“Flexible” Nematic Polymers: Stiffening Near the Clearing Point

Polymers composed of a nematogenic backbone plus a relatively long “spacer”, are quite flexible in the isotropic fluid state. However, when the melt is cooled down towards the nematic transition point Tc, each monomer aligns the surrounding medium, and this induced alignment reacts on other monomers from the same chain. A simple (mean field) estimate of this effect gives a persistence length lp increasing like the square of the nematic correlation length ξ. Thus, near Tc, the chains must be rigid. This may explain why “flexible” polymers show pretransitional properties which differ from those of the monomer, and are rather of the Onsager Flory type, together with a large jump of the order parameter at Tc.