Thermal growth of organic supramolecular crystals with screw dislocations

We report here a simple pathway to thermally assemble acene-based molecules into large crystals without modification of their chemical structures. Differential scanning calorimetry was used to characterize properly thermal events occurring during successive heating and cooling processes. More interestingly, observations by means of polarized light microscopy (POM) revealed that a spontaneous formation of screw dislocations within crystals during the isothermal treatment triggered a structural reorganization by forming large and well-defined spiral architectures. After this reorganization, new crystals showed an excellent ordering in both vertical and horizontal directions. Due to the richness in pi-electrons of acene-based molecules, we expect this work of importance to organic electronics, especially in the design of new molecular building blocks and investigation of their assembly into sophisticated supramolecular structures.     

Thermal properties of 4-alkyl-4′-cyanotolanes: a new series of liquid crystals

The thermal properties of a new series of liquid crystals, the 4-alkyl-4′-cyanotolanes, were measured. The DuPont 990 thermal analyser was used to measure the temperatures and heats of the transitions. The entropy changes in both the crystal to mesophase and mesophase to isotropic liquid transitions are found to be a function of the length of the alkyl chain on the molecules.     

Thermal expansion of a viscoelastic gel

It was previously shown [J. Non-Cryst. Solids, 130, 157 (1991)] that the permeability of a saturated gel can be determined from a measurement of its rate of expansion during a change in temperature. The existing analysis assumes that the solid network of the gel behaves elastically, but in many cases the gel is likely to be viscoelastic, especially when the gel exhibits syneresis. In this paper, the expansion kinetics are determined for a gel with a viscoelastic (VE) network. Sample calculations for a silica gel containing ethanol indicate that a naive application of the elastic analysis could result in errors exceeding a factor of two in the estimate of the permeability of a VE gel. However, in such cases, there are qualitative features in the experimental data that alert the experimenter to the existence of significant VE relaxation. Therefore, it is possible to avoid errors by careful examination of the data.     

Thermal properties of organic latent cold storage materials

The thermal properties of organic compounds intended to be applied as latent cold storage substances were investigated by methods of analysis according to Tammann, and by DTA and DSC. Findings were made concerning the melting and solidification behaviour, subcooling, specific melting enthalpies and specific heat capacities of paraffin mixtures, low and high-molecular glycols, ethylenediamine hydrates and formamide. Ethylenediamines and ethylene glycol, with high storage densities of 200–300 MJ·m^–3, appear to be promising cold storage substances if subcooling is reduced. Through the use of congruently melting low-molecular glycols and formamide, storage densities of about 170 MJ·m^–3 are obtained. Paraffin mixtures and high-molecular glycols melt in wide ranges of temperatures and give values of about 100 MJ·m^–3, which are not relevant to cold storage units.

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Zusammenfassung Mittels DTA, DSC und dem Analysenverfahren von Tammann wurden die thermischen Eigenschaften von organischen Substanzen untersucht, die als Kältespeichersubstanzen verwendet werden sollen. Dabei wurden Aussagen bezüglich des Schmelz- und Erstarrungsverhaltens, der Unterkühlung, der spezifischen Schmelzenthalpien und spezifischen Wärmekapazitäten von Paraffingemischen, nieder- und hochmolekularen Glykolen, Ethylendiaminhydraten und von Formamid getroffen. Ethylendiamine und Ethylenglykole mit einer hohen Speicherkapazität von 200–300 MJ·m^–3 scheinen gute Kältespeicherstoffe zu sein, wenn die Unterkühlung unterbunden wird. Bei der Verwendung von kongruent schmelzenden niedermolekularen Glykolen und von Formamid wurden Speicherkapazitäten von 170 MJ·m^–3 erreicht. Paraffingemische und hochmolekulare Glykole schmelzen in einem weiten Temperaturbereich und sind mit einer Speicherkapazität von 100 MJ·m^–3 für Kühlbatterien ungeeignet.

     

Thermal expansion effects in a silo

We consider a vertical container filled with granular material and exposed to temperature cycles which induce changes in the pressure distribution. Our starting point is an elastic model for the grains leading to classical Janssen behaviour in isothermal conditions. We assume complete mobilisation of the wall frictions. The dynamics superposes static friction plus a viscous friction. After a T jump favouring an expansion, we predict that the resulting overpressures are relaxed only in a region (of size comparable to the Janssen screening length λ) near the top. When T returns to the initial value, we expect decohesion in a certain layer near the top (for rapid cooling). For slow cooling, we expect a smooth return to the original Janssen state. We also discuss a different, but related, effect: instead of changing the temperature, we switch off the gravity effects (on a small column) by suddenly rotating the column from vertical to horizontal. Here, a rarefaction wave should invade the whole column.     

Thermal expansion of a polydiacetylene single crystal

Summary Fractural length changes l/l of a polydiacetylene (toluene sulfonate) single crystal and its monomer have been measured along the crystallographica-,b-, andc-direction in the temperature range 100 to 400 K. Small steps in the temperature dependence of l/l at 194 K for the polymer and 159 K for the monomer are assumed to be caused by a well known phase transition. From the volume coefficient of thermal expansion the bulk Grüneisenparameter _b has been calculated. Temperature dependence as well as absolute values of _b for the polymer crystal are found to be in accordance with current theories assuming that both inter and intrachain vibrations strongly influence the thermoelastic behaviour of polymer crystals.

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Zusammenfassung Die relative Längenänderung l/l eines Polydiacetylen (Toluolsulfonat) Einkristalls und des zugehörigen Monomerkristalls wurde entlang der kristallographischena-,b-undc-Richtung im Temperaturbereich 100 bis 400 K gemessen. Kleine Stufen im Temperaturverlauf von l/l bei 194 K für den Polymerkristall und bei 159 K für den Monomerkristall werden als Einfluß einer bekannten Phasenumwandlung gedeutet. Aus dem Volumenausdehnungskoeffizienten wurde der Bulk-Grüneisenparameter _b berechnet. Sowohl die Temperaturabhängigkeit als auch absolute Werte von _b für den Polymerkristall befinden sich in Einklang mit theoretischen Modellen, wenn man annimmt, daß sowohl Inter- als auch Intrakettenschwingungen das thermoelastische Verhalten von Polymerkristallen beeinflussen.

     

Discotic liquid crystals: a new generation of organic semiconductors

Discotic (disc-like) molecules typically comprising a rigid aromatic core and flexible peripheral chains have been attracting growing interest because of their fundamental importance as model systems for the study of charge and energy transport and due to the possibilities of their application in organic electronic devices. This critical review covers various aspects of recent research on discotic liquid crystals, in particular, molecular design concepts, supramolecular structure, processing into ordered thin films and fabrication of electronic devices. The chemical structure of the conjugated core of discotic molecules governs, to a large extent, their intramolecular electronic properties. Variation of the peripheral flexible chains and of the aromatic core is decisive for the tuning of self-assembly in solution and in bulk. Supramolecular organization of discotic molecules can be effectively controlled by the choice of the processing methods. In particular, approaches to obtain suitable macroscopic orientations of columnar superstructures on surfaces, that is, planar uniaxial or homeotropic alignment, are discussed together with appropriate processing techniques. Finally, an overview of charge transport in discotic materials and their application in optoelectronic devices is given.     

交联酶晶体的性能及其在有机合成中的应用

交联酶晶体提供了酶(高活性,高选择性,易处理,反应条件温和)及固相催化剂(环境适应性大,能循环利用)的复合特征。它在稳定性、活性、机械强度方面均有突出的优势。交联酶晶体在有机合成中非常有用,如在手性化合物拆分,肽合成,及碳-碳键的形成等方面有着比较成功的应用。     

Negative thermal expansion of polymer crystals: Lattice model

The lateral thermal motion of atoms in an inextensible polymer chain would cause a shortening along the chain direction. This effect could explain the observed negative thermal expansivity α along the chain direction of a polymer crystal. Using this idea, the expansivity α is calculated for a lattice of parallel linear chains. A value of ?1.3 × 10^?5 K is obtained for chains with a carbon backbone, in good agreement with data for polyethylene.     

Thermal expansion of polyoxymethylene

The thermal expensivities of polyoxymethylene crystals in the direction parallel (α_|^c) and perpendicular (α_‖^c) to the chain axis have been measured from 160 to 400 K using wide-angle x-ray diffraction. Although polyoxymethylene has a helical chain structure, it exhibits a thermal expansion behavior similar to that of polymer crystals with planar zigzag chains, namely that α_‖^c is negative while α_|^c is positive and larger by an order of magnitude. The negative α_‖^c arises from the shortening along the chain axis caused by the torsional and bending motions of the chain, whereas the large and positive α_|^c reflects the weak interaction across the chains. Combining the crystal data with dilatometric measurements on semicrystalline samples, the thermal expansivity is found to vary linearly with crystallinity, thus allowing the expansivity of the amorphous phase to be derived by extrapolation. With the thermal expansivities of the crystalline and smorphous phases known, the draw ratio dependence can be calculated in terms of existing models and is found to agree reasonably with experimental data.     

第一性原理研究有机分子晶体中电声子耦合的压强效应

电声子耦合系数是决定有机分子体系电荷传导过程的最重要因素之一,研究电声子耦合对理解有机分子晶体中的本征电荷传输机制有重要的意义.从实验得到的晶体结构出发,使用密度泛函理论计算出电子结构和声子模式,再投影到紧束缚模型,通过数值拟合和有限差分,计算出各个声子模的电声子耦合系数,进一步还可以研究随压强变化的关系.以萘晶体为例,计算表明,声子频率随压强增加,但电声耦合强度随压强增大而减小,对迁移率起到促进作用.     

Lateral thermal expansion of chitin crystals

Measurements of the thermal expansion coefficients (TECs) of chitin crystals in the lateral direction are reported. We investigated highly crystalline α chitin from the Paralithodes tendon and an anhydrous form of β chitin from a Lamellibrachia tube from room temperature to 250 °C, using X‐ray diffraction at selected temperatures in the heating process. For α chitin, the TECs of the a and b axes were α_a = 6.0 × 10⁻⁵ °C⁻¹ and α_b = 5.7 × 10⁻⁵ °C⁻¹, indicating an isotropic thermal expansion in the lateral direction. However, the anhydrous β chitin exhibited an anisotropic thermal expansion in the lateral direction. The TEC of the a axis was constant at α_a = 4.0 × 10⁻⁵ °C⁻¹, but the TEC of the b axis increased linearly from room temperature to 250 °C, with α_b = 3.0–14.6 × 10⁻⁵ °C⁻¹. These differences in the lateral thermal expansion behaviors of the α chitin and the anhydrous β chitin are due to their different intermolecular hydrogen bonding systems. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 168–174, 2001     

Guanine crystals: a first principles study

We report first principles density functional theory studies on the basic ground state characteristics, dynamic properties, and the electronic structure of guanine crystals. The effect of water molecules within the crystal is studied in detail, and we discuss their influence on the structural, vibrational, and electronic properties. The geometries calculated for various crystal structures are compared with gas-phase calculations and available experimental data. Phonon frequencies and eigenvectors are predicted for intermolecular and intramolecular lattice vibrations. Vibrational and electronic density-of-states are presented and analyzed. The electronic band structure near the fundamental gap is calculated from the Kohn-Sham approach. We find that the former molecular HOMO states form a dispersive band in the pi-pi stacking direction upon condensation resulting in a large bandwidth of 0.83 eV. Consequences for the charge transport in layered van der Waals bonded organic molecular crystals are discussed.     

Thermal expansion of irradiated polyoxymethylene

The thermal expansion coefficient of gamma irradiated polyoxymethylene has been measured in the temperature range 80–340 K by using a three terminal capacitance technique. The radiation induced changes are measured by recording the i.r. spectra of the irradiated samples. The change in crystallinity caused by irradiation is measured by an X-ray technique. The thermal expansion coefficient increases with radiation dose below 170 K due to the predominant effect of degradation. Above 170 K, this trend reverses and the expansion coefficient decreases with radiation dose due to the increased crystallinity caused by irradiation.     

Thermal expansion of ultradrawn polystyrene

Measurements on the linear thermal expansivities α∥ and α⊥ parallel and perpendicular to the extrusion direction, respectively, have been carried out between ?160 and 30°C for atactic polystyrene (aPS) with extrusion ratio 1 ≤λ≤ 15 and isotactic polystyrene (iPS) with 1 ≤λ≤ 7.5. For both aPS and iPS, α∥ decreases sharply with increasing λ whereas α⊥ shows only a slight increase. Below λ = 5 the anisotropy α⊥/α∥ is nearly the same for both, but α⊥/α∥ for iPS becomes much larger at higher λ. This is accompanied by an abrupt rise in crystallinity and probably results from the increase in the number and tautness of intercrystalline tie molecules. The birefringences of aPS and iPS increase with λ and have nearly the same values for λ < 5. At higher λ, however, there is a sharp rise in the birefringence of iPS which is probably associated with the sudden increase in crystallinity. With an aggregate model, the chain orientation function for aPS calculated from thermal expansivity is found to be in reasonable agreement with the corresponding value obtained from birefringence.     

Thermal expansion of irradiated polytetrafluoroethylene

The thermal expansion coefficient of gamma-irradiated Polytetrafluoroethylene (PTFE) has been measured in the temperature range 80–340 K by using a three-terminal capacitance technique. The samples are irradiated in air at room temperature with gamma rays from a Co⁶⁰ source at a dose rate of 0.26 Mrad/h. The change in crystallinity is measured by an x-ray technique. The expansion coefficient is found to increase with radiation dose below 140 K owing to the predominant effect of degradation. Above 140 K, the expansion coefficient is found to decrease with radiation dose because of the enhanced crystallinity. The temperatures of the two first-order phase transitions are also found to shift to lower temperatures owing to degradation of PTFE.     

Some effects of structural disorder on the optical properties of organic molecular crystals

We investigate the effects of structural disorder on the absorption line shapes of the lowest exciton states in molecular crystals at zero temperature by the application of the average t-matrix approximation to a system characterized by a gaussian distribution of site excitation energies. The line shape exhibits asymmetric line broadening with the high energy portion being close to lorentzian, while the low energy edge is sharp. The energy dependence of the low energy tail is determined by the dimensionality of the exciton band structure.