Phase Behaviors and Structures of Supramolecules Formed by 4-Vinylpyridine-Based Polymers and Fan-Like Benzoic Acid Derivatives

In this work, we investigated the supramolecular complexes of fan-like small molecules and 4-vinylpridine based polymers via hydrogen bonding interactions. The fan-like molecules are the carboxylic acid derivatives with different number of hexadecyl tails, including 3,5-bis(hexadecyloxyl)benzoic acid (BA2) and 3,4,5-tris(hexadecyloxyl)-benzoic acid (BA3). We firstly used FTIR to investigate the degree of small molecules associated onto pyridine groups in the complexes of BA and homopolymer poly(4-vinylpyridine)(P4VP). It is interesting to find that for P4VP(BA3)x, where x is the molar ratio of added BA3 to P4VP repeating unit, the degree of complexation is not monotonically increased with x but reaches a maximum at x ~ 0.7 and then decreased as x is increased. Since BA3 is highly crystalline and tends to self-aggregate, an excess BA3 induces a macrophase separation that leads to the formation of a pure BA3 crystalline phase, thus reducing the number of BA3 molecules hydrogen-bonded with pyridine. The corresponding thermal behaviors of the supramolecules are confirmed by differential scanning calorimetry (DSC) measurements and the resulting structures are verified by X-ray scattering and polarized optical microscopy (POM) measurements.
In addition, we have studied the morphology of the supramolecules. Around the upper limit of complexation without macrophase separation, the homopolymer supramolecules P4VP(BA3) form small-scaled hexagonally packed cylinders, ~ 5 nm. When BA3 is incorporated into polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP), a variety of block copolymer-scaled structures, including lamella, perforated lamella, cylinder, and sphere are formed dependent on the amount of incorporated BA3. Furthermore, the small-scaled hexagonally packed cylinders can be maintained in the P4VP(BA3) blocks and a rarely seen hierarchical cylinder-within-sphere can be obtained. The present studies provided a better understanding of supramolecules containing highly crystalline components and demonstrated the feasibility of tailoring supramolecules without macrophase separation.