New insights into the fragility dilemma in liquids

A compilation of data for small molecule organic, polymeric, and inorganic glass-forming liquids shows that the original expectation, that there be a positive correlation between the thermodynamic measure of fragility $C_p^l{\mathrm{/C}}_p^g$ (or $C_p^l{\mathrm{/C}}_p^c)$ and the dynamic fragility index m, is not generally true. The results are consistent with three classes of behavior: (1) a decrease in m with increasing $C_p^l{\mathrm{/C}}_p^g$ for the polymeric glass formers; (2) a nearly constant value of m independent of $C_p^l{\mathrm{/C}}_p^c$ for small molecule organics and hydrogen bonding small molecules; (3) an increasing value of m with increasing $C_p^l{\mathrm{/C}}_p^c$ for inorganic glass formers as originally considered by Angell.