Comparison of dynamic mechanical measurements and Tg determinations with two different instruments

Abstract

The results of dynamic mechanical measurements in tension, made by using two commercial dynamic mechanical instruments, a DMTA (Polymer Laboratories) and a DMA (Perkin Elmer), are compared and discussed. Differences in Tg values of approximately 10°C for an unfilled and a carbon-black-filled natural rubber (NR) were obtained, the higher temperature being associated with the DMTA instrument. For an MDPE material, temperature differences of approximately 14°C for the γ transition and approximately 8°C for the α; transition were noticed. These temperature differences were apparently due to temperature calibration errors and show the need to have an acceptable calibration standard for this type of measurement. The modulus values showed good agreement except for very stiff materials such as NR at temperatures below Tg, where the modulus values obtained with the DMTA were lower than those obtained with the DMA instrument. Incorrect sample geometry giving too stiff a structure could explain this low modulus. Different ways to characterize Tg by thermal analysis are also compared and discussed, with the emphasis on frequency-dependence.