The Glass Transition Temperature Measured by Different TA Techniques. Part 1: Overview
Introduction
The glass transition is an important thermal effect that can be used to characterize plastics and other amorphous or semicrystalline materials (e.g. inorganic glasses) [1]. For this reason, the determination of the glass transition temperature (Tg) is one of the most frequently performed TA applications. At the glass transition, changes can be observed in physical quantities such as the specific heat capacity, the thermal coefficient of expansion and the mechanical modulus. In view of the fact that the measurement principles involved in DSC, TMA and DMA are different, the question immediately arises as to which technique should be used and to what extent the measured glass transition temperatures can be compared.
A comparison of glass transition temperatures measured under different conditions may show temperature differences of several Kelvin. In practice, an understanding of the origin of these differences is of great importance, in particular when comparing different materials, e.g. in quality assurance. A special point to note is that a glass is an amorphous solid and is not in thermodynamic equilibrium. The transition to the liquid or rubbery state is a relaxation process [2] and is therefore under kinetic control. The glass transition does not therefore occur at a specific temperature as with melting, but rather over a broad temperature range. To nevertheless determine temperatures that can be compared numerically, different evaluation procedures and corresponding standard methods have been developed. The DSC evaluations and several standard methods are described in reference [3]. The interpretation of DSC, TMA or DMA glass transition measurement curves is discussed in several previous UserCom articles [4].
The Nature of the Glass Transition
On cooling, at the glass transition, a material passes from a supercooled liquid or rubbery-like state to a glassy solid state. The glass transition also occurs in the reverse direction on heating. In the liquid state, molecules are able to move relative to one another. So-called cooperative rearrangements occur. The volume of the region involved in the rearrangement is several nm3 . In the glassy state, the cooperative rearrangements are frozen in (see the analogy in the text box).
Cooperative rearrangements take place at a certain rate and hence have a characteristic frequency. The frequency of the rearrangements is lower at lower temperatures, i.e. the rearrangements take place more slowly
The Deborah number (D) can be used to characterize time- or frequency-dependent events. D is the ratio of the characteristic time of the cooperative rearrangement, τa, and the observation time, tb, so that D = τa / tb. τa is shorter at higher temperature. tb depends on the measurement parameters (cooling rate, frequency). For D < 1, the characteristic time of the cooperative rearrangement is shorter than the observation time. The material appears to be liquid or rubbery-like. In the glassy state D > 1. The cooperative rearrangements are so slow that they do not occur during the measurement. They therefore appear to be frozen. From these considerations, it is apparent that the glass transition depends on the measurement or observation conditions.
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Summary: Comparison of TA Techniques to Measure Glass Transitions
An overview of measurement techniques and their application is given in Table 1. The sensitivity of the techniques is summarized in Table 2. In Part 2 of this study, measurements of the glass transition obtained using the different TA techniques will be described and the results compared. Polystyrene is used as an example.
The Glass Transition Temperature Measured by Different TA Techniques. Part 1: Overview | Thermal Analysis Application No. UC 171 | Application published in METTLER TOLEDO Thermal Analysis UserCom 17