Tensile Fracture of Molecular Glasses Studied by Differential Scanning Calorimetry: Reduction of Heat Capacity by Lateral Constraint

Molecular glasses indomethacin and ortho-terphenyl were formed and fractured by cooling a liquid on a less thermally expansive substrate. In-plane tension was created by the mismatch of thermal expansion coefficients and accumulated to cause catastrophic network fracture. Differential scanning calorimetry was used to characterize the process. The heat of fracture exceeds by 10 times the strain energy released, and matches the excess enthalpy stored by an elastic film that is cooled under lateral constraint. The constrained film has a smaller heat capacity than a free-standing film, by approximately 0.01 J/g/K or 1%. This allows the constrained film to reach higher enthalpy on cooling and the excess enthalpy is released at fracture.