Abstract
Common epoxy thermosets are glassy at ambient temperatures and are characterized by a densely crosslinked microstructure. Under normal use conditions they generally fail by brittle fracture mechanisms. The influence of network microstructure on glassy fracture is largely undetermined in spite of a sizeable literature. This can be attributed to a lack of studies on structurally characterized networks and the often complicated microstructure of typical epoxy systems. To address these problems we examine structure-fracture relationships in simple epoxy systems whose structural variables are systematically controlled. Densely crosslinked networks may be characterized by equilibrium modulus measurements above Tg. Application of rubber elasticity theory yields very reasonable average network chain molecular weights (Mc); surprising in view of the expected non-Gaussian character of short epoxy network chains. Rubbery fracture energy increases with Mc when compared at equivalent temperatures above Tg. In fact, the dependence approximates a M 1/2 c ordering, suggesting that the influence of a threshold fracture energy persists well into nonthreshold testing conditions. Often, glassy fracture is characterized by brittle, unstable crack propagation leading to initiation and arrest fracture energies. The initiation values increase with temperature and generally increase with Mc. In comparison, the arrest values are independent of temperature and display a strong proportionality to M 1/2 c . A theory presuming material devitrification at a sharp crack tip is consistent with this observation.
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LeMay, J.D., Kelley, F.N. (1986). Structure and ultimate properties of epoxy resins. In: Dušek, K. (eds) Epoxy Resins and Composites III. Advances in Polymer Science, vol 78. Springer, Berlin, Heidelberg. https://doi.org/10.1007/BFb0035359
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DOI: https://doi.org/10.1007/BFb0035359
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