Abstract
The temperature dependence of the Kohlrausch-Williams-Watts (KWW) stretching exponent β of shear relaxation kinetics is determined for a wide variety of deeply supercooled glass-forming liquids with fragility index m ranging between ∼ 30 and 90 using small amplitude oscillatory shear parallel plate rheometry. Intriguingly, and contrary to conventional wisdom, β and m are observed to be uncorrelated at temperatures close to the glass transition Tg. However, a clear pattern emerges when the variation of β is considered as a function of the α-relaxation timescale τα. In particular, β is observed to increase rapidly (slowly) for relatively strong (fragile) liquids with decreasing τα upon increasing temperature above Tg. Consequently dβ/d(log τα) is found to be negatively correlated with m near and above Tg. A possible origin of this intriguing trend is discussed within the frameworks of the energy landscape and the elastic facilitation models of relaxation of supercooled liquids.
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