Thermomechanical analysis of chain-extended PET and PBT

Two series of samples, one of PET and another of PBT, were received after chain extension at different reaction times with two new chain extenders (diimidodiepoxides). These samples showed different intrinsic viscosity and degree of branching or crosslinking. The effect of this differentiation on thermal properties was studied by thermomechanical analysis (TMA). The parameters studied were the glass transition temperature (Tg), melting temperature (Tm), and the linear expansion coefficient (α). It is remarkable that in the case of PET amorphous or semicrystalline samples, two peaks appeared next to the Tg in the TMA thermogram. The first peak appeared at a temperature very close and lower to the Tg, and the other peak, at higher temperature into the “cold crystallization region.” The presence of two such peaks was not detected in the DSC thermogram of PET samples either in the TMS or DSC thermograms of PBT. The Tg values were found to agree to within ±1°C of those obtained from DSC; on the contrary, the Tm values varied significantly from those received from DSC. The linear expansion coefficient of samples was found to increase with the degree of chain extension.