Supramolecular side-chain liquid-crystal polymers. Part 1.—Thermal behaviour of blends of a low molar mass mesogenic acid and amorphous polymers

Abstract

The thermal behaviour of blends of 6-(4-n-butylazobenzene-4′-oxy)hexanoic acid (BABOHA) with three Polymers, polystyrene (PS), poly(2-vinylpyridine)(P2VP) and poly(4-vinylpyridine)(P4VP) has been characterised using differential scanning calorimetry and polarised light microscopy. BABOHA is a monotropic nematogen and its thermal behaviour is determined largely by hydrogen-bonded dimeric species. BABOHA is effectively immiscible with PS and thus, the thermal behaviour of BABOHA in the PS–BABOHA blends was essentially identical to that of pure BABOHA. In contrast, the transition temperatures of BABOHA are dramatically reduced in the blends with both P4VP and P2VP. The nematicisotropic transition temperature is reduced to a greater extent than the crystal–isotropic transition temperature. The driving force for miscibility in these systems is the formation of a hydrogen bond between the acid and pyridine groups. This thermal behaviour cannot be rationalised using the non-covalently attached side-chain liquid-crystal Polymer model suggested by Bazuin and Brandys (Chem. Mater., 1992, 4, 970). Instead we propose that the behaviour of the P2VP–BABOHA and P4VP–BABOHA blends may be understood by considering partial complexation of the Polymer by the acid, with this species acting as an isotropic solute in the liquid-crystal phase thus reducing the melting and clearing temperatures.