation impurities, cation vacancies and anion vacancies have been studied in the extrinsic and intrinsic temperature ranges for the KClSr²⁺ system. Pair correlation functions have been derived for the various defect interactions distinguished by their site symmetries and these show the existence of associated divalent cation—cation vacancy and cation—anion vacancy pairs even at high temperatures. Pair formation gives way to dissociation at low impurity concentrations and high temperatures.     

Epitaxial crystallization of polyesters on inorganic and organic substrates

A series of low-molecular-weight linear polyesters were epitaxially crystallized from dilute solution and the melt on a variety of organic and inorganic substrates. The rod-like polyester crystals which were formed assumed general orientations in alignment with substrate geometry. This yielded monodirectional orientation of these crystals on one of the organic substrates, and bidirectional orientation on the other surfaces. Heterogeneous nucleation on the organic substrates, trioxane and naphthalene, induced the growth of much larger epitaxial crystals than have previously been observed on inorganic substrates. Those polyesters, with a high percentage of methylene units per chain repeat, crystallized in an unusual polymorphic form when in contact with the organic substrates. Polymorphic transformation to the normal form was not possible under thermal treatment or with increased crystal thickness. The expected relationship between dipolar alignment in the polymer crystal and this polymorphism was established.     

Impurity-vacancy interactions in the alkali halides

The interaction energies of vacancies with divalent impurities are calculated using recently developed efficient computer simulation methods. The results show that nearest neighbour and next nearest neighbour complexes are roughly equally bound, thus emphasising the importance of including both types of defect pair in analyses of spin resonance and other experimental data on doped alkali halides. We also find that a simple Coulomb expression for defect interactions gives energies close to those obtained by the full calculation, for all but the nearest neighbour complex. This result encourages the use of Debye-Hückel treatments of defect activities in alkali halide crystals.     

Crystal lattice energies of alkali metal halides

An empirical relation for crystal binding energy in terms of the valence level splitting of the anion and the radius of the cation has been proposed. The results are much closer to the experimental values and comparable with those obtained by different methods. The anomalously high binding energy of fluorides has been discussed.     

Epitaxial crystallization of aliphatic polyesters on trioxane and various aromatic hydrocarbons

Linear polyesters of the diacid-dialcohol type are observed to crystallize epitaxially onto substrate crystals of trioxane, condensed aromatic hydrocarbons, and linear polyphenyls. In line with results previously reported for PE, lattice matching requirements for epitaxy lead to a change in the polymer contact plane on the different substrates. The highly oriented, sometimes single-crystal-like polymer films are formed of stacks of lamellae seen edge on which appear well suited to investigations of the lamellar structure.     

On the fractionation of homopolymers during crystallization from the pure melt

The question as to whether fractionation occurs during the crystallization of homopolymers from the pure melt has been addressed by studying linear polyethylene mixtures. Two methods were used. In one procedure, mixtures of well-defined fractions were crystallized under controlled conditions and the subsequent fusion process was analyzed. In the other procedure, a selective extraction was carried out on an isothermally crystallized polydisperse whole polymer, and the residue was analyzed by gel permeation chromatography. Both methods lead to the same conclusion, namely that during isothermal crystallization only the very low molecular weight species crystallize separately. An extreme upper limit to fractionation corresponds to M = 7000. More likely, fractionation in linear polyethylene during bulk crystallization is restricted to molecular weights of 5000 or less.     

The enthalpies and entropies of dilution of alkali halides and tetraalkylammonium halides in N-methylacetamide

The enthalpies of dilution of some alkali and tetraalkylammonium halides have been measured in N-methylacetamide at 35°C. The results approach the Debye-Hückel limiting law at low concentrations. Excess free energies and entropies were calculated from the present results and previous freezing point measurements. The excess enthalpies of the alkali halides in N-methylacetamide are in the same range as the excess enthalpies in water. The effect of changing anions is quite small in N-methylacetamide. The cation order is Li⁺?Na⁺>Cs⁺>K⁺. The excess enthalpies of the tetraalkylammonium halides in N-methylacetamide are very different from the excess enthalpies in water, confirming the conclusion that in water the large excess enthalpies are due to hydrophobic bonding and that in N-methylacetamide this effect is not present.     

Epitaxial crystallization of polyoxymethylene during cationic polymerization of trioxane

Epitaxial crystallization of polyoxymethylene (POM) during cationic polymerization of trioxane in ethane dichloride was studied. Nascent POM crystals grew epitaxially on the surface of drawn, high-molecular-weight POM and on the (001) surface of KCl single crystals. Half-hexagonal crystals several thousand angstroms in thickness were formed on these substrates. No discrete diffraction maximum could be observed in small-angle x-ray scattering patterns, but diffuse streaks were present on both the equator and the meridian of the pattern. These diffuse streaks are due to inhomogeneities in the nascent crystals. It is proposed that the crystals contain voids due to occlusion of reactive chain ends in the crystal. The optical diffraction pattern from a model made up of rectangles agreed qualitatively with the actual x-ray pattern.