Concept of Molecular Nucleation

The concept of molecular nucleation was developed to explain some observations not in agreement with the above model of crystallization limited by secondary nucleation [29]. In Fig. 3.73 the molecular length is plotted which is rejected by a crystal growing from the melt or from solution (curves 1 and 2, respectively). These data were obtained by measuring the molar mass of the noncrystaUized molecules of a... 

Abstract. We review how the nucleation mechanism of polymer crystallization could be assigned to intramolecular processes and what are the preliminary benefits for understanding some fundamental crystallization behaviors. The speculative concept of molecular nucleation and the theoretical model of intramolecular nucleation have been elucidated in a broad context of classical nucleation theory. The focus is on explaining the phenomenon of molecular segregation caused by polymer crystal growth. 

In a broad sense, the concept of molecular nucleation can be applied to the primary crystal nucleation too, since the sizes of primary nuclei are usually smaller than the coil size of single macromolecules [37]. As a matter of fact, both experiments and simulations have provided evidences for the crystallization within a single homopolymer chain [38-42]. 

This concludes the discussion of the nucleation dynamics of macromolecules. It shows that the usually assumed constant number of heterogeneous nuclei and linearly increasing number of homogeneous nuclei is a simplification, and secondary nucleation as the basis for crystal growth is a doubtful concept. Finally, molecular nucleation is not a well enough understood concept to quantitatively explain facts such as the molar mass dependence of crystallization. The further work needed to understand the basis of nucleation in polymers is a big challenge for new research in solid-state polymer science. 

Carter and Ward have identified a surface-mediated nucleation mechanism that involves a geometric shape match between planes of a ledge site on the substrate and planes of prenucleation aggregates. They have applied these concepts to the directed nucleation of polymorphs. ° This work provides us with the attractive possibility that a library of organic seeds can be used to control polymorphism, or to search for unknown polymorphs. Interpretations of molecular assemblies between solute and additive (seeds, substrate surfaces, excipients, and solvents) molecules may prove beneficial in selectively crystallizing or screening polymorphic systems. 

The nucleus is assumed to have the same structure, density, etc. as the bulk phase, and have the same interaction energies with adjacent phases (surface tension, etc.). This conception has proved valuable in general, but a few of its assumptions are difficult to prove or incorrect on the molecular level (Talanquer and Oxtoby 1994). Other assumptions are practically impossible to test (ten Wolde and Frenkel 1997), as critical nucleus formation is a fleeting event that may not be amenable to any direct imaging method. However, as nanoparticles are of the same size domain as theoretical critical nuclei, an understanding of the nucleation process is essential to defining and possibly controlling the pathways for nanoparticle formation. 

Molecular clusters are formed due to weakly attractive forces between molecules, the Van der Waals forces. Except under conditions of low temperature, it is difficult to observe and study in the laboratory clusters containing more than a few molecules, so that details about their properties are sparse. Our understanding of the nucleation process consequently relies mainly on theoretical concepts based largely on the principles of statistical mechanics. The theory of homogeneous nucleation developed by Volmer and Weber (1926), Flood (1934), Becker and Doering (1935), and Reiss (1950) assumes that certain thermodynamic properties, such as the molar volume or the surface tension, that can be determined for bulk material... 

The mechanisms of crystal phase formation are a key problem in materials science that has not clear comprehension still now. At present, the study of this problem is especially important in connection with the development of nanostructured materials. There are two different approaches to consideration of crystal nucleation/growth as well as crystal melting/dissolution processes [1,2], In accordance with the first approach based on the atomic-molecular theory, the individual atoms or molecules take the leading part in these processes (the role of clusters is ignored). In accordance with the second approach based on the cluster theory, these processes are carried out mainly by means of clusters. Till recently the atomic-molecular theory was generally accepted. However, today many scientific data vote for the cluster theory. The aim of this paper is to analyze the main statements of the cluster conception of crystal phase formation and as a result to consider the nature of nanocrystal. 

The perspective upon which the concept of a superheat limit emerges is based on random molecular density fluctuations within the hulk of a liquid producing hole-like regions of moleculardimensions that act as bubbles. A phase transition occurs when abubble, formed by these molecular processes, grows to a size such that it is in unstable equilibrium (i.e.. a critical size nucleus ) with the surrounding liquid. The "nucleation rate ,/, refers to the mean rate of forming such vapor nuclei in units of nuclei/(volume time). 

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