Mechanical relaxation mechanism of epoxide resins cured with acid anhydrides. II. Effect of the chemical structure of the anhydrides on the β relaxation mechanism

The mechanism of low-temperature mechanical relaxation of acid-anhydride-cured epoxide resins has been investigated in detail. One mechanical relaxation, denoted as the β relaxation, is observed from ?80 to ?50°C for all epoxide resin systems cured with aromatic, alicyclic, and aliphatic anhydrides. The β relaxation increases in peak height and shifts to higher temperature with increasing molecular volume of the diester segments formed by the reaction of acid anhydrides. From these results, it is concluded that the β relaxation for anhydridecured systems is due to the motion of the diester segment, and that the intensity and peak position of the β relaxation depend on the molecular volume of this segment. Moreover, it was shown that the tensile impact strength of the anhydride-cured systems is governed by the intensity of the β relaxation of these systems when the parameters T_g and v of these systems are nearly constant.