Two series of samples, one of PET and another of PET, were received af
ter chain extension at different reaction times with two new chain ext
enders (diimidodiepoxides). These samples showed different intrinsic v
iscosity and degree of branching or crosslinking. The effect of this d
ifferentiation on thermal properties was studied by thermomechanical a
nalysis (TMA). The parameters studied were the glass transition temper
ature (T-g), melting temperature (T-m), and the linear expansion coeff
icient (alpha). It is remarkable that, in the case of PET amorphous or
semicrystalline samples, two peaks appeared next to the T-g in the TM
A thermogram. The first peak appeared at a temperature very close and
lower to the T-g 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 PET. The T-g values were found to agree to within +/-1
degrees C of those obtained from DSC; on the contrary, the T-m values
varied significantly from those received from DSC. The linear expansi
on coefficient of samples was found to increase with the degree of cha
in extension. (C) 1996 John Wiley & Sons, Inc.