Tethered chains in theta solvent conditions: An experimental study involving Langmuir diblock copolymer monolayers

Langmuir monolayers of polydimethylsiloxane-polystyrene (PDMS-PS) diblock copolymers on dioctyl phthalate (DOP) at 22 degrees C are employed as a model tethered chain system in theta solvent conditions. The segmental concentration profile of the tethered PS block is obtained over an order of magnitude in both surface density (sigma) and molecular weight (M) by neutron reflection. A depletion layer of PS segments is observed at the air-liquid interface which increases with M and is independent of sigma. The variation of the tethered layer height with sigma and M is consistent with h similar to sigma(0.18)M(0.74) over the range of reduced surface density (sigma pi R-g(2)) from 1 to 11. These dependencies, along with the form of the profile, indicate that the asymptotic limit is not achieved for sigma pi R-g(2) less than or equal to 11 in theta solvent conditions. The upper limit of surface density is limited by the interaction of the submerged blocks, which leads to a sharp rise in surface pressure (Pi). The increase of Pi with sigma far exceeds theoretical predictions, even in the asymptotic limit, and is attributed to distortion of chain configurations arising from limited lateral interpenetration. Anchoring energies compare well with differential chain energies at the desorption transition when the surface PDMS blocks an noninteracting. Comparisons are made with results obtained previously in good solvent conditions. (C) 1998 American Institute of Physics. [S0021-9606(98)50110-9].