Neutron Reflectivity Study of End-Adsorbed Bimodal Polymer Systems under Static Conditions and Shear Flow

The behavior of end-adsorbed bimodal PS-PEO block copolymer systems was studied with neutron reflectivity experiments, both under static conditions (zero shear rate) and under a stepwise-increasing shear flow, up to a critical shear rate where desorption occurs. It is found that when the height of one kind of chain in the bimodal brush is significantly greater than that of the other, the longer chains desorb first, while if the heights of the two constituent kinds of chains in the brush are similar, then the bimodal system responds as a whole and both kinds of chain desorb simultaneously. In particular, when the chain height ratio of the two components of the bimodal system is less than 0.80, the reflectivity spectra under static conditions are best fitted by two parabolas, and under shear flow each component desorbs at a different shear rate. Above this value, the bimodal system reacts en bloc, and desorption occurs at the same shear rate for both copolymers. It is further observed that the critical shear rate for desorption, in all bimodal systems studied in the present investigation, scales with s(-3), where s is the mean interanchor distance, in accord with the behavior of monodisperse brushes reported previously [Anastassopoulos et al. Macromolecules 2006, 39, 8901-8904]. In addition, careful measurements under shear flow up to and in the vicinity of the critical shear rate for desorption show no evidence of any measurable change in the brush height or volume fraction profile.