The chemical nature of the changes in a cyanate ester-novolac epoxy resin blend caused by hygrothermal ageing and the effects of the residue reaction in the blends on the hygrothermal ageing resistance were studied. The blends with epoxy molar fraction less than 0.5 absorb much less moisture in boiling water than does the homopolymer of the cyanate ester. The moisture uptake of the blend decreases with the extent of cure. The results from dynamic mechanical thermal analysis (DMTA) and Fourier transform infrared spectroscopy (FTIR) indicate that the blends possess superior hygrothermal stability compared to homopolycyanate. The results also indicate that the long-term hygrothermal ageing may cause substantial changes in the chemical nature of the blend when the cure extent is not sufficiently high.
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