Local thermal analysis

Sanders GHW, Roberts CJ, Danesh A, et al. Discrimination of polymorphic forms of a drug product by localized thermal analysis. J Microsc (Oxf) 2000 198 77-81. 

Microthermal analysis is a recently introduced thermoanalytical technique that combines the principles of scanning probe microscopy with thermal analysis via replacement of the probe tip with a thermistor. This allows samples to be spatially scanned in terms of both topography and thermal conductivity, whereby placing the probe on a specific region of a sample and heating, it is possible to perform localized thermal analysis experiments on those regions. 

Figure 9. Optical micrograph of a cross-section of a gel particle embedded in a 75 pm low density polyethylene film. Also visible are craters remaining following localized thermal analysis of die specimen.

The society wasfounded in 1968. its progam includes an annual conference. In 2002 the 30 NATAS Conference was held in Pittsburgh, PA. The conference proceedings and a quarteriy bulletin, the NATAS Notes, are the publications of the society. A Tutorial at the conference is part of the educational program. An increasing number of local thermal analysis sections are affiliated with NATAS. As an interested student of thermal analysis you should consider becoming a member. 

The principle of microcalorimetry is iUustraled with Fig. A.10.7 (pTA of TA Instruments, Inc.). The tip of an atomic force microscope, AFM, is replaced by a Pt-wire that can be heated and modulated, as is illustrated in detail with Fig. 3.96. A typical resolution is about 1.0 pm with heating rates up to 1,000 K min. A temperature precision of +3 K and a modulation frequency up to 100 kHz has been reached. The figure shows the control circuit for localized thermal analysis. In this case the probe contacts the surface at a fixed location with a programmed force, controlled by the piezoelectric feedback of the AFM. A reference probe is attached next to the sample probe with its tip not contacting the sample, allowing for... 

Recent developments have been in the area of microthermal analysis using thermal conductivity with thermal diffiisivity signals or AFM to visualize specific areas or domains in the material and perform localized thermal analysis studies (183,184). Relaxational behavior over time and temperature is related to changes in free volume of the material. Positron annihilation lifetime spectroscopy (PALS) measurements of positron lifetimes and intensities are used to estimate both hole sizes and free volume within primarily amorphous phases of polymers. These data are used in measurement of thermal transitions (185,186) structural relaxation including molecular motions (187-189), and effects of additives (190), molecular weight variation (191), and degree of crystallinity (192). It has been used in combination with DSC to analyze the range of miscibility of polymethyl methacrylate poly(ethylene oxide) blends (193). 

Localized thermal analysis (LTA micro- or nano-thermal... 

The form of SThM most relevant to the subject of this discussion is carried out using near-field electrical resistance thermometry, and this method has been adopted in the work reported in this chapter. This is because miniaturized resistive probes have the considerable advantage that they can be used both in passive mode as a thermometer and as an active heat source. This enables local thermal analysis (L-TA see text below) as well as SThM to be carried out. At present the most common type of resistive probe available is the Wollaston or Wollaston Wire probe, developed by Dinwiddle et al. (1994) and first used by Balk et al. (1995) and Hammiche et al. (19%a) The construction details of this probe are illustrated in Fig. 7.3. A loop of 75-pm-diameter coaxial bimetallic Wollaston wire is bent into a sharp V-shaped loop. The wire consists of a central 5-pm-diameter platinum/10% rhodium alloy core surrounded by silver. The loop is stabilized with a small bead of epoxy resin deposited approximately 500 pm from its apex. The probe tip or sensor is made... 

Figure 7.16. Craters made in a PET film by local thermal analysis with nanoprobes (a) before L-TA and (b) right after L-TA (reproduced with permission of Anasys Instruments Inc.).

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