An Evaluation of Local Thermal Analysis of Polymers on the Sub-Micrometer Scale Using Heated Scanning Probe Microscopy Cantilevers

A basic understanding of thermal properties of polymers is of fundamental importance for the development of advanced polymers. However, up to now, mainly bulk properties have been investigated. To characterize local softening processes in polymers, a local thermal analysis (LTA) technique is applied as an add-on to a scanning probe microscope. The development of a new generation of heatable cantilever probes enables thermal analysis in the sub-μm range. This method is based on an appropriate temperature calibration, which provides a reliable correlation of the applied voltage heating the tip and the actual temperature at the tip–sample interface. As the presented technique is more susceptible to environmental changes than comparable macroscopic methods, different parameters that might influence its performance are evaluated like a strong dependence on sample temperature. It is shown that the measured softening temperature on a polystyrene (PS) sample decreases from 102.2 to 66.4 °C as the temperature of the substrate is increased by 50 °C. The interaction between heat from the cantilever and the substrate is the reason for local sample softening, which opens new perspectives to understand the temperature calibration process using the melting standard method. A stepwise guideline for a suitable temperature calibration is provided

Assessing the Nanoscale Structure and Mechanical Properties of Polymer Monoliths used for Chromatography

Concerning polymeric monolithic materials utilized in separation science, the structural and mechanical characteristics from the nanoscopic to the macroscopic scale remain of great interest. Suitable analytical tools are urgently required to understand the polymer monolith’s constituent structure, particularly in the case of nanoscale polymer properties that tend to develop gel porosity in contact with a mobile phase ultimately affecting the chromatographic performance. Herein described are our first findings from a characterization of commercially available analytical polymer monoliths based on styrene/divinylbenzene and methacrylate chemistries utilizing confocal Raman spectroscopy imaging and atomic force microscopy (AFM). Confocal Raman spectroscopy can be used to generate a three-dimensional representation of monoliths in both dry state and in contact with solvent. AFM force–indentation measurements on individual cross-sectioned globular features permit detailed assessment of mechanical properties of the stationary phase. This approach allowed so far unprecedented insight and identification of a heterogeneous cross-link density distribution of polymer material within individual globular features on a submicrometer scale