Heat Capacity Signature Resembling the Glass Transition in Crystalline Solids: Challenging the Classical View of Melting

Melting and glass transitions are classically viewed as distinct phenomena, the former in crystalline materials and the latter in amorphous systems. Here we show that fully crystalline, low-molecular-weight solids can display heat-capacity changes during melting that closely mimic those seen at the glass transition. Using temperature-modulated differential scanning calorimetry, we detect continuous, reversible steps in heat capacity in crystalline roxithromycin and glucose that mirror those of their amorphous counterparts. Similar behavior is observed during the crystallization of semicrystalline poly(3-hydroxybutyrate). These findings provide direct experimental evidence for a transition exhibiting a <i>C</i>_<i>p</i> signature similar in shape and magnitude to that observed at the glass transition, a transition-like thermodynamic signature in the melting of crystalline materials. This discovery expands the conceptual boundary of glass transitions and suggests that large-amplitude molecular motions underlie both melting and vitrification.