Crystalline substructures

Just prior to Rubin s publication, another article appeared focusing on substructures of graphdiyne [63]. Like the other researchers in the PDM area, the Haley team was intrigued by the predictions of useful materials properties and technological applications for this and similar carbon-rich systems [5c, 50,52]. In particular, topochemical polymerization of a crystalline substructure of this network could produce an environmentally robust material with a large third-... 

BET studies of both the commercial and laboratory scale particles discussed above indicate that there is little internal area accessible to BET adsorbate molecules. This holds for both amorphous and polycrystall ine particles. If the individual particles are composed of multiple crystalline substructures, internal defects capable of adsorption would be expected. However, the BET measurements. show that internal pore.s, if they are present, are not accessible to adsorbate gases. A possible explanation is that annealing by solid-state diffusion occuin sufftcienily rapidly al the temperatures of formation to block access of the external gas to dislocations and grain boundaries. However, the origins of the crystallites within the particles and the mechanisms of crystallization tire not understood al present. 

In one case, each of the multiple melting peaks can be attributed to the various crystalline substructures, such as unit cells, lamellae, or spherulites, present in the polymer. In the alternative interpretation, multiple melting was attributed to the melting of thinner lameUae/crystals, recrystallization to thicker ones, and remelting of the thickened lamellae during DSC scanning. 

Crystalline Substructures. Figure 4 indicates that a number of structures are conceivable based on the two alternative arrangements of the three-fold hyckogen bond pattern connecting the melamine and barbiturate (isocyanurate) units. Isomerism around these sets of bonds leads to various possible substructures in the solid state, from the straight tape at the bottom of the figure to the cyclic hexamer at the top. Unless the substituents are tailored to fill interstitial voids efficiently, we suspect that... 

Figure 5. Crystalline substructures obtainable in 1 1 co-crystals of dmvatives of barbituric acid (B) and melamine (M). T = the number of B-M dimers that constitute a translational repeat unit along a tape (boxed) S = the number of tapes that constitute a translational repeat unit in a sheet 6 = the angle between tape axes in adjacent sheets. Tlie structures in this figure are representative examples of possible geometries and not an exhaustive list of all possible orientations of M andB.

In this context, cooperativity does not necessarily mean that different parts of the system depend on and need to interact with each other to change the macrostate (or the phase). Rather, local parts of the system can react individually in the same way upon a weak change of the environmental conditions. In the freezing transition of water, nucleation cores form independently and attract other molecules in the local environment of each nucleus to join. This leads to macroscopic crystalline structures which finally bind to each other in order to reduce instabilities due to surface effects. However, the individual growth of the nucleation centers also causes dislocations that typically appear at the boundaries of these crystalline substructures,... 

Mesoscale simulations model a material as a collection of units, called beads. Each bead might represent a substructure, molecule, monomer, micelle, micro-crystalline domain, solid particle, or an arbitrary region of a fluid. Multiple beads might be connected, typically by a harmonic potential, in order to model a polymer. A simulation is then conducted in which there is an interaction potential between beads and sometimes dynamical equations of motion. This is very hard to do with extremely large molecular dynamics calculations because they would have to be very accurate to correctly reflect the small free energy differences between microstates. There are algorithms for determining an appropriate bead size from molecular dynamics and Monte Carlo simulations. 

Another type of fibril substructure in PET fibers, besides the microfibrillar type already discussed, is the lamellar substructure, also referred to as the lateral substructure. The basic structural unit of this kind of substructure is the crystalline lamella. Formation of crystalline lamellae is a result of lateral adjustment of crystalline blocks occurring in neighboring microfibrils on the same level. Particular lamellae are placed laterally in relation to the axis of the fibrils, which explains the name—lateral substructure. The principle of the lamellar substructure is shown in Fig. 2. 

Figure 2 The lamellar substructure of a fibril. (a) Reciprocal positions of crystalline lamellae as a result of fiber annealing. (b) The situation after relaxation of stress affecting TTM. ai.2 - average angle of orientation of TTM CL - crystalline lamellae CB - crystalline blocks (crystallites) mF -border of microfibrils and F - fibril. In order to simplify it was assumed that (1) there are the taut tie molecules (TTM) only in the separating layers, (2) the axis of the fibril is parallel to the fiber axis.

Draw ratio Density of the amorphous material da) (g/cm-" ) Amorphous orientation function fa) Crystallite length Oc) (nm) Long period (L) (nm) Degree of crystallinity (X=>) Substructure parameter (A) Axial elastic modulus ... 

On warming to — 30°C., the spectrum of the crystalline samples changed to a sextet with a doublet substructure. By analogy with poly( 1-butene) this spectrum was interpreted as arising from allylic radicals (XXII), but this assignment needs further justification. 

Figure 2. (a) The amino acid sequence of the enzyme lysozyme (from egg white). Blocks enclosing two cysteines (Cys) denote intramolecular covalent cross-links (disulfide bonds). This molecule in crystalline form has the three-dimensional structure sketched in part (b). Note the helical subdomains and sheet substructures formed by nearby extended segments. Reprinted by permission from C. C. F. Blake, Structure of Hen Egg-White Lysozyme, Nature vol. 206 p. 757. Copyright (c) 1967 Macmillan Magazines Ltd. 

For various reasons, the superior properties of most liquid-crystalline materials contained in LCD displays depend critically on the presence of fluorinated substructures [103]. In particular, perfluorinated groups have displaced cyano groups for their role in inducing polarity. 

Active carbon is a composite of amorphous and microcrystalline substructures and exhibits semiconducting properties. An interpretation of the mechanism of electronic conduction in active carbon is more complicated than that for crystalline (graphitized) carbon. The conduction processes are very different becau.se the current carriers in active carbon are assumed to be localized by disorder, which introduces randomness into the potential-energy bands for electrons. 

Aerosol product properties of interest include primary particle size (and/orsize distribution) and substructure (grain boundary, pore size, and defect concentrations and crystalline... 

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