Neutron reflectivity study of diblock formation during reactive blending processes

In a series of neutron reflectivity experiments, we studied the fundamental process of diblock formation during reactive blending processes of an immi scible blend comprised of normal polysulfone (hPSU) containing 30% reactive end group-modified deuterated polysulfone (dPSU-R) and polyamide (PA). Dib lock formation (dPSU-b-PA) and dPSU-R enrichment at the interface between t he incompatible polymers were monitored in thin bilayer films using neutron reflectivity. These results are compared to experimental results obtained with bilayer films of pure nonreactive deuterated PSU (dPSU) and PA and pur e reactive dPSU-R and PA, respectively. The interfacial width in the pure r eactive system is slightly larger than that in the pure nonreactive system, indicating the formation of a diblock copolymer at the interface of the re active system. The results for the diluted system (30% dPSU-R + 70% hPSU) s how an enrichment of the deuterated species at the interface. The amount of dPSU-R at the interface rises from 30 vol % initially, up to an equilibriu m value of 47 vol % after annealing at 210 degrees C within about 30 min. T his is interpreted as the formation of diblock copolymer out of the reactiv e components, dPSU-R and PA, as an interfacial reaction. Annealing at T = 2 10 degrees C for substantially longer times reveals no further evolution of the interfacial profile, indicating that the diblock, once formed, stays l ocalized at the interface. The formation of a diblock monolayer with comple te coverage of the interfacial area is not observed. This is probably due t o steric hindrance and strong segregation of the diblock between dPSU-R and PA. The block copolymer layer once formed at the interface suppresses the approach of additional dPSU-R homopolymer toward the interface due to the c onformational entropy costs to the homopolymer and block copolymer already at the interface. For these reasons, it is possible to diminish but not eli minate the interfacial tension (gamma > 0) between the PSU and PA, as it is not possible to build up a large enough normalized surface excess, z*/R-g < 1, of dPSU-b-PA.