Hybrid liquid crystal polymers from the self-assembly of poly(vinylpyridine) and polyoxometalates via multiple non-covalent bonds

Abstract

A quaternary ammonium surfactant (S) carrying a phenol terminal was grafted on the surface of nano-sized polyoxometalates, H₄SiW₁₂O₄₀ (PM-1), K₆P₂W₁₈O₆₂ (PM-2) and Na₁₀[Co₄(H₂O)₂(VW₉O₃₄)₂] (PM-3), respectively, by ionic self-assembly. The obtained complexes were connected with the poly(vinylpyridine) (P4VP) backbone via hydrogen bonds, resulting in the formation of nano-hybrid supramolecular polymers. Differential scanning calorimetry, polarized light microscopy, temperature-dependent X-ray diffraction, and transmission electron microscopy confirmed that the hybrid polymers containing PM-1 exhibited a thermotropic smectic C phase, where the alkyl chains of the surfactant are equally distributed on either side of PM-1 in a compact manner with only interfacial hydrogen bonding interactions between the phenol groups and the pyridine units of P4VP. Detailed investigation revealed that the liquid crystal properties are dependent on the molar ratio between PM-1 and P4VP, the molecular weight of P4VP and the charges of PMs. An increase of charge from 4 to 6 results in a loss of liquid crystal behaviour. Although the liquid crystal structure of the hybrid polymers is independent of the molecular weight of the P4VP backbone, the transition temperatures are strongly related to the change of the molecular weight of P4VP. The UV-vis and XPS measurements reveal that these hybrid liquid crystal polymers show reversible photochromic properties because of the multi-electronic redox activity of PM-1. The present article opens up a door for developing nano-hybrid liquid crystal polymers via a multi-step supramolecular self-assembly strategy, and provide an insight into understanding the nature and mechanisms of the influence of nano-objects on the self-assembly behaviour of hybrid polymers.