Findings of sub-\(T_\mathrm{g}\) endotherm in vapor-deposited ultrastable phenolphthalein glass

We have performed differential scanning calorimetric and synchrotron x-ray diffraction studies to elucidate the nature of vapor-deposited ultrastable phenolphthalein glass. As a result, we found that phenolphthalein forms the ultrastable glass by depositing at 313 K, which is about 0.86 times the ordinal glass transition temperature of 361 K. As previous ultrastable glass studies reported, this ultrastable state involved an anisotropic structure. In addition, we found that a large endotherm (sub-\(T_\mathrm{g}\) endotherm) was observed in the temperature range between deposition and ordinal glass transition temperatures. We have assessed the stability of deposited states thermodynamically and found that those states are much more stable than those crystalline states when the deposition rate is small enough. The total enthalpies associated with the sub-\(T_\mathrm{g}\) endotherm are roughly proportional to the powers of the inversed thickness of the deposited glass. Despite the thermodynamical evidence, wide-angle x-ray diffraction of the structure associated with the sub-\(T_\mathrm{g}\) endotherm was unchanged. Following our findings, we have proposed a scenario in which ultrastable vapor-deposited phenolphthalein glass is rooted in locally superstable structure. Our locally oriented scenario would be universal for forming stable structures in other vapor-deposited glasses.