Studies on polycarbonate-modified epoxies

A typical epoxy resin based upon the diglycidyl ether of bisphenol-A (DGEBA) was modified with polycarbonate (PC). Prior to aromatic amine cure with diaminodiphenyl methane (DDM), the possible uncatalyzed reactions in the PC/DGEBA blends were investigated. It was shown that at 200$\sp\circ$C, the secondary hydroxyl groups in the epoxy resin reacted with the carbonate groups in PC through transesterification, resulting in degraded PC chains with phenolic end groups and PC/DGEBA copolymers. The secondary hydroxyl groups were regenerated by the addition reaction between the epoxide groups and the phenolic end groups which were formed from the transesterification and the hydrolysis of PC. Thus, by using a melt blending process at 200$\sp\circ$C, a copolymer network structure of PC-modified epoxy was formed. The fracture toughness was increased by increasing the capability for plastic deformation due to the incorporation of ductile PC chains into the network. Other properties such as the thermal and mechanical properties were not sacrificed.^ Antiplasticization behavior was found in the polycaprolactone (PCL)/PC-modified epoxy system, cured with DDM. The initial modulus increased and the fracture toughness and the elongation at break decreased with the addition of the PCL/PC (1/10) modifier up to 15 phr. The glass transition temperature (Tg) slightly decreased. The antiplasticization phenomenon can be explained by the formation of hydrogen bonding between the carbonyl groups in the PCL/PC and the hydroxyl groups in the epoxy. The hydrogen bonding proportion, as analyzed from Fourier transform infrared spectra, increased with the addition of PCL/PC (1/10) up to 15 phr. It is suggested that for antiplasticization to occur within the miscibility range, a strong molecular interaction is necessary to restrict molecular motion, and in turn to decrease the free volume. Dynamic mechanical analysis revealed an increase in the activation energy and a decrease in the peak area for the $\beta$ relaxation process, which also can be explained by the hydrogen bonding. The results indirectly support the hypothesis that the motion of the 2-hydroxypropyl ether moiety is responsible for the $\beta$ relaxation process.