Using rapid chip calorimetry to investigate nano-gram quantities of material

摘要

One of the advantages of rapid chip calorimetry is that one can measure the behavior of very small quantities of material in order to investigate the material properties in nano-metric confinement, viz., ultrathin polymer films. In addition, the method can be used to investigate the calorimetric response of materials that are produced with great difficulty and in extremely small quantities, such as samples made by vapor deposition processes. We have been pursuing investigations in both areas. First, we have used the commercial Flash DSC system to investigate the glass transition response of ultrathin polymer films as a function of both cooling rate and film thickness [1]. Of particular interest is that the behavior of polycarbonate shows a strong reduction in the glass transition temperature as the films get thinner, with the magnitude of the reduction being greater for the slowest cooling rates. In addition it is observed that the temperature dependence of the response is also a function of film thickness with both fragility and activation energy decreasing with decreasing film thickness. In the case of materials that are very difficult to make or are made very slowly, we have used a vapor deposition process to create small quantities (30 to 250 ng) of an amorphous fluoropolymer and established that, under the right conditions of deposition, the fictive temperature is approximately 57 oC below the glass transition temperature [2]. By determining the cooling rate dependence of the fictive temperature of the rejuvenated sample we were able to determine the nominal Vogel [3] temperature or Kauzmann [4] temperature TK and find that the vapor deposited glass had a fictive temperature equal to the value of TK. This opens up novel perspectives in examining polymers in the deep glassy state [5] which we are exploring. Finally, the chip calorimeter can also be used to perform very rapid cooling and heating rate experiments, hence offering the opportunity to make glasses from materials that are otherwise difficult to vitrify due to intervening crystallization during ordinary cooling rates typical of conventional DSC. One such material is TNT for which we were able to determine the glass temperature ranges from 247.5 K to 239.3 K over the cooling rate from 1000 K/s to 10 K/s [6]. At slower cooling rates the sample was found to crystallize and no glass transition was observed. In addition, we found that at cooling rates above 100 K/s no cold crystallization was observed when heating at 600 K/s. Of interest in this case is that in working with energetic materials it is often a great benefit to be able to test small quantities of material because of safety concerns. The importance of the results will be discussed.