Polymer single crystals defects

Some important aspects of topochemical polymerizations can be understood by inspection of Eq. (1), All reactivity comes about by very specific rotations of the monomers and by 1,4-addition of adjacent units and an extended, fully conjugated polymer chain is formed. The unique feature of the topochemical polymerization of diacetylenes is the fact that in many cases the reaction can be carried out as a single phase process. This leads to macroscopic, defect-free polymer single crystals which cannot be obtained, in principle, by crystallization of ready-made polymers by conventional methods. Thus, polydiacetylenes are ideal models for the investigation of the behaviour of macromolecules in their perfect three dimensional crystal lattice. 

Epitaxy and Twinning in Polymer Single Crystals. Epitaxy and twinning are frequently observed in polymer single crystals. In most cases, they are always associated with traditional crystal defects. Epitaxy is generally applied to oriented growth of one crystalline substance on a substrate of a different substance. However, it is also common that epitaxy may occur within one crystalline substance at different hkl) crystalline planes. By contrast, the twin contains an added element of symmetry over and above that which either component possesses. In other words, epitaxy only needs lattice match whereas twinning introduces an additional symmetry axis. 

The crystalline state of a polymeric material is the most efficient manner in which molecules can be packed into a dense state. However, polymers rarely exist as single crystals -defects and heterogeneities are almost always present. Such heterogeneities are inherent... 

In the case of polymer single crystals, the situation is not very different from that of regular crystals these entities undergo fusion or melting at a rather well-defined temperature because the dimensions of the crystalline domains are relatively large. The only structural defects are those corresponding to chain folding, hairpin turns, loops, and chain ends. 

The essential difference between the traditional concept of a crystal structure and crystalline polymers is that the former is a single crystal whilst the polymer is polycrystalline. A single crystal means a crystalline particle grown without interruption from a single nucleus and relatively free from defects. The term polycrystallinity refers to a state in which clusters of single crystals are involved, developed from the more or less simultaneous growth of many nuclei. 

And indeed we do. Of course, we cannot make perfect materials, there are always defects of some kind. But we can get close. What we would like to do for a polymer is grow true single crystals, where the chains are all fully extended. Remember, most crystalline solids are not single crystals, but collections of smaller crystals that grow from separate nucleation events until their boundaries impinge on one another. Furthermore,... 

However, disorder appears to be crucial in the occurrence of photoexcited paramagnetic centers since irradiation of single crystal samples, e.g. poly TS, has no effect. The energy levels in the low energy absorption tail of the disordered samples play an important part. It is not clear if these derive from excitonic or interband transitions but it should be noted that in principle any distortion of the polymer chain can lead to defect states in the optical gap ( ) and that the weak absorption tail in crystals is a major factor determining photoconduction (9 ). Thus, it appears that a basis exists for the correlation of structure and the intrinsic absorption spectra but that further efforts are required to obtain a better understanding of defect states and their spectroscopic properties. 

A single crystal of monomer becomes a nearly defect-free single crystal of the polymer . The two most commonly used monomers are the phenylurethane and tosylate derivatives of 2,4-hexadiyne-l,6-diol, 233a and 233b, but the reaction has been accomplished with a variety of symmetrical and unsymmetrical diacetylene derivatives . 

Simultaneous polymerization emd crystallization is another approach to memroscopic, defect-free single crystals of macromolecules (59). Recent examples include a preparative method for mixed metal coordination polymers (60), emd M. Hemack emd coworkers have reacted hemipotphyreizine (6 with iron (II) acetate in nitrobenzene to obtain single crystals of em oxygen-bridg polymer with iron in a -i- 4 oxidation state. 

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