Examining the Initiation of the Polymerization Mechanism
and Network Development in Aromatic Polybenzoxazines
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Abstract
Three bis-benzoxazine monomers based
on the aniline derivatives of bisphenol A (BA-a), bisphenol F (BF-a),
and 3,3′-thiodiphenol (BT-a) are examined using a variety of
spectroscopic, chromatographic, and thermomechanical techniques. The
effect on the polymerization of the monomers is compared using two
common compounds, 3,3′-thiodiphenol (TDP) and 3,3′-thiodipropionic
acid (TDA), at a variety of loadings. It is found that the diacid
has a greater effect on reducing the onset of polymerization and increasing
cross-link density and <i>T</i><sub>g</sub> for a given
benzoxazine. However, the addition of >5 wt % of the diacid had
a detrimental effect on the cross-link density, <i>T</i><sub>g</sub>, and thermal stability of the polymer. The kinetics
of the polymerization of BA-a were found to be well described using
an autocatalytic model for which values of <i>n</i> = 1.64
and <i>m</i> = 2.31 were obtained for the early and later
stages of reaction (activation energy = 81 kJ/mol). Following recrystallization
the same monomer yielded values <i>n</i> = 1.89, <i>m</i> = 0.89, and <i>E</i><sub>a</sub> = 94 kJ/mol
(confirming the influence of higher oligomers on reactivity). The
choice of additive (in particular the magnitude of its p<i>K</i><sub>a</sub>) appears to influence the nature of the network formation
from a linear toward a more clusterlike growth mechanism
