The proper glass transition temperature of amorphous polymers on dynamic mechanical spectra

Glass transition is crucial to the thermal and dynamical properties of polymers. Thus, it is important to detect glass transition temperature (T (g)) with a sensitive and proper method. Dynamic mechanical analysis (DMA) is one of the most frequently used methods to determine T (g) due to its advantage of high sensibility. However, there is controversy in the past literatures to determine the proper glass transition temperature among three transition temperatures, i.e., T (g1), T (g2) and T (g3) in the dynamic mechanical spectra, which correspond to the temperature abscissa of intersect value of two tangent lines on storage modulus (E'), the peak of the loss modulus (EaEuro(3)) and the peak of the loss tangent (tan delta). In this work, these three transition temperatures were compared with the glass transition temperature determined by DSC (T (gDSC)). Based on the discussion of different modes of molecular motion around the glass transition region, it is demonstrated that T (g1) and T (g2) have the same molecular mechanism as T (gDSC), i.e., local segmental motion which is enthalpic in nature and determines the proper glass transition temperature, while T (g3) is assigned to the transition temperature of entropic Rouse modes, thus cannot be used as the proper glass transition temperature.