(ICE-036-2014) Thermal analysis of nanofibres: the importance of heating rate

This study describes the importance of heating rate when analysing nanofibres using differential scanning calorimetry and illustrates some pitfalls related to the use of conventional rates (-20 K min-1). The melting behaviour of several polyamide (PA) types was researched using a fast-scanning calorimeter (RHC, T A Instruments) capable of controlled heating rates above 1000 K min-1. Past heating suppresses kinetic processes such as cold crystallization, crystal reorganization and crystalline perfectioning, which can be necessary for a correct-analysis of crystal structures, especially in oriented polymorphous structures such as PA nanofibres. For PA69, for instance, analysis at higher rates reveals that nanofibres have a much more stable crystal structure than bulk material (only 15% of low-melting phase vs. 71 % in bulk). This difference cannot be seen at 20 K min-1 since this crystal phase recrystallizes during heating, indicating the importance of a higher rate. However, high heating rates may result in thermal lag and a reduced resolution. Our results indicate that this effect becomes significant for all samples at rates above 500 K min-1 due to the high porosity of nanofibrous sample (-90%) and thus low thermal conductivity. Finding the golden mean between suppression of kinetic processes and limitation of thermal lag is key and a comparison of the melting behaviour at high and low scan rates is recommended