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Well annealed specimens of polycarbonate film were first subjected rapidly to a static elongation of 2.6% at 13 temperatures from −110 to 110°C. During stress relaxation at each temperature, the static strain was modulated with a sinusoidal tensile strain of small amplitude (⩽ 0.1%), usually from 0.4 to 25 Hz, to determine the (differential) storage modulusE′. Such measurements were repeated intermittently at values of the elapsed (ageing) timete during periods of several hours or more. During such periods,E′ increased continuously, a reflection of physical ageing, a hardening process. Initially, the storage modulusE′0 was significantly higher than after the static elongation was applied; the latter produces de-ageing, a softening process. From the data, the ageing rate d logE′/d log te was evaluated and found to be independent of frequency andte. The ageing rate showed a minimum at about 30°C, a pronounced maximum at –40°C and then became almost zero at –110°C. Also discussed is the temperature dependence of d logE′d log f, wheref is the frequency, and the effect at two temperatures of different values of the static elongation on both the ageing rate and the frequency dependence ofE′. From all data, the ageing-rate parameter μ was also evaluated and discussed. |
