Effects of Added CO2 on the Dynamics of Poly(dimethylsiloxane) Oligomers Dissolved in a ϑ Solvent and a Poor Solvent

Contrasting ground- and excited-state intramolecular aggregation in choline chloride-based deep eutectic solvents versus a liquid polymer

Although the pyrenyl (Py) groups in several dipyrenyl (or bispyrenyl) compounds do not dimerize in the ground-state, they are known to intramolecularly aggregate exclusively in the excited-state to form excimers in common organic solvents. We present contrasting intramolecular aggregation behaviour of such a prototypical compound, 1,3-bis(1-pyrenyl)propane [1Py(3)1Py], dissolved in judiciously selected liquids having relatively high dynamic viscosities (η). Specifically, the intramolecular aggregation of 1Py(3)1Py is investigated in a liquid polymer polydimethylsiloxane with number average MW 2000, PDMS2000 (η^293.15K = 21.4 mPa s), and is compared with aggregation of 1Py(3)1Py in deep eutectic solvents (DESs) constituted of the H-bond acceptor (HBA) choline chloride (ChCl) and H-bond donors (HBDs) urea and glycerol in a 1 : 2 mole ratio of ChCl : urea (η^293.15K = 1372.0 mPa s) and ChCl : Gly (η^293.15K = 473.0 mPa s), respectively, in 293.15 to 363.15 K temperature range. The HBD constituent of ChCl : Gly, glycerol (Gly) (η^293.15K = 1412.0 mPa s) is also investigated for comparison purposes. It is found that while in PDMS2000, 1Py(3)1Py intramolecularly aggregates exclusively in the excited-state, thus forming a classical excimer, ground-state heterogeneity is clearly evident in both the DESs and Gly. High viscosity, a consequence of the extensive H-bonding in DESs/Gly, appears to induce the two Py units, both in the ground-state, to exist partly in a configuration where the interaction, albeit a weak one, takes place between the two. This ground-state interaction is not present in relatively low viscosity media PDMS2000 as observed in the common organic solvents. The role of viscosity/H-bonding of the solubilizing milieu on differential intramolecular aggregation in ground- and excited-states is highlighted.

Classifying deep eutectic solvents for polymer solvation via intramolecular dimer formation

Deep eutectic solvents (DESs) have emerged as versatile and inexpensive solubilizing media with widely varying physicochemical properties. Establishing characteristics of a novel solvent milieu for polymer dissolution is an important...

Temperature-Dependent Tail−Tail Dynamics of Pyrene-Labeled Poly(dimethylsiloxane) Oligomers Dissolved in Ethyl Acetate

We have determined the tail-tail dynamics and energetics of linear poly(dimethylsiloxane) (PDMS) polymers (M(n) = 3100; M(w)/M(n) = 1.07) that have been terminally labeled with pyrene (Py-PDMS-Py) in ethyl acetate between 255 and 323 K. The upper critical solution temperature (theta(u)) for the PDMS/ethyl acetate system is 278 K. The Py-PDMS-Py photophysics follow the Birks model at all of the temperatures studied. However, there is evidence for an increase in the local composition of PDMS surrounding the Py- residues in Py-PDMS-Py below the theta temperature. The recovered activation energy for Py-PDMS-Py tail-tail cyclization is the same as the activation energy for diffusion in ethyl acetate. The activation energy for cyclization is about fourfold larger in comparison to the barrier for PDMS internal backbone rotations. The estimated internal activation barrier for Py-PDMS-Py in ethyl acetate is in line with the internal barrier rotation for PDMS backbone rotation. The intramolecular excimer binding energy for Py-PDMS-Py in ethyl acetate is similar to the intermolecular pyrene excimer. In the ground state, the mean distance between the Py-PDMS-Py tail segments does not change over the temperature range studied. A comparison of these temperature-dependent Py-PDMS-Py/ethyl acetate results with previous results on Py-PDMS-Py in pure supercritical CO2 and CO2-dilated liquids shows that the addition of CO2 to the liquid phase offers the way to tune to Py-PDMS-Py tail-tail dynamics over the greatest range.