[Image above] Sketch of the enthalpy–temperature behavior of differently annealed glasses at the annealing temperature Ta. Credit: Schawe and Wrana, Polymers (CC BY 4.0)
This month’s Glass: Then and Now topic, Glass relaxation, is highly related and complementary to last month’s discussion of viscosity and fictive temperature.
As mentioned last month, the glassy state is a nonequilibrium state. The changes to viscosity lock glassy materials into energy states that are greater than those had the material been allowed to equilibrate during the cooling process. Over time, the high energy forces the atoms and molecules to rearrange into lower energy (more stable) states, i.e., relax.
Our guest columnist for glass relaxation, Minoru Tomozawa, comes to us from Rensselaer Polytechnic Institute. Tomozawa, a Fellow and Distinguished Life Member of ACerS, received his Ph.D. from University of Pennsylvania (where I received my master’s degree) after working at Nippon Electric Company. He has published extensively in the area of glass science and previously served as chair of the Glass & Optical Materials Division.
“Glasses are metastable materials, meaning they will change over periods of time. Because these changes move glasses toward more stable configurations, this process is called relaxation.
There are many types of relaxation of glasses. The best known are structural and stress relaxation. Structural relaxation involves the change of fictive temperatures while, as the name suggests, stress relaxation reduces the residual stress, which plays an important role in glass annealing.
Both relaxation types affect the viscosity of glass over long periods of time. The relaxation time of structural relaxation is given by bulk viscosity/bulk modulus while that of stress relaxation is given by shear viscosity/shear modulus. These relaxations are measured through experiments performed near the glass transition temperature.
A third relaxation type—mechanical relaxation—takes place under applied stress and occurs much more rapidly than the structural and stress relaxations. Mechanical relaxation involves alkali and alkali earth ionic motion and mixed alkali ionic motion, as well as water molecule motion by applied mechanical stress. These mechanical relaxations involve change of elastic constants, which can be measured well below the glass transition temperatures.
Alkali ionic motion can also take place by applied electric field and rise to dielectric relaxation. The dielectric relaxation involves change of dielectric constants. Both mechanical relaxation and dielectric relaxation, additionally, involve energy loss component.
These relaxations cause time-dependent changes of glass properties. Thus, it is important to understand these relaxation kinetics in order to understand the time dependence of glass properties.”
– Minoru Tomozawa, Rensselaer Polytechnic Institute
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