Influence of chain flexibility on polymer mesogenicity

Abstract

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 temperatures 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 from 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.