Structure—Property Relations of Polyethertriamine-Cured Bisphenol-A-diglycidyl Ether Epoxies

Abstract

The relations between the chemical and physical network structure, the deformation and failure pro cesses and the tensile mechanical properties of polyethertriamine cured bisphenol-A-diglycidyl ether epoxies are reported for a series of epoxy glasses prepared from a range of polyethertriamine concen trations. Near infrared spectroscopy indicates that these glasses form exclusively from epoxide-amine addition reactions. Their Tg exhibits a maximum and their swell ratio a minimum at the highest crosslink density. Stress-birefringence studies reveal that these highly crosslinked glasses are ductile and undergo necking and plastic deformation. The plastic deformation initially occurs homogen eously, but ultimately becomes inhomogeneous and shear bands develop. Tensile failure occurs in the high strain shear band region. The ultimate tensile strain of these epoxies attains a maximum of 15% for the highest crosslinked glass. Off-stoichiometric networks fail at lower strains because such networks inherently contain more defects in the form of unreacted ends. The density, yield stress, tensile strength, and modulus of these glasses all decrease with increasing polyethertriamine concentration as a result of increasing free volume because of the poor packing ability of the amine molecule. A minimum is superimposed on this downtrend in density and mechan ical properties with increasing amine content at the highest crosslink density because of geometric con straints imposed on segmental packing by the net work crosslinks. The ability of these crosslinked glasses to undergo deformation is discussed in terms of the free volume and the crosslinked network topo graphy. Network failure is considered in terms of stress-induced chain scission which is determined by the concentration and extensibility of the least extensible network segments.

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Application of Mechanistic Chemical Kinetics to Thermoset Composites Processing

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