Dielectric and specific heat relaxations in vapor deposited glycerol

Recently (Nat. Commun. 3:1233), vapor deposited glasses of glycerol have been found to recover their super-cooled liquid state via a meta-stable, ordered liquid state (MROL) characterized by a tremendously enhanced dielectric strength along with a slow-down of the relaxation rate of the structural relaxation. The aim of this paper is to investigate the calorimetric signature of this phenomenon through the measurement of the specific heat in addition to the dielectric relaxation behavior. For this purpose we have developed a modified OMBD (organic molecular beam deposition) setup that allows the deposition of glassy thin films of glycerol, and the subsequent measurement of dielectric and specific heat relaxation spectra as the function of temperature. Here, interdigitated comb electrodes were used to measure the dielectric relaxation spectra, while the frequency depending specific heat was monitored by a pair of Si-based calorimeter chips in a differential setup to provide highest sensitivity for ultrathin (100 – 200nm) film samples. Heating of the as deposited glass just above the bulk Tg and subsequent cooling/reheating revealed an increase in cp by about 9% indicating that slow deposition of glycerol indeed yields a low enthalpy glass. A second, minor gain in the “glassy” cp was observed after full transformation of the ordered glycerol phase (-60°C) to bulk-like glycerol (OL), which is consistent with the idea that MRCO regions possess a lower cp that the OL state. Comparing the dielectric and specific heat relaxation times during the MRCOàOL transformation showed a gradual speed-up in the dielectric relaxation while no discontinuity was observed in the cp-relaxation times. This finding gives a first clue about the actual (small) volume fraction of MRCO structures coexisting with the OL being unable to cause a significant shift in the calorimetric relaxation spectrum. Finally we discuss possible crystallisation and reorganisation effects as they manifest in the specific heat data at higher temperatures.status: publishe