Effects of fluid density on a poly(dimethylsiloxane)-based junction in pure and methanol-modified carbon dioxide

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 CO ₂ at 308 K as a function of CO ₂ density and (ii) a binary mixture of CO ₂ 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 CO ₂ 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 CO ₂ density increases the solvent quality, and PDMS segments become better solvated by the CO ₂. At the highest CO ₂ 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 CO ₂ densities. As the CO ₂ density is increased, these aggregates are disrupted, and isolated DJ molecules appear to be the major species in the fluid phase above a reduced CO ₂ density of ∼1.5 and up to ∼1.9. DJ aggregates are present in the CO ₂/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.