Structural relaxation in thin glass fibers

Structural relaxation is the process of gradual approach to equilibrium of a liquid following changes in temperature. This phenomenon forms the basis of the liquid to glass transition and of the memory effects in glasses.;Enthalpy relaxation is examined in samples of a soda-potasia-lime-silicate composition and a similar single-alkali composition with potassium replaced by sodium. For each composition, two types of samples were studied: those near equilibrium (slowly-cooled bulk) and those in conditions far from equilibrium (rapidly-cooled thin fibers and splat-cooled samples). The heat capacity of samples were measured by heating at a constant rate in a differential scanning calorimeter. Data were analyzed in terms of the Tool-Narayanaswamy (TN) model.;Results were similar for both compositions. Fibers begin to relax at about 420 K, exhibiting large exotherm and negligible endothermic glass transition peaks. Bulk samples do not relax below 650 K, exhibiting negligible exotherms and large endothermic peaks. Both types of samples gave the same temperature independent liquid state heat capacity and the same glassy state heat capacity whose temperature dependence is described by the Quasi-Harmonic model.;Good fits of the TN model were observed for the data on all bulk samples. However, the model fails to fit the data on fiber or splat-cooled samples. No composition difference between fiber and bulk samples was found. The anomalous fiber behavior is also not because of the mixed alkali effect.;Several modifications of the TN model were tried: (i) a fictive temperature dependent exponent in the stretched exponential relaxation function, (ii) non-linear driving force, and (iii) a set of N-Independent-Processes. None gave a satisfactory fit to the fiber data.;Inclusion of non-Newtonian viscosity in the TN model describes relaxations in fiber, splat-cooled, and bulk samples. It also explains the density relaxation data of Hara and Suetoshi at low temperatures where failure of the TN model was reported by Scherer.;Based on the present work, it is concluded that a non-linear description of the structural relaxation time similar to the non-Newtonian description of viscous flow, adequately describes the structural relaxation kinetics under conditions far from equilibrium.