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 carboxylato- and sulfonato-(semi)telechelic polymers to form liquid crystalline halato(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 mesogen. The second method is based on the proton-transfer from a sulfonic or carboxylic 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 calorimetry (DSC), polarized optical microscopy (POM), small-angle X-ray scattering (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 results from the interplay of two competitive effects: the dipolar interaction of the ion-pairs known for multiplets formation and the propensity of the mesogenic moiety to form mesophases. The outcome of this competition depends on the mobility of the mesogenic counterion, i.e. on the strength of the dipolar interactions, the mobility of the polymer backbone and the mesogen/polymer ratio, which is controlled by the polymer molecular weight. A rod-like organization of the multiplets and a stretching of the polymer chains in the very close vicinity of the mesogenic core has been found in sulfonate polystyrenes, in agreement with the Eisenberg, Hird and Moore model for ionomers. The thickness of this region of restricted mobility has been estimated to 1 nm, which is the order of magnitude of the persistence length of polystyrene. Finally, LC H(S)TPs have been tested as interfacial agents in polystyrene/liquid crystal dispersions. The dipolar interactions of the ion pairs are clearly favorable to the additive localization at the polymer/LC interface. The higher polarity of the ammonium sulfonate pairs compared to the parent ammonium carboxylate ion pairs accordingly accounts for a higher interfacial activity.