Polymer crystallography

Figure 5.25 shows the unit ceU of isotactic poly(l-butene). It illustrates a special problem to polymer crystallography. Its space group requires simultaneous presence of centers of inversion and glide planes between the chains. One produces isoclined neighbors, the other anticlined neighbors, an obvious impossibility. One solution to... 

The 7-modification of iPP (y-iPP) may form in degraded, low molecular weight iPP or in samples crystallized under high pressure [5, 6]. Certain propylene copolymers with low comonomer content (4-10 wt.%) crystallize preferentially in y-form, as well. y-iPP has a face-centred ortho-rombic unit cell with parameters a = 0.85 run, b = 0.993 nm and c = 4.241 nm containing isochiral helices. The cell structure proposed by Bruckner et al. [5] is unique in polymer crystallography the chain axes in adjacent crystal layers are not parallel. The angle between the chain stems is about 80°. y-iPP is not usually observed as an independent phase, but crystallizes with and within the a-spherulites. According to Lotz et al. [7], the positive spherulites observed in samples with mixed polymorphic composition of a- and y-iPP are probably made of a... 

A central postulate of polymer crystallography is that the conformation of the polymer chains within the crystals is that of the lowest possible energy. Different crystal structures (so-called polymorphs) of a given compound may arise due to different types of packing of the low-energy chains. The chain... 

One question which immediately arises is that of the structure at the surface of polyethylene films produced in this manner. The question of polymer crystallography and morphology has been extensively discussed and documented by Wunder1ich. For the most part, discussion has centered around samples crystallized from solution or produced from the melt under pressure. There are two considerations which make it difficult to extrapolate from this work. 

Recently, new interesting phenomena that control the mode of packing of polymers have been found, and it has been shown that the basic principles of polymer crystallography are, in some cases, violated. In particular, (1) an atactic polymer can crystallize this is, for instance, the case of polyacrylonitrile [107] (2) in a crystalline polymer the chains can be nonparallel for instance, the structure of the yform of iPP is characterized by the packing of nearly perpendicular chains [108, 109] (3) the principle of entropy-driven phase formation may be violated and the high local symmetry of the chains is lost in the limit-ordered crystkhne lattice of polymers (symmetry breaking). 

In this chapter we review, briefly, the essential features of polymer structures. They are more complicated than those of metal crystals, and there is no formal framework (like that of crystallography) in which to describe them exactly. But a looser, less precise description is possible, and is of enormous value in understanding the properties that polymers exhibit. 

In this chapter the topochemical [2+2] photoreactions of diolefin crystals are reviewed from the viewpoints of organic photochemistry, analysis of reaction mechanism, and crystallography as well as in terms of synthetic polymer chemistry and polymer physics. 

In general it has been found that polypyrrolc is extremely poorly crystalline, limiting the information that can be obtained from direct structural techniques such as X-ray crystallography, and hence much of our knowledge has been obtained from indirect measurements and/or experiments on model compounds (e.g. X-ray studies on pyrrole trimers and dimers). It is now generally accepted that the ideal structure of the polymer is a planar (a a>bonded chain in which the orientation of the pyrrole molecules alternates. 

Organic Chemistry, X-ray Crystallography, and Modern Polymer Theory... 

Through a combination of X-ray crystallography and solid-state NMR studies, they were able to correlate chromic transitions with motion, and an increase in disorder within the backbone of the polymer [1]. As the temperature increased, so did disorder within the polymer, resulting in a blue-shift of the UV-vis spectra (Fig. 4). 

Dorset, D.L. (1992.) Electron crystallography of linear polymers direct determination for zonal data sets Macromolecules 25,4425-4430. 

In the preceding Sect. I have tried to illustrate the problems and developments of polymer stereochemistry from both the historical and logical points of view. A clear connection exists between synthetic and stmctural aspects For the solution of problems yet unsolved an interdisciplinary approach is required involving not only polymer chemistry but also spectroscopy, crystallography, statistical thermodynamics, solid state physics, and so on. 

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