Polyoxymethylene extended-chain crystals

It is known that in a solid state polymerization occurs along the certain axis of the monomer crystal and extended chain crystals are formed [146]. In the polymerization from liquid phase, on the other hand, lamellar crystals are formed. This indicates that back-biting reaction proceeds not at random but in a specific manner, forced by the nature of crystals. The model of growth of lamellar crystals of polyoxymethylene is known [147]. According to this model, the subsequent layers are formed on the surface of the crystal by growth of folded chain as shown schematically below ... 

A characteristic feature emerges Crystals formed in solution or by melt polymerization are more perfect than those formed upon crystallization of the preformed polyoxymethylene from solution or melt. If extended chain crystals with >95% crystallinity are reprecipitated from hexafluoroacetone sesquihydrate, the degree of crystallinity decreases to 67.5 %76>. 

Other Thermoplastics. Polyoxymethylene (POM) was imaged by afm, revealing oriented polymer chains parallel with the machine axis of sample extrusion (Fig. 15). Atomic scale resolution of the chains demonstrated the helical nature of the polymer chains. Long-range correlation between poljuner chains was observed as well (97). Imaging of extended chain crystals of POM closely matched molecular models for this material, allowing for the molecular packing and order in the extended chain crystal to be well understood. The authors were able to describe the polymer chain orientations with respect to the crystal (98). 

Direct Measurement of when Equilibrium Samples are Available. Extended-chain crystals have been prepared. For a limited number of polymers. Polyethylene was the first (Geil et al. 1964 Prime and Wunderlich 1969 Prime et al. 1969) in addition, extended-chain crystals have been prepared from polyoxymethylene (Jaffe and Wunderlich 1967 Iguchi 1976) poly(tetrafluoroethylene), poly(ethylene terephthalate)... 

Snetivy D, Vancso GJ. Selective visualization of atoms in extended-chain crystals of oriented polyoxymethylene by atomic force microscopy. Macromolecules 1992 25 3320. 

Figure 5.1 (a) The solution-grown polyoxymethylene single crystal, (b) spherulites of melt-cooled polyethylene sample and the hierarchical structure, (c) extended-chain crystals of polyoxymethylene (whisker), and (d) the shish-kebab structure. (See color insert.)... 

Moreover, if polyoxymethylene is recrystallized from nitrobenzene and the usual chain-folded lamellae of ca. 100 A thickness is treated with TXN in the presence of BF3, more perfect extended-chain-type crystals are formed. These facts confirm the conclusion formulated -by Wunderlich, that post-crystallization leads to chain-folded crystals because of kinetic restrictions, while crystallization in the polymerizing system gives thermodynamically more stable extended-chain crystals77). The formation of the thermodynamic product in polymerization is due to the growth of crystals at equilibrium polymerization conditions. Thermodynamically less stable crystals may redissolve as a result of depropagation, and crystals may thus grow further under equilibrium conditions. This (at least partly) eliminates kinetic restric-... 

The high axial elastic modulus of polyethylene and polyamide 6 is due to the fact that these polymers have a preferred conformation that is fully extended, i.e. all-trans. The elastic deformation is caused by the deformation of bond angles and by bond stretching, both showing high elastic constants. Isotactic polypropylene and polyoxymethylene crystallize in helical conformations and therefore exhibit a maximum stiffness which is only 20% of the maximum stiffness of the all-trans polymers. The elastic deformation of a helical chain involves, in addition to the deformation of bond angles and bond stretching, deformation by torsion about the G bonds. The latter... 

A general picture emerges concerning the values of chain direction moduli of polymer crystals. They tend to be high if the molecule is in the form of a planar zig-zag rather than a helix. For example, polyethylene is stiffer than polyoxymethylene or polytetrafluoroethylene which both have molecules in helical conformations (Table 4.1). The helices can be extended more easily than the polyethylene planar zig-zag. Also the presence of large side groups tend to reduce the modulus because they increase the separation of molecules in the crystal. This causes an increase in the area supported by each chain. 

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