Observation of Surface Ordering of Alkyl Side Chains in Polystyrene/Polyelectrolytes Diblock Copolymer Langmuir Films

Abstract

Block polyelectrolytes P(Sx-b-VP/RXy) composed of polystyrene (PS) blocks (x = 200, 260) and alkylated (R = Cn) poly(vinylpyridine) (PVP) (X = I or Br; y = 119−270; n = 4, 10, and 18) have been studied using the Langmuir film balance technique and in situ X-ray and neutron reflectivity measurements. Previous studies showed that the self-assembled surface micelles (with PS core and RPV+PX- corona) at the air/water interface revealed a characteristic transition (when n > 6) without any significant hydration or the submersion of the corona chains. Combined X-ray and neutron reflectivity studies of P(Sx-b-VP/RXy) polyelectrolytes show that although the polyelectrolyte block is water soluble, it remains adsorbed to the water surface. The thickness of the adsorbed layer measured by X-ray reflectivity is in agreement with that measured using neutron reflectivity. This agreement indicates that the counterions are closely associated with the alkylated PVP chains, localized at the air/water interface, and are not distributed into the subphase. Reduction of the water subphase surface tension (using 1−5 vol % n-butanol) yields no change in the two-dimensional ordering of the surface micelles but increases the thickness of the corona layer to ca. 100 Å during film compression, indicating that the alkylated blocks become submerged into the subphase under these conditions. In situ Fourier transform infrared spectroscopy performed at the air/water interface establishes that for n = 10 and 18, the alkyl chains adopt an ordered, all-trans state. However, no ordering is observed for n = 4. These results clearly indicate that transition observed in the π−A isotherms is related to an order−disorder process of the alkyl side chains.