Supermolecular structure general

In this Chapter I claim that the temporal aspect of chemical structure has been neglected relative to the energetic one and offer an historically based explanation for this neglect. I then discuss recent work that enriches our conception of molecular and supermolecular structure by attending more closely to their temporal dimension. The final pages contain my assessment of why this story in particular and chemistry in general might merit the attention of philosophers. 

Neat isotactic polypropylene (iPP) crystallized from melt exhibits spherulitic morphology of the crystalline phase (72,73). In some cases and under very specific conditions, cylindrites, axialites, quadrites, hedrites, and dendrites may be formed of iPP (74). In general, crystallization from quiescent melts results in spherulitic morphology, whereas crystallization fi-om melts subjected to mechanical loads results in cylindrites (75). Crystalline supermolecular structure caused by oriented crystal growth from heterogeneous surfaces is commonly termed transcrystallinity (76). 

In this review the crystal structure and the super-molecular structure of the most used polyolefins is discussed. In particular the latest papers on the morphology of polyethylene, isotactic and syndiotactic polypropylene, isotactic poly(l-butene), and finally isotactic poly(4-methylpentene-l) are summarized and integrated with the fundamental work on the topic. After a short general introduction, the first part of the chapter is dedicated to the analysis of the order at the molecular level (the crystal structure), and the second part deals with the supermolecular structures. 

In addition to the above interpretation of the electron diffraction data on the supermolecular structure of Kevlar [81], models with fibrils consisting of molecular chains in a straightened conformation are also proposed in a numba of other studies [82]. The macromolecules pass through the crystallites and periodically arranged defective layers of the polymer. Generalized data on the structure of PPTA are contained in [83, 84]. It was hypothesized that PPTA fibers consist of a core and a thin shell. Planes p pendicular to the axis of the fiber in which the ends of the macromolecules are concentrated are located in the core with a periodicity of 200-250 nm. The basic (layered) structure of the core is formed by cylindrical crystallites with a diameter of 60 nm and a length of 200-250 nm. 

Table I summarizes some general features of the mesomorphic order in liquid crystalline me-sophases. In polymers as well as low molecular materials the mesomorphic order is characterized by the nature of the positional and orientational order of segments (or molecules) within an oriented domain. The supermolecular structure of the mesomorphic materials is characterized by the positional (and sometimes orientational) order of the domains themselves. In the case of liquid crystals, in contrast to crystals, the molecular (or segmental) positional order is lost to some degree while the orientational order subsists.

In order to establish the existence of supermolecular structures in the test sample. X-ray diffraction techniques suggested by Alexander [23] were used. Using a General Electric X-ray unit, rubber samples were exposed at various stretch ratios and temperatures to a 0.025 -diameter beam of nickel filtered Cu Ka radiation (50KV, 20MA). X-ray diffraction photographs (5cm film-to-target distance) were taken at one hour exposures. The samples were... 

Although the chemical composition is an influential factor in maximizing properties, it is important to realize that a hierarchy of structures exists from atomic dimensions to the macroscopic shape of the sample. Fracture is generally initiated at the surface of impurities (which may be as small as 1000 a) whereas the process of crack propagation and accompanying fracture energy are dominated by supermolecular structures in the network. 

Nylon crystallites consist of sheets of chains that are hydrogen-bonded to their neighbors. On a supermolecular scale, crystallites have a lamellar structure, that is they are many times longer and broader than they are thick. When nylon crystallizes from an isotropic molten state, it generally forms spherulites, which consist of ribbon-like lamellae radiating in all directions... 

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