Mixtures of an acid-functionalized mesogen with poly(4-vinylpyridine)

A thermotropic dialkoxy biphenyl liquid crystal, functionalized by a carboxylic acid group at one extremity, is mixed to varying proportions with poly(4-vinylpyridine) (P4VP). The liquid crystal, LC-4,6A, is characterized by a monotropic smectic A phase and a highly ordered or crystalline lamellar phase formed primarily of acid dimers. In the mixtures with P4VP (studied up to acid/pyridine molar ratios of 1.0), the fluid mesophase is suppressed. A solid-isotropic phase transition is present above a molar ratio of about 0.2, whereas for lower LC-4,6A contents the mixtures form an apparently miscible, amorphous material. The glass transition temperature, T-g, of the latter is strongly depressed, at a rate of 7 degrees C/mol % LC-4,6A up to a molar ratio of 0.1, after which the T-g remains essentially constant. As supported by infrared analysis, the plasticization behavior is attributed to strong hydrogen bonding between the acid and pyridine moieties, which solubilizes the LC-4,6A molecules within the polymer matrix. When the solubility limit is reached, the excess small molecule solidifies into an ordered phase that is essentially identical to that of neat LC-4,6A. This phase can be modified by quenching from the melt, which freezes in a greater proportion of the acid-pyridine interactions and leads to incorporation into this phase of significant amounts of P4VP.