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

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 nematic-isotropic 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.