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

We have employed Langmuir monolayers of highly asymmetric polydimethylsilox ane-polystyrene (PDMS-PS) diblock copolymers on dioctyl phthalate (DOP) at temperatures ranging from 22 to -35 degrees C as a model system for tethere d chains in poor solvent conditions. The thicknesses of the tethered PS lay ers extending into the DOP subphase, measured by neutron reflection, decrea se with decreasing temperature (T) over this entire range. However, the var iation with T becomes weak below -20 degrees C. At the lowest T, the layer thicknesses are 55%-75% of the values at the theta condition (T-theta = 22 degrees C). The contraction of the layer with decreasing T is determined as a function of surface density and molecular weight, and these data are com pared to universal scaling forms. The PS segments are depleted from the nea r surface region over the entire T range, with the thickness of the depleti on layer increasing slightly with decreasing T. The free energy of the surf ace layer is probed by surface tension measurements. With decreasing T, neg ative surface pressures are observed at low coverages for both PDMS-PS and PDMS monolayers, indicating metastability toward lateral phase separation. Evidence for a transition from a dispersed phase to a condensed phase with decreasing T was observed in the reflectivity for very low PDMS-PS coverage . At high coverage where the submerged blocks are strongly interacting at 2 2 degrees C, only a modest decrease in surface pressure is observed over th e experimental range of T despite the strong contraction. This latter resul t is discussed in terms of the relative contributions of enthalpic and entr opic effects to the surface pressure. (C) 1999 American Institute of Physic s. [S0021-9606(99)51501-8].