Interplay of Ionic, Hydrogen-Bonding, and Polar Interactions in Liquid Crystalline Complexes of a Pyridylpyridinium Polyamphiphile with (Azo)phenol-Functionalized Molecules

Abstract

Most supramolecular side-chain polymers investigated to date are based on a single type of noncovalent link and involve systems that do not test its selectivity in the presence of other potentially interfering interactions. In this paper, we study the selectivity of hydrogen-bond complexation in the presence of both ionic moieties and highly polar groups and evaluate the effect of these potentially interacting functions on the self-assembly and thermotropic behavior of the intended side-chain liquid crystal (LC) polymer complexes. To this end, the weakly mesomorphic polyamphiphile, poly(ω-pyridylpyridinium dodecyl methacrylate) bromide (P12PP), where the side chains are terminated by a hydrogen-bond acceptor in close proximity to an ion pair, is mixed in equimolar proportion with a series of phenol-functionalized mesogenic complexants (X), some of which have highly polar tails, while others have chiral tails and most contain an azo moiety for potential optical responsiveness. It is found that, in all cases, hydrogen-bond complexation is successful and essentially complete, leading to supramolecular homopolymeric complexes P(X) with well-defined thermotropic LC characteristics. These complexes, which are partially crystalline initially, have above-ambient glass transitions and smectic LC mesophases that are monotropic, enantiotropic or effectively enantiotropic, depending on the molecular characteristics of X. The presence of a polar tail hampers recrystallization after melting, with a cyano tail suppressing it completely and also strongly retarding the development of the LC mesophase from the isotropic phase. This might be related to enhanced electrostatic interactions and increased viscosity in the melt. The mesophase periodicity of P12PP is unchanged by complexants with a flexible alkyl tail, suggesting that they can adapt to the packing structure of P12PP (previously reported to be partial bilayer smectic A-like), whereas more rigid albeit shorter complexants lead to a periodicity increase (indicative of an effective single-layer smectic A mesophase). Possible molecular packing models are presented and discussed.