Effects of Fluid Density on a Poly(dimethylsiloxane)-Based Junction in Pure and Methanol-Modified Carbon Dioxide

Abstract

We report on the behavior of a three-armed, poly(dimethylsiloxane) (PDMS)-based junction that is site-selectively labeled with a single fluorescent dansyl reporter group (dansyl junction, DJ) in (i) pure CO2 at 308 K as a function of CO2 density and (ii) a binary mixture of CO2 and 3 mol % methanol at 313 K as a function of mixture density. The DJ results are compared to those of dansylpropylsulfonamide (DPSA)a dansyl reporter molecule without any PDMS segments. In pure CO2 at low fluid densities, the local microenvironment sensed by the dansyl residue in DJ appears to be rich in PDMS, indicating that the solvent quality is poor and the PDMS segments interact strongly with the dansyl residue. Increasing the CO2 density increases the solvent quality, and PDMS segments become better solvated by the CO2. At the highest CO2 densities studied, the solvent quality begins to decrease again, resulting in a local increase in the PDMS composition around the average dansyl residue. Results from steady-state anisotropy measurements suggest the presence of DJ aggregates in the fluid phase at low CO2 densities. As the CO2 density is increased, these aggregates are disrupted, and isolated DJ molecules appear to be the major species in the fluid phase above a reduced CO2 density of ∼1.5 and up to ∼1.9. DJ aggregates are present in the CO2/methanol mixture at all fluid densities studied. At low mixture densities there is enrichment in the local methanol concentration surrounding the dansyl residue in DJ in comparison to isolated DPSA molecules under equivalent mixture densities. Thus, it appears as if the PDMS segments in DJ augment/enhance the methanol concentration surrounding the dansyl reporter group in DJ. Together these results demonstrate the dramatic role fluid density and composition can play in tuning the local microenvironment that surrounds a polymeric junction point.