Haruhiko Yao | Search arXiv

arXiv2026

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

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.

cond-mat.soft↗

arXiv2021

Towards the ideal glass transition by pinning in a dimer-polymer mixture

We use a mixture of a polymer and its dimer to control dynamics in a manner inspired by \emph{pinning} a fraction of the system. In our system of $α$-methyl styrene, where the polymer has a glass transition at higher temperature than the dimer, at intermediate temperatures, the polymer acts to "pin" the dimer. Within this temperature range, we use differential scanning calorimetry to infer a point-to-set length which we find to be profoundly influenced by the degree of pinning. We determine the dynamics of the system with dielectric spectroscopy and find that while the dynamics are very substantially slowed by the "pinning", the fragility exhibits only a small change relative to the precision of our measurements. This may indicate that in the approach we have used, fragility has a relatively weak dependence on quantities such as the point--to--set length. % than one might expect, However, an alternative explanation is that the dimer may act to \emph{plasticize} the polymer and thus open routes to relaxation that may be inaccessible to fully pinned systems.

cond-mat.soft↗