Tethered chains in poor solvent conditions: An experimental study involving Langmuir diblock copolymer monolayers
- Sandia National Laboratories, Albuquerque, New Mexico 87185 (United States)
- Los Alamos National Laboratories, Los Alamos, New Mexico 87545 (United States)
- Lab. Leon Brillouin, CEN-Saclay, 91191 Gif-sur-Yvette Cedex (France)
- NIST Center For Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 (United States)
We have employed Langmuir monolayers of highly asymmetric polydimethylsiloxane-polystyrene (PDMS-PS) diblock copolymers on dioctyl phthalate (DOP) at temperatures ranging from 22 to {minus}35thinsp{degree}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 ({ital T}) over this entire range. However, the variation with {ital T} becomes weak below {minus}20thinsp{degree}C. At the lowest {ital T}, the layer thicknesses are 55{percent}{endash}75{percent} of the values at the theta condition (T{sub {theta}}=22thinsp{degree}C). The contraction of the layer with decreasing {ital 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 {ital T} range, with the thickness of the depletion layer increasing slightly with decreasing {ital T}. The free energy of the surface layer is probed by surface tension measurements. With decreasing {ital 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 {ital T} was observed in the reflectivity for very low PDMS-PS coverage. At high coverage where the submerged blocks are strongly interacting at 22thinsp{degree}C, only a modest decrease in surface pressure is observed over the experimental range of {ital 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. {copyright} {ital 1999 American Institute of Physics.}
- OSTI ID:
- 304233
- Journal Information:
- Journal of Chemical Physics, Vol. 110, Issue 7; Other Information: PBD: Feb 1999
- Country of Publication:
- United States
- Language:
- English
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