Hydrogen-bonded polyurethane complexes based on 4-alkoxybenzoic acids as the low molar mass components

Supramolecular polyurethane complexes based on a polyurethane with a pendant pyridyl unit (PUPy) as the hydrogen bond acceptor and 4-octyloxybenzoic acid (OOBA) or 4-butyloxybenzoic acid (BOBA) as the hydrogen bond donor were prepared. The primary driving force for the formation of these complexes is the hydrogen bonding between the pyridyl polyurethane group and the carboxylic acid group, while the urethane group only contributes to the binding of acid molecules to a minor extent. Phase separation in the BOBA/PUPy and OOBA/PUPy systems occurred at low molar ratios: BOBA and OOBA formed amorphous polyurethane complexes at a molar ratio of 0.2, while the OOBA/PUPy complex exhibited a glassy smectic mesophase at a molar ratio of 0.4. In comparison to the previously studied complexes of PUPy and 4-dodecyloxybenzoic acid (DOBA), where side-chain ordering prevailed, OOBA/PUPy complexes showed a high tendency of the main-chain crystallization at molar ratios of 0.4 or higher. This is due to weak overall van der Waals interactions between the short octyl tails and effective intra- and/or intermolecular hydrogen bonding between the urethane and pyridyl groups of the polymer backbone. (C) 2004 Society of Chemical Industry.