Polyoxymethylene, helical conformation

Fig. 5.23 The 9s helical conformation of polyoxymethylene (a) viewed along the chain axis and (b) viewed perpendicular to the chain axis and to the line AB.

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... 

The molecular chain of trigonal polyoxymethylene (-CHj-O-jp takes the 9s helical conformation [Tadokoro, Yasumoto, Murahashi, and Nitta (I960)]. There are four atoms per repeat unit of CH2O and accordingly twelve branches of chain vibrations. 

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. 

The shape of macromolecules within a folded lamella is not the same for all polymers. In crystalline polyethylene, for example, the chains assume a planar zigzag conformation, but in some other polymers like polypropylene and polyoxymethylene the chains prefer a helical shape, as in proteins. The helix might have three, four, or five monomer units per turn, i.e., the helices are three-, four-, or five-fold (Fig. 1.12)... 

An accurate evaluation of the energetics of different conformations in pol3nners is of basic importance for an understanding of their physical properties. We therefore investigated also other periodic conformations of polyethylene and polyoxymethylene including extensive geometry optimization. Results on the energies of (t), (g), and (tg) helices are collected in table 6,... 

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