NEUTRON REFLECTIVITY STUDY OF END-ADSORBED DIBLOCK COPOLYMERS - CROSS-OVER FROM MUSHROOMS TO BRUSHES

We report neutron reflectivity data on polystyrene-poly(vinyl-2-pyridine) (PS-PVP) diblock copolymers adsorbed onto quartz from the selective solvent toluene (a good solvent for PS, but a poor one for PVP). The PVP > block adsorbs strongly to form a thin layer on the quartz substrate, while the PS chains dangle into the solvent. The grafting density of the non-adsorbing PS chains is varied by varying the size of the PVP block, while the PS molecular weight is kept constant. The PS adsorbance decreases systematically with increasing PVP molecular weight. The form of the polymer density profile normal to the substrate is found to change with the PS grafting density, and a transition from a > to a > conformation is observed as the PVP molecular weight is increased. For the copolymers with a high PVP content, the dependence of the maximum in the polymer density profile on the mean distance, s, between anchor points obeys the scaling law expected of mushrooms, while s increases with increasing molecular weight of the anchor block also in a manner consistent with a mushroom model.