Macromolecular crystals properties

In this paper, we make use of molecular modelling techniques, particularly the AMI semiempirical molecular orbital method, to study the intermolecular interactions that are important for determining the manner in which crystal formation takes place. We are particularly inter ested in compounds that can potentially exhibit nonlinear optical properties. The calculational techniques are directed towards providing insight into the manner in which the desired nonlinear optical properties can be op timized in the macromolecular crystal state.(1)... 

Macromolecules are intricate physical-chemical systems whose properties vary as a function of environmental influences such as temperature, pH, ionic strength, contaminants, and solvent composition, to name only a few. They are structurally dynamic, often microhetero-geneous, aggregating systems, and they change conformation in the presence of ligands. Superimposed on this is the limited nature of our current understanding of macromolecular crystallization phenomena and the forces that promote and maintain protein and nucleic acid crystals. 

The first year, Jim and I believed we could simply crack ahead with the practical aspects of the subject, assuming that the students knew the fundamentals of diffraction theory, crystal properties, and the basic concepts of solving the structures of macromolecular crystals. It quickly became apparent to both of us that we were sadly naive. 

The high solvent content of macromolecular crystals leads to another way to modify the electron density. The electron density map is the space average of all the unit cells in the crystal, so atoms that are in random positions (as in liquid water) in different unit cells will not show up as peaks. Why They do not obey the periodicity of the crystal (remember that the FTs concerned periodic functions) and so are called disordered. The crystal then consists of ordered molecules, where the electron density is the same in each unit cell, and so visible, and disordered solvent, where the electron density averages to zero. In order to make physical sense, the electron density also has to be positive a property not imposed by the FT. Consequently, electron density peaks outside the macromolecule are noise and can be got rid of, as can negative electron density inside the macromolecule. We can therefore apply these conditions modify the initial electron density so that it is zero outside the molecules and positive within them. This, as for noncrystallographic averaging, alters the electron density map to conform to what must be true, and the map is thus a better representation than the initial map. 

The solid state, finally, has gained by the understanding of macromolecular crystals with helical molecules, their defect properties, mesophases, small crystal size, glass ttansitions, and rigid-amorphous fractions (Chaps. 5 and 6). 

If, however, the stress applied to the solid exceeds its elastic limit, the response is plastic deformation. This deformation persists when the stress is removed, and the unstressed solid no longer has its original properties. Plastic deformation is a kind of hysteresis, and is caused by such microscopic behavior as the slipping of crystal planes past one another in a crystal subjected to shear stress, and conformational rearrangements about single bonds in a stretched macromolecular fiber. Properties of a solid under plastic deformation depend on its past history and are not unique functions of a set of independent variables an equation of state does not exist. 

Melting is an endothermic event, and shows up in the DSC curve as an endothermic peak. One important task in DSC measurements is determination of the melting point and heat of fusion of both low-molecular-mass and macromolecular crystals. In addition to melting of polymers, we briefly describe here the melting of low-molecular-mass substances. Every thermal analyst must be familiar with this if for no other purpose than for calibration of the instruments with metal standards and for measuring melting properties of low-molecular-mass substances used in the plastics industry. 

R.G. Alamo and C. Chi, Crystallization behavior and properties of polyolefins. In Y. Morishima, T. Norisuye and K. Tashiro (Eds.), Molecular Interactions and Time-Space Organization in Macromolecular Systems, Springer, New York, 1999, p. 29. 

CNTs own excellent materials properties. DNA is an excellent molecule to construct macromolecular networks because it is easy to synthesize, with a high specificity of interaction, and is conformationally flexible. The complementary base-paring properties of DNA molecules have been used to make two-dimensional crystals and prototypes of DNA computers and electronic circuits (Yan et al., 2002 Batalia et al., 2002). Therefore functionalization of CNTs with DNA molecules has great potential for applications such as developing nanodevices or nanosystems, biosensors, electronic sequencing, and gene transporters. 

The most relevant property of stereoregular polymers is their ability to crystallize. This fact became evident through the work of Natta and his school, as the result of the simultaneous development of new synthetic methods and of extensive stractural investigations. Previously, the presence of crystalline order had been ascertained only in a few natural polymers (cellulose, natural rubber, bal-ata, etc.) and in synthetic polymers devoid of stereogenic centers (polyethylene, polytetrafluoroethylene, polyamids, polyesters, etc.). After the pioneering work of Meyer and Mark (70), important theoretical and experimental contributions to the study of crystalline polymers were made by Bunn (159-161), who predicted the most probable chain conformation of linear polymers and determined the crystalline structure of several macromolecular compounds. 

Nowadays attention is turned also to the supermolecular level, that is, to the morphologic aspects, to the nature of interfaces, to the formation of new phases, or of particular aggregates (liquid crystals, gels, etc.). Interest has also been directed to the study of chain mobility for its influence on frictional properties of polymers. In recent years there have been many successful approaches to a microscopic theory (in contrast to a phenomenological approach) of the physi-comechanical behavior of macromolecular materials. 

As can be seen in H, Kelkers l) excellent review on the history of liquid crystals, investigations on liquid crystalline polymers already exist before F. Reinitzer in 1888 gave the very first description of a low molar mass liquid crystal (1-l.c.). While, however, 1-l.c. s have become an extensive field of research and application during the past decades, these activities on l.c. polymers have come rather late. The research on l.c. polymers during the last years is mainly joined with activities in material science and tries to realize polymers with exceptional properties. These exceptional properties are expected because of the combination of the physical anisotropic behavior of l.c. and the specific properties of macromolecular material. 

The macromolecular nature provides an interesting feature of LC polymeric cholesterics, namely the possibility of obtaining monochromic films. Thus for polymeric liquid crystals the helix pitch is practically not altered with temperature below Tg, when a cholesteric phase is frozen in a glassy matrix (Fig. 23a). This implies that fast cooling of polymeric films from a mesomorphic state (shown with arrows) fixes their optical properties, which makes it possible to use them at ordinary temperatures as selective monochromic reflectors. On the other hand, such polymeric films display the extraordinary polarizing properties of cholesterics, i.e. the different absorption... 

This book is concerned mainly with the study of the viscoelastic response of isotropic macromolecular systems to mechanical force fields. Owing to diverse influences on the viscoelastic behavior in multiphase systems (e.g., changes in morphology and interfaces by action of the force fields, interactions between phases, etc.), it is difficult to relate the measured rheological functions to the intrinsic physical properties of the systems and, as a result, the viscoelastic behavior of polymer blends and liquid crystals is not addressed in this book. 

We will consider below self-assembled superstructures, like collagen, plant cell walls, starch and the contractile macromolecular complex. These super structures are also liquid crystals, evident from their birefringence and rheological properties. With increasing knowledge of the molecular mechanisms behind function, however, there has been an unfortunate tendency to neglect the role of overall structure. 

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