An energy transfer study of homopolymer localization in block copolymers

We have investigated the localization of homopolystyrene (hPS) in its blends with poly(styrene-b-butyl methacrylate) (PS-b-PBMA), at 140 degreesC, measuring the quantum efficiency of direct energy transfer (Phi(ET) of DET) between hPS randomly labeled with phenanthrene moieties and PS-b-PBMA labeled at the junction with anthracene. The molecular weight of hPS was similar to the molecular weight of the PS block of PS-b-PBMA, and the hPS content was kept to a maximum of 26 vol %, to preserve the lamellar morphology of PS-b-PBMA. We have found very low rates of DET, which implies that hPS is almost totally localized between the PS brushes of PS-b-PBMA. In addition to the experimental measurements, we also calculated Phi(ET) values by simulating fluorescence decay curves, based on Vavasour and Whitmore's numerical self-consistent-field (NSCF) formalism. The latter theory confirmed that we were dealing with a "dry brush" situation, but the NSCF results suggested a higher level of penetration of hPS than the experiments. We attribute this discrepancy to uncertainties about inputs to the simulations, particularly the Flory-Huggins chi parameter and the Kuhn length for the PBMA block.