Effects of thermal history on the polymerisation mechanism and network development in aromatic polybenzoxazines

The effect of heating rate (2, 8 and 15 K min-1) during the initial stages of cure of 2,2-bis(3,4-dihydro-3-phenyl-2H-1,3-benzoxazine)propane is examined. The rate of heating has a marked effect on the observed modulus, measured by DMTA, with the higher heating rate giving rise to an increase in storage modulus of ca. 1000 MPa, although this is not accompanied by an increase in glass transition temperature. The thermal stability of the resulting polybenzoxazines also differs with the slower heating rate giving rise to less thermally stable structures. Data obtained from Raman spectroscopy (when combined with principal components analysis) suggest subtle changes in the mechanism during the early stages of reaction associated C-N-C and C-O moieties, some of which persist following a higher temperature postcure step leading to a crosslinked network with higher aliphatic character.