Fragility and mechanical moduli: do they really correlate?

The correlation between the fragility of glass-forming systems and the ratio of instantaneous bulk to shear moduli, K ∞/G ∞, of the corresponding glasses is analyzed for bulk metallic and polymeric materials. We emphasize the importance of the free electron contribution to the bulk modulus of metals and a specific intramolecular contribution to fragility in the case of polymers for some long molecules. We argue that these two contributions are the main reasons for the failure of the correlation between fragility and K ∞/G ∞ observed for these materials.