Thermal lag and its practical consequence in the dynamic mechanical analysis of polymers

The thermal lag associated with the temperature ramping experiment in a dyn amic mechanical analysis is demonstrated on carbon black filled vulcanised rubber samples by using a DMTA MkLIl from Rheometric Scientific. The import ance of achieving thermal equilibrium on the sample before starting the mea surement is emphasised. By running a combination of isothermal and ramping steps, the impact of thermal lag on the obtained dynamic mechanical propert ies can be estimated. For the measurement in tensile mode at a rate of heat ing of 2 degrees C/min and vertical orientation of the driveshaft motion, t he isothermal step applied at the sample glass transition temperature revea led that the temperature equilibration of the sample takes at least 15 min. Consequently, the storage modulus measured during the heating step is over estimated by around two times and the glass transition temperature is shift ed to higher values. For the given chamber geometry, the bending mode of me asurement exhibits a lower thermal lag than the tensile one. The experiment al data indicate that the major factors responsible for thermal lag are hea t transfer from the sample environment to the sample surface (which is give n by the chamber design), sample position in the chamber and the mutual pos ition of the sample and thermometer Pt-100. Based on the results presented, a recommendation for selecting the experimental conditions is given in ord er to recognise and minimise the effect of thermal lag on the data obtained by dynamic mechanical analysis. (C) 2000 Elsevier Science Ltd. All rights reserved.