Two- and Three-Dimensional Imaging of Multicomponent Systems Using Scanning Thermal Microscopy and Localized Thermomechanical Analysis

Abstract

The aim of this study was to develop a novel approach to the spatial characterization of multicomponent samples, based on the emergent technique of microthermal analysis. More specifically, we present an assessment of the use of scanning thermal microscopy as a means of component mapping via thermal conductivity; we include a new statistical approach to data handling, which allows reduction of topographic effects. We also introduce a novel three-dimensional mapping technique based on localized thermomechanical analysis. Tablets of paracetamol and hyproxypropyl methylcellulose (HPMC) and 50:50 mixes of the two were prepared and the materials characterized in scanning and localized modes using a TA Instruments 2990 microthermal analyzer with a Thermomicroscopes Explorer AFM head and Wollaston wire thermal probe. L-TMA studies of the pure components indicated markedly differing thermal responses, with the paracetamol showing a sharp melting accompanied by a probe pull-in effect, while HPMC showed only thermal expansion over the temperature range studied. Thermal conductivity and topographic images indicated that two-dimensional differentiation between the components was possible in scanning mode. A means of delineating the relative contribution of the topographic and conductivity effects was developed based on a regression analysis of the thermal conductivity measurements on a set of terms representing the local surface curvature. The results of three-dimensional imaging using a grid of L-TMA measurements is presented. This technique utilized the distinct thermal responses of the two components to allow the probe to melt through the paracetamol down to the underlying HPMC. The advantages and limitations of this novel imaging method are discussed in the context of pharmaceutical and broader uses of the approach.