Local dynamics of bismaleimide adhesives in an aggressive environment

Molecular aspects of chemical and physical changes in bismaleimide (BMI) adhesive joints caused by absorbed moisture were investigated. The focus was on the early (pre-damage) stage that precedes the formation of voids and microcracks. Local dynamics were investigated by broad-band dielectric relaxation spectroscopy (DRS) and the changes in the chemical state of the matter were monitored by Fourier transform infrared spectroscopy (FTIR). Absorbed water interacts with the BMI network and gives rise to a fast relaxation process (termed γ*), characterized by an increase in the dielectric relaxation strength, an Arrhenius temperature dependence of the average relaxation time, and an activation energy of 50 kJ/mol. The γ* dynamics are slower than the relaxation of bulk liquid water because of the interactions between the absorbed water and various sites on the network (the ether oxygen, the hydroxyl group, the carbonyl group, and the tertiary amine nitrogen). One particularly significant finding is that the average relaxation time for the γ* process above 20°C is of the order of nanoseconds or less and, hence, the detection and monitoring of this process hinges upon the ability to perform high precision DRS at frequencies above 1 MHz. This is an important consideration in the ongoing efforts aimed at the implementation of DRS as a non-destructive inspection (NDI) tool for adhesive joints. FTIR spectra reveal the presence of non hydrogen-bonded water and hydrogen-bonded water, the latter bonded to one and/or two sites on the BMI network. A good agreement was found between the calculated ratio of non hydrogen-bonded to total absorbed water from DRS and FTIR data.