Crystals properties

Independently of crystallization properties, it is useful to characterize the behavior of a product with respect to the following criteria ... 

Hagler A T and S Lifson 1974. Energy Functions for Peptides and Proteins. II. The Amide Hydrogen Bond and Calculation of Amide Crystal Properties. Journal of the American Chemical Society 96 5327-5335. 

Other complex thiosulfates have been prepared to study crystal properties, eg, cadmium ammonium thiosulfates (90), NaAgS202 H2O [37954-66-8] (91), I C2Mg(S203)2 -6H20 [64153-76-0] (92), and (NH 2[Ag(S203)JCl2 [12040-89-0] (93). 

Most materials scientists at an early stage in their university courses learn some elementary aspects of what is still miscalled strength of materials . This field incorporates elementary treatments of problems such as the elastic response of beams to continuous or localised loading, the distribution of torque across a shaft under torsion, or the elastic stresses in the components of a simple girder. Materials come into it only insofar as the specific elastic properties of a particular metal or timber determine the numerical values for some of the symbols in the algebraic treatment. This kind of simple theory is an example of continuum mechanics, and its derivation does not require any knowledge of the crystal structure or crystal properties of simple materials or of the microstructure of more complex materials. The specific aim is to design simple structures that will not exceed their elastic limit under load. 

As examples of the successful application of the PIMC method outlined in Sec. IV D 1, we focus here on studies of crystal properties where either Ar, Ne atoms or N2 molecules occupy sites on an fee lattice. 

Chemical development Proof of structure and configuration are required as part of the information on chemical development. The methods used at batch release should be validated to guarantee the identity and purity of the substance. It should be established whether a drug produced as a racemate is a true racemate or a conglomerate by investigating physical parameters such as melting point, solubility and crystal properties. The physicochemical properties of the drug substance should be characterized, e.g. crystallinity, polymorphism and rate of dissolution. 

The raw materials from which di-D-fructose dianhydrides can be obtained in appreciable yield are readily available from comparatively inexpensive agricultural feedstocks. Thus, these compounds are attractive as chiral-starting materials for chemical synthesis. Their stability to acid and heat, and their relative rigidity, because of the conformational constraints covered here, are also features that might be exploited during syntheses.119 A series of variously substituted di-D-fructose dianhydrides has been prepared,119 starting from 6,6 -dideoxy-6,6 -di-halosucroses. The properties of these and other derivatives of di-D-fructose dianhydrides are summarized in Tables XIV-XX. Two of these derivatives, 48 and 56, exhibit thermotropic liquid-crystal properties.119... 

Currently, the effect of the molecular quadrupoles on liquid crystal properties is not clearly understood though there is some evidence to suggest that they influence surface properties and phase stability [28, 60]. 

A compound which displays liquid crystal properties is referred to as a mesogen and said to exhibit mesomorphism. Liquid crystals may be considered either as disordered solids or ordered liquids, and their properties are very dependent on temperature and the presence or absence of solvent. In thermotropic liquid crystals the phases of the liquid crystals may be observed to change as the temperature is increased. In lyotropic liquid crystals the ordered crystalline state is disrupted by the addition of a solvent, which is very commonly water. For these systems temperature changes may also be... 

The 3,6-bis(2-arylethenyl)-l,2,4,5-tetrazines 97 have been synthesised and have been shown to have liquid crystal properties <95JPR(337)641 >. 

Several mixtures of hexanethiol capped gold nanopartides and triphenylene based discotic LCs have been studied. These mixtures display liquid crystal behavior (columnar mesophases) and an enhancement in the DC conductivity, due to the inclusion of gold nanoparticies into the matrix of the organic LC [70]. Other studies of mixtures of gold nanoparticies with mesogens indude a series of cholesteryl phenoxy alkanoates. The inclusion of the nanopartides does not change the inherent liquid crystal properties of the cholesteryl derivative but the mesophases are thermally stabilized [71]. 

The studies show that the observed crystal volume is in fact composed of the fractional contributions from the unit cell volumes of the HS and LS isomers of the compound and a linear volume change with temperature as expressed in Eq. (128). Similarly, the observed lattice constants are formed from a deformation contribution proportional to the HS fraction and a contribution from thermal expansion following Eq. (131). This is a convincing demonstration that it is the internal variation of the molecular units occurring in the course of the spin-state transition which determines, at least in principle, the observed crystal properties. 

Lin, /. and Sherrington, D. C Recent Developments in the Synthesis, Thermostability and liquid Crystal Properties of Aromatic Polyamides. Vol. Ill, pp. 177-220. 

However, the hydrogen-bonded mesogens that are of most interest in the context of this article are those elaborated initially by Kato and Frechet in the early 1990s [24-33]. In this approach, a pyridine, which may or may not have liquid crystal properties, was hydrogen bonded with a 4-substituted benzoic acid to form a new species with its own, distinct mesomorphism. For example, complex 9 shows a SmA phase that persists to 238 °C (n = 2, m = 4), while its free component pyridine is nematic to 213 °C the component benzoic acid is also nematic (as the H-bonded dimer) to 147 °C (although note that the notional monomer would not be liquid crystalline). 

The liquid crystal properties of the complexes were characterised using polarised optical microscopy and showed a nematic phase for n = 4 and 6 and a SmA phase for n = 6, 8, 10 and 12. The mesophases were monotropic for n = 4 and 6 and enantiotropic for the others the progression from a nematic phase for shorter chain lengths to SmA at longer chain lengths is quite typical for simple, polar mesogens. 

Blaha, P., Schwarz, K., Dufek, P. et al. (1995) WIEN95 A Full Potential Linearized Augmented Plane Wave Package for Calculating Crystal Properties. Technical University, Vienna. 

Cieplak P, Kollman P, Lybrand T (1990) A new water potential including polarization - application to gas-phase, liquid, and crystal properties of water. J Chem Phys 92(11) 6755-6760... 

The influence of crystal defects on crystal properties provides indirect methods of studying defects. The following techniques may be employed ... 

AF1L Chow, PKK Chow, W Zhongshan, DJW Grant. Modification of acetaminophen crystals Influence of growth in aqueous solutions containing p-acetoxyacetanilide on crystal properties. Int J Pharm 24 239-258, 1985. 

Kevlar has the highest tensile strength and is often used as a reinforcing fibre in composites with, e.g., epoxy, PEEK. The thermotropic liquid crystal polymer Vectran is made by melt polymerisation of p-acetoxybenzoic acid and 6-acetoxy-2-naphthoic acid, (the corresponding hydroxy acids decompose on melting). Because of its liquid crystal properties the polymer can be spun into fibres from the melt. Kevlar is spun from a solution in concentrated sulfuric acid, and can be melt drawn to give a high modulus (stiff) polymer. Vectran ... 

The procedure of Lifson and Warshel leads to so-called consistent force fields (OFF) and operates as follows First a set of reliable experimental data, as many as possible (or feasible), is collected from a large set of molecules which belong to a family of molecules of interest. These data comprise, for instance, vibrational properties (Section 3.3.), structural quantities, thermochemical measurements, and crystal properties (heats of sublimation, lattice constants, lattice vibrations). We restrict our discussion to the first three kinds of experimental observation. All data used for the optimisation process are calculated and the differences between observed and calculated quantities evaluated. Subsequently the sum of the squares of these differences is minimised in an iterative process under variation of the potential constants. The ultimately resulting values for the potential constants are the best possible within the data set and analytical form of the chosen force field. Starting values of the potential constants for the least-squares process can be derived from the same sources as mentioned in connection with trial-and-error procedures. 

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