Interface between a polysulfone and polyamide as studied by combined neutron reflectivity and small-angle neutron scattering techniques

Interfaces between the two phases formed in immiscible polymer pairs were s tudied by means of neutron reflectivity (NR) combined with small-angle neut ron scattering (SANS). The interfacial widths evaluated by the two methods were compared for two systems: a nonreactive system composed of polyamide ( PA) and polysulfone (PSU) and a reactive system composed of PA and PSU with a phthalic anhydride reactive end group (PSU-R) in which PSU-b-PA block co polymers can be formed at the interface. SANS measurements were made on bul k mixtures, and the NR measurements were performed on thin film bilayer sta cks of PA and PSU or PSU-R. The intrinsic interfacial widths, WI,(diffuse), obtained from the two methods were compared for the samples that underwent similar annealing protocols and were quenched below the glass transition t emperatures Tg's to room temperature. A consistent result for W-I,W-diffuse can be obtained for the two methods only after taking into account the fol lowing important factors: (i) the SANS intensity should be corrected for th e frozen density heterogeneities within each phase as well as the frozen th ermal composition fluctuations; (ii) the contribution of capillary wave flu ctuations to a net (or observed) interfacial width, Wi,obs, is more signifi cant for NR than SANS. Without the former correction, the SANS profiles at high scattering vector, q, showed asymptotic behavior of q(-n) (n < 4), giv ing an erroneous conclusion on WI,obs The WI,obs values obtained for the re active system were found to be larger than those for the nonreactive system for both methods, reflecting block copolymer formation at the interfaces. The interfacial area density, <Sigma>, for the bulk mixture of the reactive system rapidly decreased with time and reached a constant value within ca. 2 min after annealing above the T-g's, while Sigma for the nonreactive sys tem kept decreasing with time. This elucidates that the growth of the phase -separating domains in the reactive system was pinned by the formation of b lock copolymers at the interfaces during early annealing times.