Glass transition under confinement-what can be learned from calorimetry

摘要

Calorimetry is an effective analytical tool to characterize the glass transition and phase transitions under confinement. Calorimetry offers a broad dynamic range regarding heating and cooling rates, in-cluding isothermal and temperature modulated operation. Today 12 orders of magnitude in scanning rate can be covered by combining dif-ferent types of calorimeters. The broad dynamic range, comparable to dielectric spectroscopy, is especially of interest for the study of kineti-cally controlled processes like crystallization or glass transition. Accuracy of calorimetric measurements is not very high. Com-monly it does not reach 0.1% and often accuracy is only a few percent. Nevertheless, calorimetry can reach high sensitivity and reproducibil-ity. Both are of particular interest for the study of confined systems. Low addenda heat capacity chip calorimeters are capable to measure the step in heat capacity at the glass transition in nanometer thin films. The good reproducibility is used for the study of glass forming materials confined by nanometer sized structures, like porous glasses, semicrys-talline structures, nanocomposites, phase separated block copolymers, etc. Calorimetry allows also for the frequency dependent measurement of complex heat capacity in a frequency range covering several orders of magnitude. Here I exclusively consider calorimetry and its application to glass transition in confined materials. In most cases calorimetry reveals only a weak dependence of the glass transition temperature on confinement as long as the confining dimensions are above 10 nm. Why these findings contradict many other studies applying other techniques to similar systems is still an unsolved problem of glass transition in confinement.