Physical aging of epoxy resin blended with poly(ether sulfone): Effect of poly(ether sulfone) molecular weight

The physical aging process of 4-4′-diaminodiphenylsulfone (DDS) cured diglycidyl ether bisphenol-A (DGEBA) blended with various molecular weights of poly(ether sulfone) (PES; M_n = 28,600, 10,600, and 6,137) was studied by DSC. For DGEBA/DDS system blended with a low MW PES-3 (M_n = 6,137), no phase separation of the polymer blend and only one enthalpic relaxation process due to physical aging was observed. Since the high MW PES-1 (M_n = 28,600) had a T_g close to that of fully cured DGEBA/DDS, the fully cured DGEBA/DDS/PES-1 blend had a broader glass transition than a neat DGEBA/DDS system. However, the DSC results showed two enthalpic relaxation processes due to the physical aging of PES-rich and cured epoxy-rich phases as the material was aged at 155 °C (30 °C below T_g). Since the T_gs of PES-1-rich and epoxy-rich phases overlapped with each other, the enthalpic relaxation processes corresponding to each phase coupled to each other in the earlier stage of physical aging. The medium MW PES-2 (M_n = 10,600) has a much lower T_g than that of fully cured DGEBA/DDS, two well separated T_gs were observed for the cured DGEBA/DDS/PES-2 blend, indicating the cured epoxy was immiscible with PES. Aging the polymer blend at 155 °C (24 °C below T_g1 of the PES-2-rich phase and 53 °C below T_g2 of the epoxy-rich phase) produced two well separated relaxation processes due to PES-2-rich and epoxy-rich phases. The experimental results suggested that aging the polymer blend at a suitable temperature would improve the phase separation between PES-1-rich and epoxy-rich phases.