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 Tg for a given benzoxazine. However, the addition of >5 wt % of the diacid had a detrimental effect on the cross-link density, Tg, 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 n = 1.64 and m = 2.31 were obtained for the early and later stages of reaction (activation energy = 81 kJ/mol). Following recrystallization the same monomer yielded values n = 1.89, m = 0.89, and Ea = 94 kJ/mol (confirming the influence of higher oligomers on reactivity). The choice of additive (in particular the magnitude of its pKa) appears to influence the nature of the network formation from a linear toward a more clusterlike growth mechanism.
Highlights
Conventional phenolics are cross-linked products of low molecular weight precursors, typically formed through a condensation reaction
The presence of the additive improves the temperature of polymerization, by encouraging the ring-opening reaction at a lower temperature, and its presence appears to improve the thermal stability of the final polymer
The selection of an initiator based on its pKa appears to be able to influence both the nature of the network architecture and potentially the final properties of the polybenzoxazine as the Tg is elevated when 3,3′-thiodipropionic acid (TDA) is used in preference to 3,3′-thiodiphenol
Summary
Conventional phenolics are cross-linked products of low molecular weight precursors, typically formed through a condensation reaction. Samples were analyzed using DSC in order to establish the relative effects of the additives, and the thermal data for the “uninitiated” polymerization of BA-a are displayed in Figure 5 along with the corresponding mixtures containing each.
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