Phase Behavior of a Hydrogen-Bonded Polymer with Lamella-to-Cylinder Transition: Complex of Poly(4-vinylpyridine) and Small Dendritic Benzoic Acid Derivative

Abstract

Phase behavior of a supramolecular system based on poly­(4-vinylpyridine) (P4VP) and 3,4,5-tris­(dodecyloxy)­benzoic acid (TDBA) [P4VP­(TDBA)_<i>x</i>, where <i>x</i> is the molar ratio of TDBA to P4VP repeating unit] was investigated by means of FTIR, differential scanning calorimetry, polarized optical microscopy, and X-ray scattering method. The intermolecular hydrogen-bonding interaction between P4VP and TDBA is confirmed by FTIR. While almost all of the added TDBA molecules are hydrogen bonded to the P4VP chains at <i>x</i> < ∼0.60, the hydrogen-bonding interaction becomes incomplete at <i>x</i> > 0.60 and saturates at <i>x</i> > 0.90. The phase structure of P4VP­(TDBA)_<i>x</i> is composition dependent. At <i>x</i> < ∼0.30, the complex is homogeneous. With ∼0.30 < <i>x</i> < ∼0.60, P4VP­(TDBA)_<i>x</i> forms a lamella phase, of which the long period is proportional to 1/<i>x</i>. Further adding TDBA causes a lamella-to-cylinder transition. At <i>x</i> > ∼0.60, the lattice parameter of the cylinder or hexagonal columnar (Φ_H) phase decreases with increasing <i>x</i>. Considering the microphase separation between the polar part and the nonpolar part of alkyl tails, the lamella-to-cylinder transition can be understood using a volumetric argument. We consider that the large nonpolar part of TDBA enhances the microphase separation of P4VP­(TDBA)_<i>x</i>, and moreover, the fan-like shape of TDBA facilitates the formation of Φ_H phase. We also roughly estimated the domain size of the P4VP chains in the microphase-separated mesophase. For both the lamellar and Φ_H phase, increasing <i>x</i> results in stronger confinement on the P4VP chains. During the lamella-to-cylinder transition the confinement imposed by the TDBA molecules may be partially released, which favors the Φ_H phase formation