Insight into pi-stacking mechanism in liquid crystalline polymer films

29 Nov 2011. Scientists at ONDL announced the mechanism for the formation of monolayer-terraced morphology in poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene] (C14–PBTTT) film is due to the formation of unentangled pi-stacks of extended polymer chains in dilute solution of its borderline solvent. (Zhao et.al. Macromol 44 (2011) 9692 )These pi-stacks polymer chains are deposited substantially parallel to the film plane during the film formation. The subsequent annealing and cooling from its liquid crystalline phases accentuates this incipient order to develop monolayer-terraced morphology. This was previously thought to be promoted by interdigitation of alkyl side-chains. They further showed that this dilute pi-stacking mechanism is general. Mono-layer terraced morphology was observed in regio-regular poly(3-hexylthiophene) (rrP3HT) film when it is deposited from a mixture of chlorobenzene and mesitylene solvents which is a borderline solvent for rrP3HT. Finally, they show that in very dilute polymer solution where incipient pi-stacks do not entangle, mono-layer terrace can also be observed. “This finding clearly showed that alkyl side-chains crystallization is not necessary for supramolecular ordering of pi-stacking polymers. Mono-layer terraced morphology can even be observed in rrP3HT films.” says Li-Hong ZHAO.