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Thermal Analysis by Structural Characterization (TASC): Structural and Thermo-Rheological Information from Hot Stage Microscopy

M. Reading Department of Pharmacy , University of Huddersfi eld , Queensgate , Huddersfi eld HD1 3DH , UK

Abstract: A new technique for thermal analysis has recently been introduced based on hot stage microscopy and software for image analysis called thermal analysis by structural characterization or TASC. It enables new measurements to be made: for a range of materials, including polymers and pharmaceuticals, we describe measuring glass transitions using 3D imaging based on z -stacking, characterizing complex melting behavior where there is a distribution of melting points, and thermo-mechanical analysis with 2D detection. In this way the range of measurements available to users of hot stage microscopy has been expanded.

Introduction T ermal methods are commonly used to characterize a wide range of materials. Measuring transition temperatures can provide a wealth of information about the structure and properties of polymers, pharmaceuticals, and ceramics. However, the most commonly used techniques for measuring glass and melting transitions—diff erential scanning calorimetry (DSC), thermo- mechanical analysis (TMA), and dynamic mechanical analysis (DMA)—provide only global information averaged over the whole sample. Local information about small areas isn’t available. Furthermore, the smallest samples are of the order of milligrams in the case of DSC and typically grams in the cases of TMA and DMA. T e introduction of micro/nano thermal analysis based on atomic force microscopy made possible, for the fi rst time, the measurement of transition temperatures in a localized way as well as measurements on nanoscale objects [ 1 ]. Whilst this represented an advance, atomic force microscopes are expensive and relatively diffi cult to use. T e largest fi eld of view is frequently only 100 µm 2 , and samples must be smooth because the maximum z displacement is typically 10 µm or less. Hot stage microscopy (HSM), also called thermomicroscopy, is a well-established method that can examine µm-sized to mm-sized objects and is frequently applied to ordered materials, such as liquid crystals. T e method struggles, however, with amorphous samples as well as many opaque samples. Transmitted or refl ected light intensity measured as a function of time and temperature can be useful, but, once again, opaque samples are challenging. T ere is usually no intuitive relationship between light intensity and the sample property being measured; for example, melting could result in an increase or decrease


in light intensity depending on the background, lighting, and the optical properties of the sample.

We have developed a method called thermal analysis by structural characterization (TASC), which directly measures changes in structure observed in a light optical microscope with a temperature-controlled stage [ 2 , 3 , 4 ]. It is a method of image analysis that is universal in that it does not depend on the presence of specifi c types of structures. A crucial capability is that it removes the eff ect of sample movement. When a sample is heated it usually moves due to its thermal expansion and/or the expansion of the sample chamber. By removing this eff ect the system algorithm returns values related only to structural change. T e details of how this is achieved have been given elsewhere [ 3 ]. It has been shown that TASC provides a means of: (a) measuring local transition temperatures, (b) analyzing very small samples (down to a few micrograms), (c) quantifying heterogeneity, and (d) character- izing dissolution behavior.

In this article we discuss for the fi rst time how TASC can be used to look at glass transitions using 3D imaging. We also present for the fi rst time an image based on transition temperatures measured using TASC (called T-Map mode) as part of a study of complex melting behavior. We also describe a TASC-related method to create a form of thermomechanical analysis within a hot stage microscope.

Figure 1 : Relaxation of fi lled polystyrene. (a) Light microscope image of a pattern of four indentations in the sample of fi lled polystyrene. (b) 3D image of surface topography obtained with extended depth of fi eld derived from z -stacking of multiple images. (c) Thermal analysis by structural characterization (TASC) plot with the topography maps from points along the relaxation process.

doi: 10.1017/S1551929517000815 • 2017 September

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