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Thermal Analysis by Structural Characterization

Figure 4 : 3D image created by mapping transition temperatures within the region shown in the inset micrograph. Image width = 500 µm. The micrograph shows an unmelted crystal surrounded by bubbling molten material from crystals that melted at lower temperatures. The bubbling is due to decomposition, and the DSC trace shows an exotherm at this point. In the 3D image this crystal appears in red because of its higher melting temperature. This illustrates the inhomoge- neous nature of the sample.

a series of indentations made with a custom tool, is created by the experimenter. Without this structure no measurement could be made because the appearance of the sample would not signifi - cantly change as it soſt ened. Because of the indentations, the transition can be detected as a consequence of the fact that, at the glass transition temperature, the material becomes fl uid and surface tension causes the material to fl ow thereby making the indentations disappear [ 2 ]. Images in 2D can provide satisfactory results for detecting transition temperatures as shown here and elsewhere [ 2 , 3 , 4 ]. Measurements of the kinetics of glass transi- tions made to date have used 2D images; this is acceptable because isoconversion points can still be identifi ed [ 2 ], but the volume of material that has fl owed and the depth of the indentation would not be known. A more rigorous approach is to acquire 3D images of the surface. In this case topographic images were acquired by z -stacking. A series of micrographs was taken as the focal plane of the microscope is moved up and down. Sophisticated soſt ware enables the focused regions in each image to be extracted, and a composite image is created in which all of the parts are in focus. T is is a well-established method in conven- tional photography and, increasingly, in microscopy. From a z -stack of images, a 3D map of the topography of the sample can be constructed as shown in Figure 1 . T e disappearance of the indentations with increasing temper- ature can be seen in the surface images associated with the graph of Figure 1c . T e TASC algorithm can equally well be applied to these topographic data, however, the process of z -stacking is more time-consuming than simply

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acquiring single 2D images. T us very rapid experiments would be precluded. In Figure 1c the topography plots are shown next to the standard 2D TASC curve and can serve as a means of calibrating 2D data so that, for example, the depths of the indentations can be calculated. T is works where the conditions under which the 3D images were acquired are identical to those under which the 2D TASC results were obtained. In this way the TASC technique can be used to obtain more quantitative measurements of the rheological behavior of the sample. Analyzing complex melting . Figure 2a shows the DSC curve for zoledronic acid. T ere is an initial dehydration step (not shown) [ 5 ], and then the DSC curve shows there is an endotherm followed by an exotherm. T e HSM data show that the sample starts as a white solid, and then a bubbling liquid is formed that coexists with some remaining solid material (inset in Figure 2 ) during the mid-stage of the process. T e picture that emerges is a complex one where melting and decompo- sition occur over a wide temperature range. T is is confi rmed by the TASC data shown in Figure 2b . Diff erent places in the sample melt at diff erent temperatures over a range of 205 °C to 225 °C, that is during both the endotherm and the exotherm. T is complexity is illustrated by the temperature map image shown in Figure 2b (inset). T e area shown in the inset image was divided into a 10 × 10 grid, and in each cell within the grid a TASC analysis was carried out. T is image then gives rise to the histogram presented in Figure 3 , which characterizes the heterogeneous nature of the melting/decomposition process. It means that the HSM data can be extended beyond qualitative observations. When the sample is distributed to an approxi- mately even depth, the histogram enables the fraction of the sample that melts to be estimated as a function of temperature. Figure 4 shows the onset temperatures of the transitions obtained from these measurements plotted in the form of a 3D image. We believe it is the fi rst time this type of transition temperature map has been acquired using light microscopy. It can be seen that there is a large particle, colored red in the image, that melts signifi cantly later (higher temperature) than the surrounding material as the temperature is gradually raised.

Figure 5 : Thermomechanical analysis (TMA) measurements of polycaprolactone (PCL) using a simple tripod device. (a) Schematic of tripod with one leg resting on a sample placed on a heated block. The microscope focal plane is set to be along the mid-point of the probe. When the sample softens, the probe moves downward and the part of the probe that is in focus moves toward the right. Movement in the orthogonal direction is also tracked. (b) The red curve shows movement corresponding to indentation, and the blue curve shows movement sideways. As the PCL softened, the tip translated toward the side of the particle, and both x and y movements were observed.


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