Structure of Crystalline Interfaces

The crystallization kinetics of commercial polyolefins is to a large extent determined by the chain microstructure [58-60]. The kinetics and the regime [60] of the crystallization process determine not only the crystalline content, but also the structure of the interfaces of the polymer crystals (see also Chapter 7). This has a direct bearing on the mechanical properties like the modulus, toughness, and other end use properties of the polymer in fabricated items such as impact resistance and tear resistance. Such structure-property relationships are particularly important for polymers with high commercial importance in terms of the shear tonnage of polymer produced globally, like polyethylene and polyethylene-based copolymers. It is seen that in the case of LLDPE, which is... 

Fig. 4. A model for the possible relationship between crystalline and disordered regions within a collagen fibril. The cross-sectional model of a 50-nm diameter fibril shows regions of crystallinity interfaced by grain boundaries. The individual crystalline unit cells are shown and the gap region is represented by a darker color. The axial projection of a single microfibrillar unit is also shown. Based on die structures developed by Hulmes et al. (1995) and adapted with permission from Hulmes et al (2002).

Polyamino acids can be considered as models for conformational studies, providing an atomistic description of the secondary structural motifs typically found in proteins [30-39]. Two-dimensional hydrogen-bonded layers and columns in the structures of crystalline amino acids can mimic S-sheets and helices in proteins and amyloids [40 5], and can be compared with two-dimensional crystalline layers at interfaces [46-58]. Nano-porous structures of small peptides can mimic cavities in proteins [24, 59-63]. One can also prepare crystals in which selected functional groups and side chains are located with respect to each other in the same way, as at recognition sites of substrate-receptor complexes, and use the systems to simulate the mutual adaptation of components of the complex responsible for recognition. 

There are two major factors which have to be considered in the process of the electrolytic metal deposition (i) the thermodynamic and growth properties of the crystalline phase which can be treated as largely independent of the presence and character of the ambient phase and (ii) the properties of the ionic solution affecting primarily the structure of the interface boundary and the kinetics of the mass and charge transfer across it. In the first part of this chapter the problems connected with the formation and growth of the crystals of the metal deposit will be discussed more closely, while the problems arising from the ionic solution side will be treated as simply as possible (see also Vol. 1 of this series). 

Phases in thermodynamic systems are then macroscopic homogeneous parts with distinct physical properties. For example, densities of extensive thermodynamical variables, such as particle number N of the fth species, enthalpy U, volume V, entropy S, and possible order parameters, such as the nematic order parameter for a liquid crystalline polymer etc, differ in such coexisting phases. In equilibrium, intensive thermodynamic variables, namely T,p, and the chemical potentials pi have to be the same in all phases. Coexisting phases are separated by well-defined interfaces (the width and internal structure of such interfaces play an important role in the kinetics of the phase transformation (1) and in other... 

The nature of the interface the solid-water interface (or solid-liquid interface in general) in systems involving particles (e.g., minerals and ceramics) or the air-water interface or liquid-liquid interfaces in systems having bubbles or oil droplets, respectively the surface charge, its hydrophobicity, and the nature of adsorption sites at the interface (e.g., exposed metal ions at the interface providing sites for chelation at interfaces). In the case of crystalline solids, the surface crystal structure of the interface plays an important role in surfactant adsorption. 

XRD techniques are used mainly to study the crystal structure of crystalline solids at the elec-trode solution interface. Structural changes, solid-state reactions, precipitation processes, dissolution processes, and intercalation processes can be followed as a function of applied potential and time. For non-crystalline samples. X-ray absorption and EXAFS allow the structure to be studied in more detail. 

The examples of multistable anchorings and of anchoring transition reported above raise numerous fundamental questions concerning the structure of the interface between the nematic layer and the crystalline support. The existence of the anchoring transitions induced by variations of the water vapor pressure indicates clearly that adsorption of water molecules assists (or induces) variations of the anchoring direction. It must be so for following reasons. For a constant temperature T, only the chemical potential of water varies as a function of the partial pressure (variations of the partial... 

While there have been some successes in this area, however, BVT principles must be applied carefully to these systems. Bond-valence parameters are calibrated on precisely known structures of crystalline solids, for instance, so the application of the BVT to liquids and solid-liquid interfaces, for which less precise structural information is available, may not be straightforward. Furthermore, BVT-based reactivity models usually involve an implicit assumption that, at least for the purpose of predicting particular types of reaction energies, bond lengths are the dominant aspect of the stmcture. This may, or may not, be the case, depending on the reactions of interest. 

EXAFS is a nondestructive, element-specific spectroscopic technique with application to all elements from lithium to uranium. It is employed as a direct probe of the atomic environment of an X-ray absorbing element and provides chemical bonding information. Although EXAFS is primarily used to determine the local structure of bulk solids (e.g., crystalline and amorphous materials), solid surfaces, and interfaces, its use is not limited to the solid state. As a structural tool, EXAFS complements the familiar X-ray diffraction technique, which is applicable only to crystalline solids. EXAFS provides an atomic-scale perspective about the X-ray absorbing element in terms of the numbers, types, and interatomic distances of neighboring atoms. 

Raman spectroscopy is a very convenient technique for the identification of crystalline or molecular phases, for obtaining structural information on noncrystalline solids, for identifying molecular species in aqueous solutions, and for characterizing solid—liquid interfaces. Backscattering geometries, especially with microfocus instruments, allow films, coatings, and surfaces to be easily measured. Ambient atmospheres can be used and no special sample preparation is needed. 

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