Fragility versus activation volume: Insight into molecular dynamics of glass-forming hydrogen-bonded liquids

In this Brief Report we show that key parameters, the fragility m(p) and activation volume Delta V, which characterize the sensitivity of molecular dynamics near the glass transition to temperature and pressure changes, respectively, reflect molecular properties in a nonequivalent way. Our comparative study of fragilities and activation volumes of isomeric pentiols provides evidence that the parameter m(p) can distinguish different H-bonded structures, even if molecular volumes are similar; however, the value of Delta V recognizes mainly the size of a relaxing molecular unit and correlates to the length scale of molecular cooperativity. Thus there is a striking difference in information given by m(p) and Delta V.