Studies Of Non-Exponentiality Of Structural Relaxation Dynamics In Glass Forming Liquids

The structural relaxation dynamics in liquids have been studied extensively to gaininsight into the nature of glass transition. The non-exponential behavior is one of threecanonical dynamic characteristics of structural relaxations in supercooled liquids, anddirectly associated with molecular interactions. The studies on non-exponentiality wouldbe of significance in offering the configurational and dynamic information for developingthe understanding of the physics of glass transition phenomena through various materials.In this work, we experimentally study the non-exponentiality of structural relaxationdyanmcis of a number of glass forming liquids covering molecular, metallic, polymeric,ionic and solution glass formers. Dielectric and enthalpy relaxation techniques are applied,and the results are compared with prevous measurements in literatures. Broadbanddielectric measurements and data analyese are outlined in details. The non-exponentialparameter (βKWW) of structural relaxation is focused by constructing βKWW-relaxationfrequency plots across wide dynamic regimes. The experimental results show that βKWWfollows the time-temperature-superposition (TTS) law at low frequency or lowtemperature for most glass forming liquids, and yet the width of the TTS regimesomewhat depends on liquid fragility. βKWWincreases rapidly with the relaxation frequencyin the intermediate region and, eventually approaches to unity (β KWW=1.0) at fastdynamic regions with a critical relaxation frequency of¹0¹¹Hz (equivalently a relaxationtime of1.6×10^-12s). The experimental measurements of liquid mixtures also show thatmixing generally generates positive or negative deviation of βKWWfrom the ideal mixedline, as evidenced in composition dependence of T_g.Finally, we discussed the correlations between βKWWand liquid fragility index as wellas the width of TTS regimes. The non-exponential dynamics between dielectric andenthalpy relaxations are also highlighted. The relationship between βKWWandJohari-Goldstein β-relaxations in diols was discussed. The observation clearly shows thatβKWWis smaller in systems where the dielectric strength of JG relaxation has a highervalue.