Ionic end-capping of (semi)telechelic polymers by mesogens: a novel route to liquid crystalline polymers

A mesogenic cation has been associated with low molecular weight carboxylat o- and sulfonato-(semi)telechelic polymers to form liquid crystalline halat o(semi)telechelic polymers (LC H(S)TPs). Two methods have been used to end- cap the linear polymer chains at one (or both) end(s) by a mesogen through ionic bonding. The first method relies on the ion-exchange reaction between the metal counterion of halato(semi)telechelic polymers and an ionic mesog en. The second method is based on the proton-transfer from a sulfonic or ca rboxylic acid end-group to a tertiary aliphatic amine, this approach being controlled by the relative pKa's of the acid and basic groups. The resulting materials have been characterized by differential scanning ca lorimetry (DSC), polarized optical microscopy (POM), small-angle X-ray scat tering (SAXS) and small-angle neutron scattering (SANS). On the basis of these experimental results, a model for the supramolecular organization of the LC H(S)TPs has been proposed. The final morphology resu lts from the interplay of two competitive effects: the dipolar interaction of the ion-pairs known for multiplets formation and the propensity of the m esogenic moiety to form mesophases. The outcome of this competition depends on the mobility of the mesogenic counterion, i.e. on the strength of the d ipolar interactions, the mobility of the polymer backbone and the mesogen/p olymer ratio, which is controlled by the polymer molecular weight. A rod-li ke organization of the multiplets and a stretching of the polymer chains in the very close vicinity of the mesogenic core has been found in sulfonato polystyrenes, in agreement with the Eisenberg, Hird and Moore model for ion omers. The thickness of this region of restricted mobility has been estimat ed to 1 nm, which is the order of magnitude of the persistence length of po lystyrene. Finally, LC H(S)TPs have been tested as interfacial agents in polystyrene/l iquid crystal dispersions. The dipolar interactions of the ion pairs are cl early favorable to the additive localization at the polymer/LC interface. T he higher polarity of the ammonium sulfonate pairs compared to the parent a mmonium carboxylate ion pairs accordingly accounts for a higher interfacial activity. (C) 2001 Elsevier Science Ltd. All rights reserved.