INFLUENCE OF CHAIN FLEXIBILITY ON POLYMER MESOGENICITY

Molecular modelling procedures have been used to determine statistical parameters of isolated chains, with the objective of achieving a simple description which defines the tendency of a molecule to form a liquid-crystalline phase, its mesogenicity. Initial attention is focused on the persistence length parameter as the most promising in this context, while a related parameter, the turn-round length, is introduced to provide a description of mesogenicity which is equally applicable to molecules which consist of rigid segments separated by flexible sequences. A range of known mesogenic molecules has been modelled using Monte Carlo routines based on carefully determined bond-rotation potentials. For each molecule type a large number of chains are built for each temperature, and the chain parameters determined as averages over the models. Evidence is presented to suggest that for liquid-crystalline polymer molecules other than those containing flexible sequences, the nematic to isotropic transition temperature occurs when the ratio of the persistence length to diameter (the persistence ratio) reaches a value of 5. The predictive possibilities of this criterion are explored in the estimation of the nematic-isotropic transition temperatures of one or two common mesogenic polymers which are too high to be accessed experimentally. It is also applied to polyethylene, giving a value of 150 K for the onset of liquid crystallinity, a transition which is of course not normally seen owing to crystallization. Modelling of a series of aromatic copolyesters which contain different lengths of flexible (alkane) sequences, shows that the critical persistence ratio, calculated at the experimentally observed transition temperature, drops from 5 to 2.5 when the flexible sequences are long enough to decouple the orientation of neighbouring rod segments, and form comparatively low-energy hairpin folds in the mesophase. The introduction of the turn-round length/diameter parameter is promising and appears to have a true predictive capability to cover series of molecules ranging from worm-like to jointed rigid rod (Kuhn)-like.