Hydrogen bonded network structure of polybenzoxazines.

摘要

The hydrogen bonding structure of polybenzoxazine is investigated by a series of benzoxazine model oligomers. By comparing the FT-IR spectra of the polybenzoxazines and model dimers, it is shown that the structures of asymmetric dimers, which have one hydroxyl group per molecule, well simulate the hydrogen bonded network structure for the polymer main chains while the structures of symmetric dimers reflect the hydrogen bonding scheme related to the end-groups of polymer chains. Bisphenol-A/methylamine based polymer (BA-m) mainly consists of −OHN intermolecular hydrogen bonding while bisphenol-A/aniline based polymer (BA-a) has a large amount of intermolecular hydrogen bonding and relatively weak hydrogen bonding groups in the polymer network structure.; To perform the further examination for more realistic longer chain oligomers, controlled-structure benzoxazine model oligomers are synthesized and characterized by nuclear magnetic resonance spectroscopy (NMR), Fourier transform infrared spectroscopy (FT-IR) and molecular modeling. The NMR resonances for model oligomers are newly assigned. Using the FT-IR spectra for model benzoxazine oligomers, the frequencies for the hydrogen bonded species in benzoxazine structure are assigned in detail. The energetically stable conformations of model oligomers due to the stable intramolecular hydrogen bonding by molecular modeling are proposed.; By investigating the FT-IR and 1H-NMR spectra of model dimers, which have different molecular size and basicity, it is revealed that the type and distribution of hydrogen bonding in polymers is closely related to the steric effect of amine functional group on the Mannich bridge as well as the basicity of the corresponding primary amine. It is demonstrated that benzoxazines based on extremely bulky amines cannot provide desirable properties. The possibility to investigate the −OHN intramolecular hydrogen bonding using 1H-NMR spectra of asymmetric dimers is shown.; The chemical stability of typical polybenzoxazines based on bisphenol-A and primary amines is studied. Using the results from the hydrogen bonding structure study, it is proposed that the nature of the primary amines is responsible for the interactions which take place between the carboxylic acid and the Mannich base model dimers. It is also found that the Mannich base is stable in carboxylic acid solution. The chemical stability of polybenzoxazines is explained by the nature of the hydrogen bonded network structure which develops upon cure. The possible network structure for BA-m and BA-a polymers is proposed and a generalized explanation for the structure-property relationships in polybenzoxazines is also discussed.