TETHERED CHAINS IN THETA-SOLVENT CONDITIONS - AN EXPERIMENTAL-STUDY INVOLVING LANGMUIR DIBLOCK COPOLYMER MONOLAYERS

Langmuir monolayers of polydimethylsiloxane-polystyrene (PDMS-PS) dibl ock copolymers on dioctyl phthalate (DOP) at 22 degrees C are employed as a model tethered chain system in theta solvent conditions. The seg mental concentration profile of the tethered PS block is obtained over an order of magnitude in both surface density (sigma) and molecular w eight (M) by neutron reflection. A depletion layer of PS segments is o bserved at the air-liquid interface which increases with M and is inde pendent of sigma. The variation of the tethered layer height with sigm a and M is consistent with h similar to sigma(0.18)M(0.74) over the ra nge of reduced surface density (sigma pi R-g(2)) from 1 to 11. These d ependencies, along with the form of the profile, indicate that the asy mptotic limit is not achieved for sigma pi R-g(2) less than or equal t o 11 in theta solvent conditions. The upper limit of surface density i s limited by the interaction of the submerged blocks, which leads to a sharp rise in surface pressure (Pi). The increase of Pi with sigma fa r exceeds theoretical predictions, even in the asymptotic limit, and i s attributed to distortion of chain configurations arising from limite d lateral interpenetration. Anchoring energies compare well with diffe rential chain energies at the desorption transition when the surface P DMS blocks an noninteracting. Comparisons are made with results obtain ed previously in good solvent conditions. (C) 1998 American Institute of Physics. [S0021-9606(98)50110-9].