In part one of this introduction, imaging with the micro-TA system was explained. In this section local thermal analysis is described.

The type of results obtained with the TA Instruments 2990 micro-TA system are illustrated below. A thermal image of polymer blend is shown that was obtained in the imaging mode described earlier. Points are then selected on the image using the mouse.

The probe is moved to the first point selected and placed on the surface of the sample. The temperature of the tip is then ramped linearly with time. The probe is used in conjunction with a reference probe in a way that is very similar to conventional Differential Thermal analysis; thus a 'calorimetric' signal can be obtained. At the same time the degree of bending of the cantilever is measured thus, as the material beneath the tip softens and the probe penetrates into the sample, this provides the micro equivalent of a thermo-mechanical analysis experiment. This is illustrated in the animation.

In the same way as Modulated Temperature DSC and AC thermal imaging, a sinusoidal modulation can be imposed on the temperature ramp and the response to this is deconvoluted from the response to the underlying heating rate. This measurement provides two additional signals, the amplitude and phase. In total, therefore, there are 4 signals from a local thermal analysis experiment, the thermo-mechancal response (the degree of bending of the cantilever), the average of the differential power (called the differential thermal analysis or DC signal) and the amplitude and phase from the AC differential thermal signal. These 4 signals are shown below for a typical polymer melt. Both the DC and AC amplitude signals are expressed in units of power as they are differential electrical power measurements. However, the weight of sample involved in the transition cannot be quantified thus these can only be qualitative measurements at this stage in the development of the technique.

It can be seen that the DC and AC power signals give step changes at the transition not peaks as would normally be expected in conventional DSC and MTDSC measurements. This is because this kind of local measurement is very different from conventional calorimetry where the entire sample is heated by a furnace that completely surrounds it. In micro-TA the heat is only applied at the point of contact. If the sample melts then a front of melting material continually moves out from that point thus melting never ends in the way that it does in conventional thermal analysis. Hence we observe a step change rather than a peak. This does not prevent the measurement from being able to identify phases from transition temperatures, however, as can be seen from the numerous examples of applications. It is often convenient to take the derivatives of the differential thermal analysis signals in order to represent the transitions as peaks. This often makes them more distinct the human eye. This is shown in below.

The concept is that the different phases of a multi-component system can be visualised on the basis of their thermal or topographic properties. Individual phases can then be identified through local thermal analysis. This is illustrated below. The sample is a paracetemol tablet, an 'over the counter' drug that was imaged without any sample preparation.

The topographic image (top left) shows no phases, but these are clearly seen in the thermal conductivity image (bottom left). Localised thermal analysis of a point in the high thermal conductivity phase (bright) shows no transitions in the region 50 to 250°C. The lower thermal conductivity region shows a melting process at 180°C, identifying it as the drug. This capability, combined with the others described here, makes micro-TA a powerful and versatile tool for characterising organic materials.