The competing effects of physical aging and moisture absorption on the relaxation behaviour of a polycarbonate/acrylonitrile-butadiene-styrene (PC/ABS) polymer blend have been investigated. Physical aging was simulated by thermal aging the blend at temperatures up to 80 °C which is below the glass transition temperature of ABS, i.e., the lower of the two components. Another set of samples was exposed to six different relative humidity and temperature combinations. Progressively aged samples in dry and hygrothermal aging conditions were then subjected to stress relaxation tests. Momentary master curves were developed by applying time/aging-time and time/moisture superposition principles for dry and hygrothermally aged specimens, respectively. The experimental data were also fitted with the Kohlrausch-Williams-Watts (KWW) model to determine the initial modulus, relaxation time constant and shape parameter. Comparisons of the first two parameters between thermally aged and hygrothermally aged conditions suggest that physical aging processes dominate absorbed moisture effects in terms of influencing viscoleastic behaviour, especially when the aging temperature approaches 80 °C.

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