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.
B i s ( 3 , 4 -d i h y d r o -3 -p h e n y l -2 H -1 , 3benzoxazine)propane (BA-a) is blended with oligomers of polyarylsulfone (PSU) and polyarylethersulfone (PES) of different low/intermediate molecular weights (3000−12 000 g mol −1 ) and terminal functionality (chloro-, hydroxyl-or benzoxazinyl-(Bz)). Fracture toughness (K IC ) is observed to increase from 0.8 MPa m 0.5 for cured BA-a to 1 MPa m 0.5 with the incorporation of 10 wt % PSU-Bz (12 000 g mol −1 ). Generally, greater improvements in K IC are observed for the PES oligomers compared with the PSU oligomers of equivalent molecular weight. The terminal functionality of the thermoplastic has a lesser effect on improving toughness than increasing the molecular weight or the nature of the polymer backbone. Surface analysis of the fractured surfaces show greater phase separation and crack pinning in the PES toughened system. Where crack pinning is less obvious, as in the case of hydroxyl-terminated PES (of 6000 g mol −1 ), this coincides with a drop in fracture toughness.
A series
of blends is prepared comprising 2,2-bis(3,4-dihydro-3-phenyl-2H-1,3-benzoxazine)propane (BA-a) with variously 5, 10, or
20 wt % of a selected oligomer represented by poly(arylsulfone) (PSU)
or poly(arylethersulfone) (PES). The oligomers, comprising either
chloro-, hydroxyl- or benzoxazinyl- (Bz) terminal functionality, are
of low molecular weight (3000–12000 g mol–1). The introduction of the oligomers is shown to initiate the polymerization
of a bisbenzoxazine monomer where the terminal functionality of the
oligomer is coreactive (e.g., hydroxyl or benzoxazine) without having
a detrimental effect on the polymerization kinetics (similar values
for the activation energy and orders of reaction are obtained). The
introduction of the nonreactive chloro-terminated oligomer appears
to favor the formation of an interpenetrating network (IPN) with a
higher energy of activation. The thermal stability of the blends is
generally increased compared with the polybenzoxazine homopolymer,
regardless of the molecular weight or thermoplastic loading. Aside
from the aforementioned PSUCl-containing IPN, the nature
of the resulting network is slightly modified by the addition of the
thermoplastic with similar or slightly elevated cross-link densities
recorded (compared with the polybenzoxazine homopolymer). The heterogeneity
of the network increases with a broadening of the tan δ response,
suggesting an improvement in the toughness of the resulting blend.
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