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

We have employed Langmuir monolayers of highly asymmetric polydimethylsiloxane-polystyrene (PDMS-PS) diblock copolymers on dioctyl phthalate (DOP) at temperatures ranging from 22 to -35 degrees C as a model system for tethered chains in poor solvent conditions. The thicknesses of the tethered PS layers extending into the DOP subphase, measured by neutron reflection, decrease with decreasing temperature (T) over this entire range. However, the variation 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 compared to universal scaling forms. The PS segments are depleted from the near surface region over the entire T range, with the thickness of the depletion layer increasing slightly with decreasing T. The free energy of the surface layer is probed by surface tension measurements. With decreasing T, negative 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 22 degrees C, only a modest decrease in surface pressure is observed over the experimental range of T despite the strong contraction. This latter result is discussed in terms of the relative contributions of enthalpic and entropic effects to the surface pressure. (C) 1999 American Institute of Physics. [S0021-9606(99)51501-8].