Interdiffusion phenomena at partially miscible polymer interfaces

Abstract

This research is focused on understanding the effect of miscibility on interdiffusion phenomena in bilayers of polystyrene (PS) and the statistically random copolymer, poly (styrene-r-4-bromostyrene) (PBS). Miscibility in such systems can be quantified by N*%, where N* is an effective degree of polymerization and % the Flory-Huggins interaction parameter. In the PS/PBS system, miscibility decreases as N % increases; specifically, systems having N*% > 2 (but less than some limiting value) are considered partially miscible, while those with N*% < 2 are miscible. The extent of interdiffusion of the two components at a bilayer interface is ultimately determined by the miscibility of the system, which controls interfacial width and fracture strength. A description of the effect of miscibility on interdiffusion phenomena, interfacial width, phase behavior, and fracture energy and a relationship between these properties has not been elucidated, thus limiting optimal design of polymer interfaces for technologically relevant applications. This work highlights the effect of miscibility on interdiffusion dynamics and interfacial behavior, employing both experimental and modeling tools. Rutherford backscattering spectroscopy (RBS) was used to quantify interdiffusion in the PS/PBS bilayers. PBS volume fraction versus depth profiles were obtained from the evolution of the bromine peak in the BIBS spectra of these bilayers as a function of annealing time. From these profiles, mutual diffusion coefficients were calculated. The results indicate that the extent of interdiffusion increases with temperature (T) and decreases with increasing volume fraction of brominated units in the PBS copolymer (f) and increasing N*. In miscible PS/PBS systems, interdiffusion of the bilayers results in a single layer of constant composition. In bilayers with PS and PBS having a disparity in N, the interface is observed