Thermal Analysis by Structural Characterization
comparisons are being made between samples, care must be taken to ensure the lighting and magnifi cation are the same. T is is usually not diffi cult to achieve because the user can refer to previous images and adjust these variables appropriately. In one of the cases described below, a whole series of TASC measure- ments was made by dividing the fi eld of view into a grid. Each square became a scanned area with the ROI being 10% less in x and y . T e results for the transition temperatures were obtained manually and presented in 3D using Matlab® (MathWorks, USA). Although the data were obtained manually in this case, it can be envisaged how this process could be automated. As part of the process for measuring glass transitions, an indentation was made in the surface of the sample. T e procedure was to raise the sample temperature above its glass transition, place the tool on the surface, cool the sample to below its glass transition, and then remove the tool. In this way there was no plastic deformation or frozen-in stress; thus the measurement was purely a relaxation event driven by surface tension.
Results
3D Measurements of glass transitions . T e fi lled polystyrene used in this experiment provides an example of how TASC can make measurements on opaque samples. In this case a structure,
Figure 2 : Melting of zoledronic acid. (a) DSC plot showing an onset temperature of 205 °C, peak temperature of 210 °C, and offset temperature of 217 °C. The endotherm is followed by an exotherm attributable to a decomposition’s reaction. (b) The TASC graph shows this complex behavior where the melting continues during the exotherm. The TASC graph follows the color coding given by the circles in the inset micrograph showing the locations of the individual measurements. Image width = 2000 µm. The blue box indicates the area shown in Figure 4 .
Materials and Methods T e apparatus was a Linkam THMS 600 Stage with Imaging Station (Linkam Scientifi c Instruments, UK) controlled by Linksys soſt ware with the TASC option. T e polymer sample was polystyrene fi lled with calcium carbonate called StripStyrene (Evergreen Corp., USA). T e 3D images were acquired using a Zeescan (Phaseview, France) fi tted to the Linkam Imaging Station. T e zoledronic acid was from Gador Laboratory (Gador S.A., Argentina). T e DSC was a prototype dual-cell DSC from Linkam. T e TMA probe was a prototype made by Cyversa Ltd (UK). T e polycaprolactone (PCL) was from Vornia Biomaterials (Ireland).
In a typical TASC experiment a series of images is collected over time as the temperature is raised at a near-constant rate. T ese images are then analyzed retrospectively by selecting an area with the curser, called the region of interest (ROI), within which is located the structure of interest. A second area is then selected around the fi rst, called the scanned area, and this represents the area within which the structure of interest is allowed to move. If there is no movement these two areas could be the same but, at high magnifi cation, some movement will almost always occur. T e amount of movement can be assessed simply by inspection of the collected micrographs. Because the actual TASC value depends on the number of pixels and light intensity, among other variables, the TASC curve is normalized. Where detailed
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Figure 3 : Data from Figure 2 can be used to create a histogram of the distri- bution of transition temperatures. This characterizes the melting process and can be compared with the DSC data to show that the melting endotherm is distorted by the exotherm resulting from decomposition.
www.microscopy-today.com • 2017 September
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