Comment on “The fragility of alkali silicate glass melts: Part of a universal topological pattern” by D.L. Sidebottom

A recent article in this journal [ D.L. Sidebottom, J. Non-Cryst. Solids 516 (2019) 63–66] proposes a “universal topological pattern” for the composition dependence of liquid fragility in terms of the number of coarse-grained topological constraints in the glass-forming network. This “universal topological pattern,” however, is reported without any physical derivation from Angell's definition of liquid fragility. Alternatively, temperature-dependent constraint theory shows that fragility is indeed governed by the underlying topology of the glass network, but in terms of the temperature dependence of the constraints rather than the absolute magnitude of those constraints. Temperature-dependent constraint theory offers quantitatively accurate predictions of the composition dependence of fragility, together with insights regarding the contributions of each individual constraint to the overall fragility.