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

Abstract

We have performed heat capacity measurements for physically vapor-deposited phenolphthalein glass using conventional and in-house high-sensitivity differential scanning calorimetry. As a result, we found that phenolphthalein forms an ultrastable glass when deposited at 313 K, about 0.86 times the ordinary glass transition temperature of 361 K. In addition, we observed a large endotherm (sub-$T_{\mathrm{g}}$ endotherm) between deposition and the ordinary glass transition temperature. From an enthalpy perspective, the integrated enthalpy of the sub-$T_{\mathrm{g}}$ endotherm increased with decreasing film thickness following an inverse-power dependence, whose exponent increased as the deposition rate decreased. In particular, the integrated enthalpy of the sub-$T_{\mathrm{g}}$ endotherm was comparable in magnitude to the enthalpy of fusion of crystalline phenolphthalein. To investigate the structural origin of the sub-$T_{\mathrm{g}}$ endotherm, we performed wide-angle X-ray diffraction and in situ atomic force microscopy. As a result, we found that while the stable structure is linked to an anisotropic amorphous structure consistent with past research on ultrastable glasses, the appearance of the sub-$T_{\mathrm{g}}$ endotherm is associated with surface morphological relaxation, as suggested by atomic force microscopy.

16 pages, 14 figures. This revised version includes new in situ AFM measurements and analyses, with a new figure and corresponding additions to the Methods, Results, Discussion, and Conclusions. Five authors have been added to reflect their contributions to the new AFM study. Submitted to The Journal of Chemical Physics