Hydrogen bonding as it relates to miscibility of high performance poly(arylene ether)s with epoxy resins

The overall goal of this research project was to synthesize poly(arylene ether)s with electron rich functional groups for use as tougheners in epoxy thermosets followed by evaluation of the miscibility of the poly(arylene ether) materials in cured epoxy thermosets. Poly(arylene ether)s were synthesized by reaction of bisphenol A with a variety of dihalo monomers including 4,4’ -difluorodiphenylsulfoxide, 4,4’- difluorodiphenylphosphine oxide, 2,6-dichloropyridine, and 4,4’- dichlorodiphenylsulfone. A novel monomer, 2,3-bis(4-fluorophenyl)quinoxaline was also synthesized and utilized to make quinoxaline-containing homopolymers and copolymers. The bisphenol A based poly(arylene ether)s: poly(arylene ether sulfoxide), poly(arylene ether pyridine) and poly(arylene ether phosphine oxide) were found to be miscible with the amine cured epoxy thermosets. Poly(arylene ether sulfone) and poly(arylene ether quinoxaline) were immiscible with the epoxy thermosets.

Polymer-polymer miscibility is governed in large part by intermolecular attractive forces. To evaluate better the role that hydrogen bonding plays in the miscibility of the poly(arylene ether)/epoxy blends, a fundamental investigation of hydrogen bonding between model compounds possessing functional groups of interest (e. g. diphenylsulfone, diphenylsulfoxide, pyridine, triphenylphosphine oxide, and diphenylmethylphosphine oxide) and water was undertaken. ¹H NMR spectroscopy of model compounds in DMSO-d₆/ water (97/3) solutions was performed to ascertain the effect of the mole fraction of model compound in solution on the shift of the water peak in the ¹H NMR spectrum. In all cases, increasing mole fractions of the electron rich model compound caused the water resonance to move downfield, revealing the existence of hydrogen bonding interactions between the model compound and water.

The ¹H NMR shift data were used to calculate the equilibrium constants of formation of the water-model compound hydrogen bonded complex and the shift of the proton hydrogen bonded to the model compound. The phosphine oxide containing model compounds followed no trend; however, the other model compounds were ranked as: pyridine > diphenylsulfoxide > diphenylsulfone in hydrogen bonding strength. This supports the thermal analysis data which showed that the poly(arylene ether sulfone) was immiscible with the epoxy thermoset while the poly(arylene ether sulfoxide)/epoxy system was miscible.