Order-disorder Transitions in Structures Containing Helical Molecules

The most commonly considered types of disorder in a crystal containing long-&& molecules are those in which the individual chain molecules are either rotated about their long axes or else translated along the fibre axis. However, since many fibrous polymers are helical in structure, we should also expect the possibility of screw disorder; that is, a combination of the above two types. A disorder of this kind can be shown to exist orientated gels of tobacco mosaic virus and in fibres of polytetrafluoroethylene (PTFE) at 20°C. R e method by which screw disorder may be deduced from X-ray diagram is described, and the order-disow transitions in the two substances discussed. The me&. anism of the transition in PTFE is of particular interest in connection with its remarkably low coefficient of friction. Attention is also drawn to a related type of disorder found in stretched fibres of deoxyribonucleic acid.