Surface Pressure of Linear and Cyclic Poly(dirnethylsiloxane) in the Transition Region

Surface pressure π has been measured as a function of surface concentration c for monolayers of linear and cyclic poly(dimethylsiloxane) (PDMS) of molecular weight 730-14 800, spread on water and tricresyl phosphate at 26.0°C. In the transition region where the surface pressure rises much more rapidly than proportional to the surface concentration, the findings for linear and cyclic PDMS were indistinguishable and independent of molecular weight. The findings in this region could be described as power laws with powers corresponding to scaling predictions for the semidilute region of concentrations (coil overlap accompanied by low overall polymer concentration) on near-θ and fairly good surface solvents, respectively. However the second virial coefficient of the surface pressure appeared to be negative for both liquid supports. These observations, the fact that the transition region occurred at quite high fractional surface coverage, and the instability of films from small oligomers suggest that recent interpretations of behavior in the transition region in terms of predictions for a semidilute surface solution are invalid in this case. The existence of a semidilute region of concentrations for polymer monolayers is uncertain in principle. In addition the comparisons of linear and cyclic polymer above the overlap concentration c* lead to the surprising conclusion that even for three-dimensional semidilute solutions, the ratio c/c* is not a universal reduced concentration.